Android ListView源碼學習

NelleVNLV 8年前發布 | 16K 次閱讀 ListView Android開發移動開發

ListView源碼分析

項目中使用ListView還是挺多的,之前看過幾次,很是容易遺忘,今特做記錄如下

Android 6.0 & API Level 23
Github: Nvsleep
郵箱: lizhenqiao@126.com
QQ: 522910000

主要從以下幾點進行源碼分析

構造函數初始化
onMeasure()
onLayout()
listview.setAdapter() 以及 adapter.notifyDataSetChanged()
onInterceptTouchEvent()和onTouchEvent()

簡要

ListView繼承之AbsListView抽象類,所以大部分分析的源碼都在這兩個類中

構造函數初始化過程

父類AbsListView的初始化:

public AbsListView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
    super(context, attrs, defStyleAttr, defStyleRes);
    // 初始化設置一些額外屬性值
    initAbsListView();

mOwnerThread = Thread.currentThread();

// 初始化XML文件中設置的某些默認屬性值
final TypedArray a = context.obtainStyledAttributes(
        attrs, R.styleable.AbsListView, defStyleAttr, defStyleRes);

final Drawable selector = a.getDrawable(R.styleable.AbsListView_listSelector);
if (selector != null) {
    setSelector(selector);
}

mDrawSelectorOnTop = a.getBoolean(R.styleable.AbsListView_drawSelectorOnTop, false);

// 初始化設置mStackFromBottom,這個影響到布局子view的順序方式,默認為false
setStackFromBottom(a.getBoolean(
        R.styleable.AbsListView_stackFromBottom, false));
setScrollingCacheEnabled(a.getBoolean(
        R.styleable.AbsListView_scrollingCache, true));
setTextFilterEnabled(a.getBoolean(
        R.styleable.AbsListView_textFilterEnabled, false));
setTranscriptMode(a.getInt(
        R.styleable.AbsListView_transcriptMode, TRANSCRIPT_MODE_DISABLED));
setCacheColorHint(a.getColor(
        R.styleable.AbsListView_cacheColorHint, 0));
setSmoothScrollbarEnabled(a.getBoolean(
        R.styleable.AbsListView_smoothScrollbar, true));
setChoiceMode(a.getInt(
        R.styleable.AbsListView_choiceMode, CHOICE_MODE_NONE));
setFastScrollEnabled(a.getBoolean(
        R.styleable.AbsListView_fastScrollEnabled, false));
setFastScrollStyle(a.getResourceId(
        R.styleable.AbsListView_fastScrollStyle, 0));
setFastScrollAlwaysVisible(a.getBoolean(
        R.styleable.AbsListView_fastScrollAlwaysVisible, false));
a.recycle();

}
private void initAbsListView() {
    // Setting focusable in touch mode will set the focusable property to true
    // 設置ListView本身可以點擊即可以消耗父View分發的事件
    setClickable(true);
    setFocusableInTouchMode(true);
    // 因為向上父類還繼承之ViewGroup,ViewGroup默認不需要重寫draw()方法, 
    // 從而setWillNotDraw(true),但是AbsListView為了滾動效果,自身重寫了View的
    // draw(),主要用于實現滾動到最底部或最頂部的非OVER_SCROLL_NEVER模式的效果
    setWillNotDraw(false);
    setAlwaysDrawnWithCacheEnabled(false);
    setScrollingCacheEnabled(true);

// 事件處理相關變量初始化
final ViewConfiguration configuration = ViewConfiguration.get(mContext);
mTouchSlop = configuration.getScaledTouchSlop();
mMinimumVelocity = configuration.getScaledMinimumFlingVelocity();
mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
mOverscrollDistance = configuration.getScaledOverscrollDistance();
mOverflingDistance = configuration.getScaledOverflingDistance();

mDensityScale = getContext().getResources().getDisplayMetrics().density;

}</code></pre> 
  
ListView的初始化: 
  public ListView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
    super(context, attrs, defStyleAttr, defStyleRes);

final TypedArray a = context.obtainStyledAttributes(
        attrs, R.styleable.ListView, defStyleAttr, defStyleRes);

final CharSequence[] entries = a.getTextArray(R.styleable.ListView_entries);
if (entries != null) {
    setAdapter(new ArrayAdapter<>(context, R.layout.simple_list_item_1, entries));
}
// 獲取item分割線的drawable對象
final Drawable d = a.getDrawable(R.styleable.ListView_divider);
if (d != null) {
    // Use an implicit divider height which may be explicitly
    // overridden by android:dividerHeight further down.
    setDivider(d);
}

final Drawable osHeader = a.getDrawable(R.styleable.ListView_overScrollHeader);
if (osHeader != null) {
    setOverscrollHeader(osHeader);
}

final Drawable osFooter = a.getDrawable(R.styleable.ListView_overScrollFooter);
if (osFooter != null) {
    setOverscrollFooter(osFooter);
}

// Use an explicit divider height, if specified.
// item分割線的高度
if (a.hasValueOrEmpty(R.styleable.ListView_dividerHeight)) {
    final int dividerHeight = a.getDimensionPixelSize(
            R.styleable.ListView_dividerHeight, 0);
    if (dividerHeight != 0) {
        setDividerHeight(dividerHeight);
    }
}

mHeaderDividersEnabled = a.getBoolean(R.styleable.ListView_headerDividersEnabled, true);
mFooterDividersEnabled = a.getBoolean(R.styleable.ListView_footerDividersEnabled, true);

a.recycle();

}</code></pre> 
  
onMeasure() 
  ListView的onMeasure()方法 
  @Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    // Sets up mListPadding
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);

final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);

int childWidth = 0;
int childHeight = 0;
int childState = 0;

mItemCount = mAdapter == null ? 0 : mAdapter.getCount();
if (mItemCount > 0 && (widthMode == MeasureSpec.UNSPECIFIED
        || heightMode == MeasureSpec.UNSPECIFIED)) {
    final View child = obtainView(0, mIsScrap);

    // Lay out child directly against the parent measure spec so that
    // we can obtain exected minimum width and height.
    measureScrapChild(child, 0, widthMeasureSpec, heightSize);

    childWidth = child.getMeasuredWidth();
    childHeight = child.getMeasuredHeight();
    childState = combineMeasuredStates(childState, child.getMeasuredState());

    if (recycleOnMeasure() && mRecycler.shouldRecycleViewType(
            ((LayoutParams) child.getLayoutParams()).viewType)) {
        mRecycler.addScrapView(child, 0);
    }
}

if (widthMode == MeasureSpec.UNSPECIFIED) {
    widthSize = mListPadding.left + mListPadding.right + childWidth +
            getVerticalScrollbarWidth();
} else {
    widthSize |= (childState & MEASURED_STATE_MASK);
}

if (heightMode == MeasureSpec.UNSPECIFIED) {
    heightSize = mListPadding.top + mListPadding.bottom + childHeight +
            getVerticalFadingEdgeLength() * 2;
}

// 有時候需要使ListView的高度等于所有子item view 可以重寫onMeasure()方法使其調用以
// 下代碼
if (heightMode == MeasureSpec.AT_MOST) {
    // TODO: after first layout we should maybe start at the first visible position, not 0
    heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
}

setMeasuredDimension(widthSize, heightSize);

mWidthMeasureSpec = widthMeasureSpec;

}</code></pre> 
  
整體上ListView的onMeasure方法比較簡單,普通. 
  onLayout() 
  ListView由adapter.getView()獲取的子view的layout方式在此實現 
  父類AbsListView的onLayout(): 
  @Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    super.onLayout(changed, l, t, r, b);

mInLayout = true;

final int childCount = getChildCount();
if (changed) {
    for (int i = 0; i < childCount; i++) {
        getChildAt(i).forceLayout();
    }
    mRecycler.markChildrenDirty();
}

// 由子類ListView 和 GridView實現,是核心布局方法代碼,也是listview與adapter交互數據
// 的主要入口函數
layoutChildren();
mInLayout = false;

mOverscrollMax = (b - t) / OVERSCROLL_LIMIT_DIVISOR;

// TODO: Move somewhere sane. This doesn't belong in onLayout().
if (mFastScroll != null) {
    mFastScroll.onItemCountChanged(getChildCount(), mItemCount);
}

}</code></pre> 
  
ListView的layoutChildren(): 
  @Override
protected void layoutChildren() {
    final boolean blockLayoutRequests = mBlockLayoutRequests;
    if (blockLayoutRequests) {
        return;
    }

mBlockLayoutRequests = true;

try {
    super.layoutChildren();

    invalidate();

    if (mAdapter == null) {
        resetList();
        invokeOnItemScrollListener();
        return;
    }

    final int childrenTop = mListPadding.top;
    final int childrenBottom = mBottom - mTop - mListPadding.bottom;
    // 每次即將進行layout子item view的時候先記錄當前listview已有的child view個數
    final int childCount = getChildCount();

    int index = 0;
    int delta = 0;

    View sel;
    View oldSel = null;
    View oldFirst = null;
    View newSel = null;

    // Remember stuff we will need down below
    switch (mLayoutMode) {
    case LAYOUT_SET_SELECTION:
        index = mNextSelectedPosition - mFirstPosition;
        if (index >= 0 && index < childCount) {
            newSel = getChildAt(index);
        }
        break;
    case LAYOUT_FORCE_TOP:
    case LAYOUT_FORCE_BOTTOM:
    case LAYOUT_SPECIFIC:
    case LAYOUT_SYNC:
        break;
    case LAYOUT_MOVE_SELECTION:
    default:
        // Remember the previously selected view
        index = mSelectedPosition - mFirstPosition;
        if (index >= 0 && index < childCount) {
            oldSel = getChildAt(index);
        }

        // Remember the previous first child
        oldFirst = getChildAt(0);

        if (mNextSelectedPosition >= 0) {
            delta = mNextSelectedPosition - mSelectedPosition;
        }

        // Caution: newSel might be null
        newSel = getChildAt(index + delta);
    }


    boolean dataChanged = mDataChanged;
    if (dataChanged) {
        handleDataChanged();
    }

    // Handle the empty set by removing all views that are visible
    // and calling it a day
    if (mItemCount == 0) {
        resetList();
        invokeOnItemScrollListener();
        return;
    } else if (mItemCount != mAdapter.getCount()) {
        throw new IllegalStateException("The content of the adapter has changed but "
                + "ListView did not receive a notification. Make sure the content of "
                + "your adapter is not modified from a background thread, but only from "
                + "the UI thread. Make sure your adapter calls notifyDataSetChanged() "
                + "when its content changes. [in ListView(" + getId() + ", " + getClass()
                + ") with Adapter(" + mAdapter.getClass() + ")]");
    }

    setSelectedPositionInt(mNextSelectedPosition);

    AccessibilityNodeInfo accessibilityFocusLayoutRestoreNode = null;
    View accessibilityFocusLayoutRestoreView = null;
    int accessibilityFocusPosition = INVALID_POSITION;

    // Remember which child, if any, had accessibility focus. This must
    // occur before recycling any views, since that will clear
    // accessibility focus.
    final ViewRootImpl viewRootImpl = getViewRootImpl();
    if (viewRootImpl != null) {
        final View focusHost = viewRootImpl.getAccessibilityFocusedHost();
        if (focusHost != null) {
            final View focusChild = getAccessibilityFocusedChild(focusHost);
            if (focusChild != null) {
                if (!dataChanged || isDirectChildHeaderOrFooter(focusChild)
                        || focusChild.hasTransientState() || mAdapterHasStableIds) {
                    // The views won't be changing, so try to maintain
                    // focus on the current host and virtual view.
                    accessibilityFocusLayoutRestoreView = focusHost;
                    accessibilityFocusLayoutRestoreNode = viewRootImpl
                            .getAccessibilityFocusedVirtualView();
                }

                // If all else fails, maintain focus at the same
                // position.
                accessibilityFocusPosition = getPositionForView(focusChild);
            }
        }
    }

    View focusLayoutRestoreDirectChild = null;
    View focusLayoutRestoreView = null;

    // Take focus back to us temporarily to avoid the eventual call to
    // clear focus when removing the focused child below from messing
    // things up when ViewAncestor assigns focus back to someone else.
    final View focusedChild = getFocusedChild();
    if (focusedChild != null) {
        // TODO: in some cases focusedChild.getParent() == null

        // We can remember the focused view to restore after re-layout
        // if the data hasn't changed, or if the focused position is a
        // header or footer.
        if (!dataChanged || isDirectChildHeaderOrFooter(focusedChild)
                || focusedChild.hasTransientState() || mAdapterHasStableIds) {
            focusLayoutRestoreDirectChild = focusedChild;
            // Remember the specific view that had focus.
            focusLayoutRestoreView = findFocus();
            if (focusLayoutRestoreView != null) {
                // Tell it we are going to mess with it.
                focusLayoutRestoreView.onStartTemporaryDetach();
            }
        }
        requestFocus();
    }

    // Pull all children into the RecycleBin.
    // These views will be reused if possible
    final int firstPosition = mFirstPosition;
    final RecycleBin recycleBin = mRecycler;
    // 只有在調用adapter.notifyDatasetChanged()方法一直到layout()布局結束,
    // dataChanged為true,默認為false
    if (dataChanged) {
        // dataChanged為true,說明當前listview是有數據的了,把當前所有的item view
        // 存放到RecycleBin對象的mScrapViews中保存
        for (int i = 0; i < childCount; i++) {
            recycleBin.addScrapView(getChildAt(i), firstPosition+i);
        }
    } else {
        // dataChanged默認為false,第一次執行此方法走這里
        recycleBin.fillActiveViews(childCount, firstPosition);
    }

    // Clear out old views
    // 清除當前listview所有的子view
    detachAllViewsFromParent();
    recycleBin.removeSkippedScrap();

    // 在我們調用listview.setAdapter()時候,已經將mLayoutMode = LAYOUT_NORMAL;
    // 所以通常情況下可認為mLayoutMode == LAYOUT_NORMAL
    switch (mLayoutMode) {
    case LAYOUT_SET_SELECTION:
        if (newSel != null) {
            sel = fillFromSelection(newSel.getTop(), childrenTop, childrenBottom);
        } else {
            sel = fillFromMiddle(childrenTop, childrenBottom);
        }
        break;
    case LAYOUT_SYNC:
        sel = fillSpecific(mSyncPosition, mSpecificTop);
        break;
    case LAYOUT_FORCE_BOTTOM:
        sel = fillUp(mItemCount - 1, childrenBottom);
        adjustViewsUpOrDown();
        break;
    case LAYOUT_FORCE_TOP:
        mFirstPosition = 0;
        sel = fillFromTop(childrenTop);
        adjustViewsUpOrDown();
        break;
    case LAYOUT_SPECIFIC:
        sel = fillSpecific(reconcileSelectedPosition(), mSpecificTop);
        break;
    case LAYOUT_MOVE_SELECTION:
        sel = moveSelection(oldSel, newSel, delta, childrenTop, childrenBottom);
        break;
    default:
        // 通常情況都走這里
        if (childCount == 0) {
            // listview第一次布局childCount必然為0走這里
            if (!mStackFromBottom) {
                // 通常我們沒有外部調用listview.setStackFromBottom()
                // 成員變量mStackFromBottom均為false都走這里
                final int position = lookForSelectablePosition(0, true);
                setSelectedPositionInt(position);
                // 從上到上布局listview能顯示得下的子view
                sel = fillFromTop(childrenTop);
            } else {
                final int position = lookForSelectablePosition(mItemCount - 1, false);
                setSelectedPositionInt(position);
                sel = fillUp(mItemCount - 1, childrenBottom);
            }
        } else {
            // 非第一次layout,之前記錄的存在的子view個數childCount不為0
            // 包括兩種情況:1.listview首次布局中的第二次執行的onlayout();
            // 2.在后續listview已經顯示存在子view然后數據改變時候調用
            // adapter.nitifyDatasetChanged()方法時候
            if (mSelectedPosition >= 0 && mSelectedPosition < mItemCount) {
                sel = fillSpecific(mSelectedPosition,
                        oldSel == null ? childrenTop : oldSel.getTop());
            } else if (mFirstPosition < mItemCount) {
                // 通常情況走這里,fillSpecific()會調用fillUp()和fillDown()布局子view
                sel = fillSpecific(mFirstPosition,
                        oldFirst == null ? childrenTop : oldFirst.getTop());
            } else {
                sel = fillSpecific(0, childrenTop);
            }
        }
        break;
    }

    // Flush any cached views that did not get reused above
    // 至此,listview已經布局完成能夠顯示得下的子view,將recycleBin可能剩余的
    // mActiveViews中view移動到mScrapViews以便于listview滑動時候復用
    recycleBin.scrapActiveViews();

    if (sel != null) {
        // The current selected item should get focus if items are
        // focusable.
        if (mItemsCanFocus && hasFocus() && !sel.hasFocus()) {
            final boolean focusWasTaken = (sel == focusLayoutRestoreDirectChild &&
                    focusLayoutRestoreView != null &&
                    focusLayoutRestoreView.requestFocus()) || sel.requestFocus();
            if (!focusWasTaken) {
                // Selected item didn't take focus, but we still want to
                // make sure something else outside of the selected view
                // has focus.
                final View focused = getFocusedChild();
                if (focused != null) {
                    focused.clearFocus();
                }
                positionSelector(INVALID_POSITION, sel);
            } else {
                sel.setSelected(false);
                mSelectorRect.setEmpty();
            }
        } else {
            positionSelector(INVALID_POSITION, sel);
        }
        mSelectedTop = sel.getTop();
    } else {
        final boolean inTouchMode = mTouchMode == TOUCH_MODE_TAP
                || mTouchMode == TOUCH_MODE_DONE_WAITING;
        if (inTouchMode) {
            // If the user's finger is down, select the motion position.
            final View child = getChildAt(mMotionPosition - mFirstPosition);
            if (child != null) {
                positionSelector(mMotionPosition, child);
            }
        } else if (mSelectorPosition != INVALID_POSITION) {
            // If we had previously positioned the selector somewhere,
            // put it back there. It might not match up with the data,
            // but it's transitioning out so it's not a big deal.
            final View child = getChildAt(mSelectorPosition - mFirstPosition);
            if (child != null) {
                positionSelector(mSelectorPosition, child);
            }
        } else {
            // Otherwise, clear selection.
            mSelectedTop = 0;
            mSelectorRect.setEmpty();
        }

        // Even if there is not selected position, we may need to
        // restore focus (i.e. something focusable in touch mode).
        if (hasFocus() && focusLayoutRestoreView != null) {
            focusLayoutRestoreView.requestFocus();
        }
    }

    // Attempt to restore accessibility focus, if necessary.
    if (viewRootImpl != null) {
        final View newAccessibilityFocusedView = viewRootImpl.getAccessibilityFocusedHost();
        if (newAccessibilityFocusedView == null) {
            if (accessibilityFocusLayoutRestoreView != null
                    && accessibilityFocusLayoutRestoreView.isAttachedToWindow()) {
                final AccessibilityNodeProvider provider =
                        accessibilityFocusLayoutRestoreView.getAccessibilityNodeProvider();
                if (accessibilityFocusLayoutRestoreNode != null && provider != null) {
                    final int virtualViewId = AccessibilityNodeInfo.getVirtualDescendantId(
                            accessibilityFocusLayoutRestoreNode.getSourceNodeId());
                    provider.performAction(virtualViewId,
                            AccessibilityNodeInfo.ACTION_ACCESSIBILITY_FOCUS, null);
                } else {
                    accessibilityFocusLayoutRestoreView.requestAccessibilityFocus();
                }
            } else if (accessibilityFocusPosition != INVALID_POSITION) {
                // Bound the position within the visible children.
                final int position = MathUtils.constrain(
                        accessibilityFocusPosition - mFirstPosition, 0,
                        getChildCount() - 1);
                final View restoreView = getChildAt(position);
                if (restoreView != null) {
                    restoreView.requestAccessibilityFocus();
                }
            }
        }
    }

    // Tell focus view we are done mucking with it, if it is still in
    // our view hierarchy.
    if (focusLayoutRestoreView != null
            && focusLayoutRestoreView.getWindowToken() != null) {
        focusLayoutRestoreView.onFinishTemporaryDetach();
    }

    // 布局完成之后的操作
    mLayoutMode = LAYOUT_NORMAL;
    mDataChanged = false;
    if (mPositionScrollAfterLayout != null) {
        post(mPositionScrollAfterLayout);
        mPositionScrollAfterLayout = null;
    }
    mNeedSync = false;
    setNextSelectedPositionInt(mSelectedPosition);

    updateScrollIndicators();

    if (mItemCount > 0) {
        checkSelectionChanged();
    }

    invokeOnItemScrollListener();
} finally {
    if (!blockLayoutRequests) {
        mBlockLayoutRequests = false;
    }
}

}</code></pre> 
  
RecycleBin是AbsListView的一個非靜態內部類,主要有一個數組成員變量View[] mActiveViews 和 ArrayList<View>[] mScrapViews.mActiveViews存放的是當前ListView可以使用的待激活的子item view,而mScrapViews存放的是在ListView滑動過程中滑出屏幕來回收以便下次利用的子item view 
  fillFromTop(): 
  /**

Fills the list from top to bottom, starting with mFirstPosition
*
@param nextTop The location where the top of the first item should be
drawn
*
@return The view that is currently selected
*/
private View fillFromTop(int nextTop) {
 mFirstPosition = Math.min(mFirstPosition, mSelectedPosition);
 mFirstPosition = Math.min(mFirstPosition, mItemCount - 1);
 if (mFirstPosition < 0) {
 mFirstPosition = 0;

 }
 return fillDown(mFirstPosition, nextTop);
}</code></pre> 
其實就是保證mFirstPosition>=0的情況下調用fillDown()從上到下依次布局子item view 
fillDown(): 
/**

Fills the list from pos down to the end of the list view.
*
@param pos The first position to put in the list
*
@param nextTop The location where the top of the item associated with pos
should be drawn
*
@return The view that is currently selected, if it happens to be in the
range that we draw.
*/
private View fillDown(int pos, int nextTop) {
View selectedView = null;
int end = (mBottom - mTop);
if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
end -= mListPadding.bottom;
}
// while循環在listview范圍內布局可見數量的子item view
// nextTop == mListPadding.top,可認為是listview的mPaddingTop
// end == mListPadding.bottom,可認為是listview的mPaddingBottom
// nextTop < end說明下一個要裝載的item view的getTop()依然可見,那當然要布局到listview中
while (nextTop < end && pos < mItemCount) {
// is this the selected item?
boolean selected = pos == mSelectedPosition;
View child = makeAndAddView(pos, nextTop, true, mListPadding.left, selected);
nextTop = child.getBottom() + mDividerHeight;
if (selected) {
 selectedView = child;
}
pos++;
}
setVisibleRangeHint(mFirstPosition, mFirstPosition + getChildCount() - 1);
return selectedView;
}</code></pre> 
第一次布局情況下,參數pos==0,即從0開始從上到下依次布局約定數量的從adapter.getView()獲取的子view 
makeAndAddView(): 
/**

Obtain the view and add it to our list of children. The view can be made
fresh, converted from an unused view, or used as is if it was in the
recycle bin.
*
@param position Logical position in the list
@param y Top or bottom edge of the view to add
@param flow If flow is true, align top edge to y. If false, align bottom
edge to y.
@param childrenLeft Left edge where children should be positioned
@param selected Is this position selected?
@return View that was added
*/
private View makeAndAddView(int position, int y, boolean flow, int childrenLeft,

 boolean selected) {

View child;
// 默認情況mDataChanged==false,在調用adapter.notifyDatasetChanged()之后的
 // layout()階段中mDataChanged == true
 if (!mDataChanged) {

 // Try to use an existing view for this position
 // 首先從mRecycler的mActiveViews數組中嘗試獲取可直接用的item view
 // 在前面layoutChildren()方法中首先如果mDataChanged==false會嘗試把item view
 // 放到mRecycler的mActiveViews中保存,mRecycler.getActiveView()中對應positon
 // 的view被獲取到之后本身就不再保存之
 child = mRecycler.getActiveView(position);
 if (child != null) {
     // Found it -- we're using an existing child
     // This just needs to be positioned
     setupChild(child, position, y, flow, childrenLeft, selected, true);

     return child;
 }

}
// Make a new view for this position, or convert an unused view if possible
 child = obtainView(position, mIsScrap);
// This needs to be positioned and measured
 setupChild(child, position, y, flow, childrenLeft, selected, mIsScrap[0]);
return child;
}</code></pre> 
主要用于從RecycleBin或者adapter.getView()中獲取子view并在setupChild()中設置以及布局之 
setupChild(): 
/**

Add a view as a child and make sure it is measured (if necessary) and
positioned properly.
*
@param child The view to add
@param position The position of this child
@param y The y position relative to which this view will be positioned
@param flowDown If true, align top edge to y. If false, align bottom
edge to y.
@param childrenLeft Left edge where children should be positioned
@param selected Is this position selected?
@param recycled Has this view been pulled from the recycle bin? If so it
does not need to be remeasured.
*/
private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
boolean selected, boolean recycled) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "setupListItem");
final boolean isSelected = selected && shouldShowSelector();
final boolean updateChildSelected = isSelected != child.isSelected();
final int mode = mTouchMode;
final boolean isPressed = mode > TOUCH_MODE_DOWN && mode < TOUCH_MODE_SCROLL &&
  mMotionPosition == position;
final boolean updateChildPressed = isPressed != child.isPressed();
// 如果child view是曾經使用過的,已經測量measure過了不需要再次measure之
final boolean needToMeasure = !recycled || updateChildSelected || child.isLayoutRequested();
// Respect layout params that are already in the view. Otherwise make some up...
// noinspection unchecked
AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
if (p == null) {
p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
}
p.viewType = mAdapter.getItemViewType(position);
if ((recycled && !p.forceAdd) || (p.recycledHeaderFooter
  && p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER)) {
// 表明該child view是曾經使用過的,只需要attch一下就行了
attachViewToParent(child, flowDown ? -1 : 0, p);
} else {
p.forceAdd = false;
if (p.viewType == AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER) {
  p.recycledHeaderFooter = true;
}
// 表明不一定是曾經使用過的,需要addview到listview中
addViewInLayout(child, flowDown ? -1 : 0, p, true);
}
if (updateChildSelected) {
child.setSelected(isSelected);
}
if (updateChildPressed) {
child.setPressed(isPressed);
}
if (mChoiceMode != CHOICE_MODE_NONE && mCheckStates != null) {
if (child instanceof Checkable) {
  ((Checkable) child).setChecked(mCheckStates.get(position));
} else if (getContext().getApplicationInfo().targetSdkVersion

  >= android.os.Build.VERSION_CODES.HONEYCOMB) {

child.setActivated(mCheckStates.get(position));
}
}
if (needToMeasure) {
// 測量item view,在viewgroup所有子view都需要測量執行view.measure()方法之后才能布局然后顯示出來
final int childWidthSpec = ViewGroup.getChildMeasureSpec(mWidthMeasureSpec,

  mListPadding.left + mListPadding.right, p.width);

final int lpHeight = p.height;
final int childHeightSpec;
if (lpHeight > 0) {
  childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
} else {
  childHeightSpec = MeasureSpec.makeSafeMeasureSpec(getMeasuredHeight(),

      MeasureSpec.UNSPECIFIED);

}
child.measure(childWidthSpec, childHeightSpec);
} else {
cleanupLayoutState(child);
}
final int w = child.getMeasuredWidth();
final int h = child.getMeasuredHeight();
final int childTop = flowDown ? y : y - h;
// 放置該子view
if (needToMeasure) {
final int childRight = childrenLeft + w;
final int childBottom = childTop + h;
child.layout(childrenLeft, childTop, childRight, childBottom);
} else {
child.offsetLeftAndRight(childrenLeft - child.getLeft());
child.offsetTopAndBottom(childTop - child.getTop());
}
if (mCachingStarted && !child.isDrawingCacheEnabled()) {
child.setDrawingCacheEnabled(true);
}
if (recycled && (((AbsListView.LayoutParams)child.getLayoutParams()).scrappedFromPosition)
  != position) {
child.jumpDrawablesToCurrentState();
}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}</code></pre> 
obtainView(): 
/**

Get a view and have it show the data associated with the specified
position. This is called when we have already discovered that the view is
not available for reuse in the recycle bin. The only choices left are
converting an old view or making a new one.
*
@param position The position to display
@param isScrap Array of at least 1 boolean, the first entry will become true if
the returned view was taken from the scrap heap, false if otherwise.
*
@return A view displaying the data associated with the specified position
*/
View obtainView(int position, boolean[] isScrap) {
 Trace.traceBegin(Trace.TRACE_TAG_VIEW, "obtainView");
isScrap[0] = false;
// 此部分忽略 
 // Check whether we have a transient state view. Attempt to re-bind the
 // data and discard the view if we fail.
 final View transientView = mRecycler.getTransientStateView(position);
 if (transientView != null) {

 final LayoutParams params = (LayoutParams) transientView.getLayoutParams();

 // If the view type hasn't changed, attempt to re-bind the data.
 if (params.viewType == mAdapter.getItemViewType(position)) {
     final View updatedView = mAdapter.getView(position, transientView, this);

     // If we failed to re-bind the data, scrap the obtained view.
     if (updatedView != transientView) {
         setItemViewLayoutParams(updatedView, position);
         mRecycler.addScrapView(updatedView, position);
     }
 }

 isScrap[0] = true;

 // Finish the temporary detach started in addScrapView().
 transientView.dispatchFinishTemporaryDetach();
 return transientView;

}
// 關鍵部分
 // 先從mRecycler的mScrapViews數組中獲取一個在滑動時候廢棄保存的子view
 final View scrapView = mRecycler.getScrapView(position);
 // 平時寫的adapter的getView()方法在此被調用了
 final View child = mAdapter.getView(position, scrapView, this);
 if (scrapView != null) {

 if (child != scrapView) {
     // Failed to re-bind the data, return scrap to the heap.
     // 獲取的不一樣就把剛才的scrapView再次加入到mRecycler的mScrapViews數組中
     mRecycler.addScrapView(scrapView, position);
 } else {
     // 保存已經獲取到廢棄view且可以使用
     isScrap[0] = true;

     // Finish the temporary detach started in addScrapView().
     child.dispatchFinishTemporaryDetach();
 }

}
if (mCacheColorHint != 0) {

 child.setDrawingCacheBackgroundColor(mCacheColorHint);

}
if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {

 child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);

}
setItemViewLayoutParams(child, position);
if (AccessibilityManager.getInstance(mContext).isEnabled()) {

 if (mAccessibilityDelegate == null) {
     mAccessibilityDelegate = new ListItemAccessibilityDelegate();
 }
 if (child.getAccessibilityDelegate() == null) {
     child.setAccessibilityDelegate(mAccessibilityDelegate);
 }

}
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
return child;
}</code></pre> 
返回的要么是adapter.getView()中調用inflate獲取的view,要么是mRecycler的mScrapViews數組中獲取的. 
fillSpecific(): 
/**

Put a specific item at a specific location on the screen and then build
up and down from there.
*
@param position The reference view to use as the starting point
@param top Pixel offset from the top of this view to the top of the
reference view.
*
@return The selected view, or null if the selected view is outside the
visible area.
*/
private View fillSpecific(int position, int top) {
boolean tempIsSelected = position == mSelectedPosition;
// 首先獲取并設置布局當前positon的item view
View temp = makeAndAddView(position, top, true, mListPadding.left, tempIsSelected);
// Possibly changed again in fillUp if we add rows above this one.
mFirstPosition = position;
View above;
View below;
final int dividerHeight = mDividerHeight;
// 然后分別調用fillUp()和fillDown()向上和向下獲取并設置布局其他item view
if (!mStackFromBottom) {
// 通常情況走這里
above = fillUp(position - 1, temp.getTop() - dividerHeight);
// This will correct for the top of the first view not touching the top of the list
// 保證listview的第一個或者最后一個item view在其paddingTop或是paddingBottom內
adjustViewsUpOrDown();
below = fillDown(position + 1, temp.getBottom() + dividerHeight);
int childCount = getChildCount();
if (childCount > 0) {
 correctTooHigh(childCount);
}
} else {
below = fillDown(position + 1, temp.getBottom() + dividerHeight);
// This will correct for the bottom of the last view not touching the bottom of the list
adjustViewsUpOrDown();
above = fillUp(position - 1, temp.getTop() - dividerHeight);
int childCount = getChildCount();
if (childCount > 0) {
  correctTooLow(childCount);
}
}
if (tempIsSelected) {
return temp;
} else if (above != null) {
return above;
} else {
return below;
}
}</code></pre> 
ListView.setAdapter() 
/**

Sets the data behind this ListView.
*
The adapter passed to this method may be wrapped by a {@link WrapperListAdapter},
depending on the ListView features currently in use. For instance, adding
headers and/or footers will cause the adapter to be wrapped.
*
@param adapter The ListAdapter which is responsible for maintaining the
data backing this list and for producing a view to represent an
item in that data set.
*
@see #getAdapter() 
*/
@Override
public void setAdapter(ListAdapter adapter) {
 if (mAdapter != null && mDataSetObserver != null) {

 mAdapter.unregisterDataSetObserver(mDataSetObserver);

}
 // 將一些成員變量還原設置為初始默認值
 resetList();
 // mRecycler的mScrapViews清空并執行listview.removeDetachedView
 mRecycler.clear();
if (mHeaderViewInfos.size() > 0|| mFooterViewInfos.size() > 0) {

 // 如果listview有headerView或者FooterView則會生成包裝adapter
 mAdapter = new HeaderViewListAdapter(mHeaderViewInfos, mFooterViewInfos, adapter);

} else {

 mAdapter = adapter;

}
mOldSelectedPosition = INVALID_POSITION;
 mOldSelectedRowId = INVALID_ROW_ID;
// AbsListView#setAdapter will update choice mode states.
 super.setAdapter(adapter);
if (mAdapter != null) {

 mAreAllItemsSelectable = mAdapter.areAllItemsEnabled();
 mOldItemCount = mItemCount;
 // listview的數據源的item個數
 mItemCount = mAdapter.getCount();
 checkFocus();

 // 重新生成一個內部類對象并將其注冊到adapter中,用于通知回調數據源改變
 mDataSetObserver = new AdapterDataSetObserver();
 mAdapter.registerDataSetObserver(mDataSetObserver);

 // 設置listview的數據源類型,并在mRecycler中初始化對應個數的scrapViews list
 mRecycler.setViewTypeCount(mAdapter.getViewTypeCount());

 int position;
 if (mStackFromBottom) {
     position = lookForSelectablePosition(mItemCount - 1, false);
 } else {
     position = lookForSelectablePosition(0, true);
 }
 setSelectedPositionInt(position);
 setNextSelectedPositionInt(position);

 if (mItemCount == 0) {
     // Nothing selected
     checkSelectionChanged();
 }

} else {

 mAreAllItemsSelectable = true;
 checkFocus();
 // Nothing selected
 checkSelectionChanged();

}
 // 會調用頂層viewRootImpl.performTraversals(),導致視圖重繪,listview重新
 // mesaure layout ...
 requestLayout();
}</code></pre> 
BaseAdapter.registerDataSetObserver(): 
BaseAdapter.notifyDataSetChanged(): 
public abstract class BaseAdapter implements ListAdapter, SpinnerAdapter {
private final DataSetObservable mDataSetObservable = new DataSetObservable();


public boolean hasStableIds() {
    return false;
}
// 注冊監聽回調
public void registerDataSetObserver(DataSetObserver observer) {
    mDataSetObservable.registerObserver(observer);
}
public void unregisterDataSetObserver(DataSetObserver observer) {
    mDataSetObservable.unregisterObserver(observer);
}
/**

Notifies the attached observers that the underlying data has been changed
and any View reflecting the data set should refresh itself.
*/
public void notifyDataSetChanged() {
 mDataSetObservable.notifyChanged();
}

/**

Notifies the attached observers that the underlying data is no longer valid
or available. Once invoked this adapter is no longer valid and should
not report further data set changes.
*/
public void notifyDataSetInvalidated() {
 mDataSetObservable.notifyInvalidated();
}

public boolean areAllItemsEnabled() {
    return true;
}
public boolean isEnabled(int position) {
    return true;
}
public View getDropDownView(int position, View convertView, ViewGroup parent) {
    return getView(position, convertView, parent);
}
public int getItemViewType(int position) {
    return 0;
}
public int getViewTypeCount() {
    return 1;
}
public boolean isEmpty() {
    return getCount() == 0;
}</code></pre> 
  
} 
  Observable.registerObserver(): 
  public abstract class Observable<T> {
/**

The list of observers.  An observer can be in the list at most
once and will never be null.
*/
protected final ArrayList<T> mObservers = new ArrayList<T>();

/**

Adds an observer to the list. The observer cannot be null and it must not already
be registered.
@param observer the observer to register
@throws IllegalArgumentException the observer is null
@throws IllegalStateException the observer is already registered
*/
public void registerObserver(T observer) {
 if (observer == null) {
 throw new IllegalArgumentException("The observer is null.");

 }
 synchronized(mObservers) {
 if (mObservers.contains(observer)) {
     throw new IllegalStateException("Observer " + observer + " is already registered.");
 }
 mObservers.add(observer);

 }
}

/**

Removes a previously registered observer. The observer must not be null and it
must already have been registered.
@param observer the observer to unregister
@throws IllegalArgumentException the observer is null
@throws IllegalStateException the observer is not yet registered
*/
public void unregisterObserver(T observer) {
 if (observer == null) {
 throw new IllegalArgumentException("The observer is null.");

 }
 synchronized(mObservers) {
 int index = mObservers.indexOf(observer);
 if (index == -1) {
     throw new IllegalStateException("Observer " + observer + " was not registered.");
 }
 mObservers.remove(index);

 }
}

/**

Remove all registered observers.
*/
public void unregisterAll() {
 synchronized(mObservers) {

 mObservers.clear();

}
}
}</code></pre> 
BaseAdapter.notifyDataSetChanged()最終會調用AdapterView中的內部類AdapterDataSetObserver.onChanged() 
class AdapterDataSetObserver extends DataSetObserver {
private Parcelable mInstanceState = null;
@Override
 public void onChanged() {

 mDataChanged = true;
 mOldItemCount = mItemCount;
 mItemCount = getAdapter().getCount();

 // Detect the case where a cursor that was previously invalidated has
 // been repopulated with new data.
 if (AdapterView.this.getAdapter().hasStableIds() && mInstanceState != null
         && mOldItemCount == 0 && mItemCount > 0) {
     AdapterView.this.onRestoreInstanceState(mInstanceState);
     mInstanceState = null;
 } else {
     rememberSyncState();
 }
 checkFocus();
 // 同樣,最終調用viewRootImpl.performTraversals(),導致視圖重繪,執行listview的 
 // measure layout 方法等
 requestLayout();

}
@Override
 public void onInvalidated() {

 mDataChanged = true;

 if (AdapterView.this.getAdapter().hasStableIds()) {
     // Remember the current state for the case where our hosting activity is being
     // stopped and later restarted
     mInstanceState = AdapterView.this.onSaveInstanceState();
 }

 // Data is invalid so we should reset our state
 mOldItemCount = mItemCount;
 mItemCount = 0;
 mSelectedPosition = INVALID_POSITION;
 mSelectedRowId = INVALID_ROW_ID;
 mNextSelectedPosition = INVALID_POSITION;
 mNextSelectedRowId = INVALID_ROW_ID;
 mNeedSync = false;

 checkFocus();
 requestLayout();

}</code></pre> 
onInterceptTouchEvent(): 
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
 final int actionMasked = ev.getActionMasked();
 View v;
if (mPositionScroller != null) {

 mPositionScroller.stop();

}
if (mIsDetaching || !isAttachedToWindow()) {

 // Something isn't right.
 // Since we rely on being attached to get data set change notifications,
 // don't risk doing anything where we might try to resync and find things
 // in a bogus state.
 return false;

}
if (mFastScroll != null && mFastScroll.onInterceptTouchEvent(ev)) {

 return true;

}
switch (actionMasked) {
 case MotionEvent.ACTION_DOWN: {

 int touchMode = mTouchMode;
 // 如果手指放下時候 listview正出于fling滾動狀態或者OVERSCROLL,則馬上攔截事件交
 // 由自身ontouchEvent()處理
 if (touchMode == TOUCH_MODE_OVERFLING || touchMode == TOUCH_MODE_OVERSCROLL) {
     mMotionCorrection = 0;
     return true;
 }

 final int x = (int) ev.getX();
 final int y = (int) ev.getY();
 mActivePointerId = ev.getPointerId(0);
 // 獲取當前觸摸事件在對應的item view的positon,在listview中實現了該方法
 int motionPosition = findMotionRow(y);
 if (touchMode != TOUCH_MODE_FLING && motionPosition >= 0) {
     // User clicked on an actual view (and was not stopping a fling).
     // Remember where the motion event started
     v = getChildAt(motionPosition - mFirstPosition);
     mMotionViewOriginalTop = v.getTop();
     mMotionX = x;
     // 記錄down事件時候的mMotionY初始值
     mMotionY = y;
     mMotionPosition = motionPosition;
     // 設置觸摸事件模式為TOUCH_MODE_DOWN
     mTouchMode = TOUCH_MODE_DOWN;
     clearScrollingCache();
 }
 // 初試設置down事件時候mLastY值
 mLastY = Integer.MIN_VALUE;
 initOrResetVelocityTracker();
 mVelocityTracker.addMovement(ev);
 mNestedYOffset = 0;
 startNestedScroll(SCROLL_AXIS_VERTICAL);
 if (touchMode == TOUCH_MODE_FLING) {
     return true;
 }
 break;

}
case MotionEvent.ACTION_MOVE: {

 switch (mTouchMode) {
 case TOUCH_MODE_DOWN:
     int pointerIndex = ev.findPointerIndex(mActivePointerId);
     if (pointerIndex == -1) {
         pointerIndex = 0;
         mActivePointerId = ev.getPointerId(pointerIndex);
     }
     final int y = (int) ev.getY(pointerIndex);
     initVelocityTrackerIfNotExists();
     mVelocityTracker.addMovement(ev);
     // 判斷是否攔截事件自己處理,此為onInterceptTouchEvent()方法核心代碼
     if (startScrollIfNeeded((int) ev.getX(pointerIndex), y, null)) {
         return true;
     }
     break;
 }
 break;

}
case MotionEvent.ACTION_CANCEL:
 case MotionEvent.ACTION_UP: {

 // touchMode恢復默認狀態
 mTouchMode = TOUCH_MODE_REST;
 mActivePointerId = INVALID_POINTER;
 // 回收速度VelocityTracker相關資源
 recycleVelocityTracker();
 reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
 stopNestedScroll();
 break;

}
case MotionEvent.ACTION_POINTER_UP: {

 onSecondaryPointerUp(ev);
 break;

}
 }
return false;
}


// 這個方法在onInterceptTouchEvent的move事件中調用,在onTouchEvent()的onTouchMove()方法
// 中開始時候也會調用
 private boolean startScrollIfNeeded(int x, int y, MotionEvent vtev) {
    // Check if we have moved far enough that it looks more like a
    // scroll than a tap
    // 得到當前事件的y值與down事件時候設置的值的差值
    final int deltaY = y - mMotionY;
    final int distance = Math.abs(deltaY);
    // mScrollY!=0即overscroll為true ,核心為distance > mTouchSlop即攔截事件自己處理
    // mTouchSlop在構造函數中初始化并賦值了
    final boolean overscroll = mScrollY != 0;
    if ((overscroll || distance > mTouchSlop) &&
            (getNestedScrollAxes() & SCROLL_AXIS_VERTICAL) == 0) {
        createScrollingCache();
        if (overscroll) {
            mTouchMode = TOUCH_MODE_OVERSCROLL;
            mMotionCorrection = 0;
        } else {
            // 設置觸摸模式為TOUCH_MODE_SCROLL,在onTouchEvent()用到
            mTouchMode = TOUCH_MODE_SCROLL;
            mMotionCorrection = deltaY > 0 ? mTouchSlop : -mTouchSlop;
        }
        // 取消子view的長按監聽觸發
        removeCallbacks(mPendingCheckForLongPress);
        setPressed(false);
        final View motionView = getChildAt(mMotionPosition - mFirstPosition);
        // listview攔截了事件本身處理,所以恢復可能設置子view的press狀態
        if (motionView != null) {
            motionView.setPressed(false);
        }
        // 通知ScrollState狀態變化回調
        reportScrollStateChange(OnScrollListener.SCROLL_STATE_TOUCH_SCROLL);
        // Time to start stealing events! Once we've stolen them, don't let anyone
        // steal from us
        final ViewParent parent = getParent();
        if (parent != null) {
            parent.requestDisallowInterceptTouchEvent(true);
        }
        // 作用如名,如果滿足條件,滾動listview
        scrollIfNeeded(x, y, vtev);
        return true;
    }

return false;

}</code></pre> 
  
onTouchEvent()之onTouchDown(): 
  private void onTouchDown(MotionEvent ev) {
    mActivePointerId = ev.getPointerId(0);

if (mTouchMode == TOUCH_MODE_OVERFLING) {
    // Stopped the fling. It is a scroll.
    // 如果已經正在出于TOUCH_MODE_OVERFLING則down事件瞬間中斷fling
    mFlingRunnable.endFling();
    if (mPositionScroller != null) {
        mPositionScroller.stop();
    }
    mTouchMode = TOUCH_MODE_OVERSCROLL;
    mMotionX = (int) ev.getX();
    mMotionY = (int) ev.getY();
    mLastY = mMotionY;
    mMotionCorrection = 0;
    mDirection = 0;
} else {
    final int x = (int) ev.getX();
    final int y = (int) ev.getY();
    int motionPosition = pointToPosition(x, y);

    if (!mDataChanged) {
        if (mTouchMode == TOUCH_MODE_FLING) {
            // Stopped a fling. It is a scroll.
            createScrollingCache();
            mTouchMode = TOUCH_MODE_SCROLL;        
            mMotionCorrection = 0;
            motionPosition = findMotionRow(y);
            // 根據y速度判斷是否馬上中斷fling
            mFlingRunnable.flywheelTouch();
        } else if ((motionPosition >= 0) && getAdapter().isEnabled(motionPosition)) {
            // User clicked on an actual view (and was not stopping a
            // fling). It might be a click or a scroll. Assume it is a
            // click until proven otherwise.
            // 設置touch模式為TOUCH_MODE_DOWN
            mTouchMode = TOUCH_MODE_DOWN;

            // FIXME Debounce
            if (mPendingCheckForTap == null) {
                mPendingCheckForTap = new CheckForTap();
            }

            mPendingCheckForTap.x = ev.getX();
            mPendingCheckForTap.y = ev.getY();
            postDelayed(mPendingCheckForTap, ViewConfiguration.getTapTimeout());
        }
    }

    if (motionPosition >= 0) {
        // Remember where the motion event started
        final View v = getChildAt(motionPosition - mFirstPosition);
        mMotionViewOriginalTop = v.getTop();
    }

    mMotionX = x;
    // 記錄mMotionY
    mMotionY = y; 
    mMotionPosition = motionPosition;
    // 記錄mLastY = Integer.MIN_VALUE
    mLastY = Integer.MIN_VALUE;
}

if (mTouchMode == TOUCH_MODE_DOWN && mMotionPosition != INVALID_POSITION
        && performButtonActionOnTouchDown(ev)) {
        removeCallbacks(mPendingCheckForTap);
}

}</code></pre> 
  
onTouchEvent()之onTouchMove(): 
  private void onTouchMove(MotionEvent ev, MotionEvent vtev) {
    int pointerIndex = ev.findPointerIndex(mActivePointerId);
    if (pointerIndex == -1) {
        pointerIndex = 0;
        mActivePointerId = ev.getPointerId(pointerIndex);
    }

if (mDataChanged) {
    // Re-sync everything if data has been changed
    // since the scroll operation can query the adapter.
    layoutChildren();
}

final int y = (int) ev.getY(pointerIndex);

switch (mTouchMode) {
    // 剛開始進入這里 touchMode為TOUCH_MODE_DOWN
    case TOUCH_MODE_DOWN:
    case TOUCH_MODE_TAP:
    case TOUCH_MODE_DONE_WAITING:
        // Check if we have moved far enough that it looks more like a
        // scroll than a tap. If so, we'll enter scrolling mode.
        // 剛開始還么有處于可滾動狀態,故進入判斷是否可以滾動,核心判斷調節為當然事件y
        // 與down事件mMotionY值的差值絕對值是否大于mTouchSlop
        if (startScrollIfNeeded((int) ev.getX(pointerIndex), y, vtev)) {
            break;
        }
        // Otherwise, check containment within list bounds. If we're
        // outside bounds, cancel any active presses.
        final View motionView = getChildAt(mMotionPosition - mFirstPosition);
        final float x = ev.getX(pointerIndex);
        if (!pointInView(x, y, mTouchSlop)) {
            setPressed(false);
            if (motionView != null) {
                motionView.setPressed(false);
            }
            removeCallbacks(mTouchMode == TOUCH_MODE_DOWN ?
                    mPendingCheckForTap : mPendingCheckForLongPress);
            mTouchMode = TOUCH_MODE_DONE_WAITING;
            updateSelectorState();
        } else if (motionView != null) {
            // Still within bounds, update the hotspot.
            final float[] point = mTmpPoint;
            point[0] = x;
            point[1] = y;
            transformPointToViewLocal(point, motionView);
            motionView.drawableHotspotChanged(point[0], point[1]);
        }
        break;
    case TOUCH_MODE_SCROLL:
    case TOUCH_MODE_OVERSCROLL:
        // 如果已經進入到listview的滾動狀態,則直接執行scrollIfNeeded根據條件判斷是否
        // 進行滾動
        scrollIfNeeded((int) ev.getX(pointerIndex), y, vtev);
        break;
}

}
private void scrollIfNeeded(int x, int y, MotionEvent vtev) {
    int rawDeltaY = y - mMotionY;
    int scrollOffsetCorrection = 0;
    int scrollConsumedCorrection = 0;
    // mLastY==Integer.MIN_VALUE表明剛達到條件進入滾動狀態
    if (mLastY == Integer.MIN_VALUE) {
        // 保證了狀態過度時候平穩滾動
        rawDeltaY -= mMotionCorrection;
    }
    if (dispatchNestedPreScroll(0, mLastY != Integer.MIN_VALUE ? mLastY - y : -rawDeltaY,
            mScrollConsumed, mScrollOffset)) {
        rawDeltaY += mScrollConsumed[1];
        scrollOffsetCorrection = -mScrollOffset[1];
        scrollConsumedCorrection = mScrollConsumed[1];
        if (vtev != null) {
            vtev.offsetLocation(0, mScrollOffset[1]);
            mNestedYOffset += mScrollOffset[1];
        }
    }
    final int deltaY = rawDeltaY;
    // 兩次連續下發事件的y差值.如果mLastY==Integer.MIN_VALUE表明剛達到條件進入滾動狀態
    // 此時incrementalDeltaY = rawDeltaY,而rawDeltaY已經在上面進行了
    // (rawDeltaY -= mMotionCorrection),保證了平穩過度
    int incrementalDeltaY =
            mLastY != Integer.MIN_VALUE ? y - mLastY + scrollConsumedCorrection : deltaY;
    int lastYCorrection = 0;
    if (mTouchMode == TOUCH_MODE_SCROLL) {
        if (PROFILE_SCROLLING) {
            if (!mScrollProfilingStarted) {
                Debug.startMethodTracing("AbsListViewScroll");
                mScrollProfilingStarted = true;
            }
        }

    if (mScrollStrictSpan == null) {
        // If it's non-null, we're already in a scroll.
        mScrollStrictSpan = StrictMode.enterCriticalSpan("AbsListView-scroll");
    }

    if (y != mLastY) {
        // We may be here after stopping a fling and continuing to scroll.
        // If so, we haven't disallowed intercepting touch events yet.
        // Make sure that we do so in case we're in a parent that can intercept.
        if ((mGroupFlags & FLAG_DISALLOW_INTERCEPT) == 0 &&
                Math.abs(rawDeltaY) > mTouchSlop) {
            final ViewParent parent = getParent();
            if (parent != null) {
                parent.requestDisallowInterceptTouchEvent(true);
            }
        }

        final int motionIndex;
        if (mMotionPosition >= 0) {
            motionIndex = mMotionPosition - mFirstPosition;
        } else {
            // If we don't have a motion position that we can reliably track,
            // pick something in the middle to make a best guess at things below.
            motionIndex = getChildCount() / 2;
        }

        int motionViewPrevTop = 0;
        View motionView = this.getChildAt(motionIndex);
        if (motionView != null) {
            motionViewPrevTop = motionView.getTop();
        }

        // No need to do all this work if we're not going to move anyway
        boolean atEdge = false;
        // trackMotionScroll()方法真正進行滾動處理
        if (incrementalDeltaY != 0) {
            atEdge = trackMotionScroll(deltaY, incrementalDeltaY);
        }

        // Check to see if we have bumped into the scroll limit
        motionView = this.getChildAt(motionIndex);
        if (motionView != null) {
            // Check if the top of the motion view is where it is
            // supposed to be
            final int motionViewRealTop = motionView.getTop();
            // 滾動到最頂部或者最底部
            if (atEdge) {
                // Apply overscroll

                int overscroll = -incrementalDeltaY -
                        (motionViewRealTop - motionViewPrevTop);
                if (dispatchNestedScroll(0, overscroll - incrementalDeltaY, 0, overscroll,
                        mScrollOffset)) {
                    lastYCorrection -= mScrollOffset[1];
                    if (vtev != null) {
                        vtev.offsetLocation(0, mScrollOffset[1]);
                        mNestedYOffset += mScrollOffset[1];
                    }
                } else {
                    final boolean atOverscrollEdge = overScrollBy(0, overscroll,
                            0, mScrollY, 0, 0, 0, mOverscrollDistance, true);

                    if (atOverscrollEdge && mVelocityTracker != null) {
                        // Don't allow overfling if we're at the edge
                        mVelocityTracker.clear();
                    }

                    final int overscrollMode = getOverScrollMode();
                    if (overscrollMode == OVER_SCROLL_ALWAYS ||
                            (overscrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS &&
                                    !contentFits())) {
                        if (!atOverscrollEdge) {
                            mDirection = 0; // Reset when entering overscroll.
                            mTouchMode = TOUCH_MODE_OVERSCROLL;
                        }
                        if (incrementalDeltaY > 0) {
                            // 頂部 OVER_SCROLL效果,draw()方法中實現
                            mEdgeGlowTop.onPull((float) -overscroll / getHeight(),
                                    (float) x / getWidth());
                            if (!mEdgeGlowBottom.isFinished()) {
                                mEdgeGlowBottom.onRelease();
                            }
                            invalidateTopGlow();
                        } else if (incrementalDeltaY < 0) {
                            // 底部 OVER_SCROLL效果,draw()方法中實現
                            mEdgeGlowBottom.onPull((float) overscroll / getHeight(),
                                    1.f - (float) x / getWidth());
                            if (!mEdgeGlowTop.isFinished()) {
                                mEdgeGlowTop.onRelease();
                            }
                            invalidateBottomGlow();
                        }
                    }
                }
            }
            mMotionY = y + lastYCorrection + scrollOffsetCorrection;
        }
        // 記錄當前事件的y
        mLastY = y + lastYCorrection + scrollOffsetCorrection;
    }
}else if (mTouchMode == TOUCH_MODE_OVERSCROLL){.....}</code></pre> 

滾動實現核心代碼trackMotionScroll(int deltaY, int incrementalDeltaY): 
  關鍵變量在第二個參數incrementalDeltaY,即兩次連續事件的y差值 
  /**
 * Track a motion scroll
 *
 * @param deltaY Amount to offset mMotionView. This is the accumulated delta since the motion
 *        began. Positive numbers mean the user's finger is moving down the screen.
 * @param incrementalDeltaY Change in deltaY from the previous event.
 * @return true if we're already at the beginning/end of the list and have nothing to do.
 */
 boolean trackMotionScroll(int deltaY, int incrementalDeltaY) {
    final int childCount = getChildCount();
    if (childCount == 0) {
        return true;
    }
    // listview 第一個item view的mTop值
    final int firstTop = getChildAt(0).getTop();
    // // listview 最后一個item view的mBottom值
    final int lastBottom = getChildAt(childCount - 1).getBottom();

    final Rect listPadding = mListPadding;

    // "effective padding" In this case is the amount of padding that affects
    // how much space should not be filled by items. If we don't clip to padding
    // there is no effective padding.
    int effectivePaddingTop = 0;
    int effectivePaddingBottom = 0;
    if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
        effectivePaddingTop = listPadding.top;
        effectivePaddingBottom = listPadding.bottom;
    }

     // FIXME account for grid vertical spacing too?
    final int spaceAbove = effectivePaddingTop - firstTop;
    final int end = getHeight() - effectivePaddingBottom;
    final int spaceBelow = lastBottom - end;

    final int height = getHeight() - mPaddingBottom - mPaddingTop;
    if (deltaY < 0) {
        deltaY = Math.max(-(height - 1), deltaY);
    } else {
        deltaY = Math.min(height - 1, deltaY);
    }
    // 限定incrementalDeltaY在合理的最值范圍內
    if (incrementalDeltaY < 0) {
        incrementalDeltaY = Math.max(-(height - 1), incrementalDeltaY);
    } else {
        incrementalDeltaY = Math.min(height - 1, incrementalDeltaY);
    }

    final int firstPosition = mFirstPosition;

    // Update our guesses for where the first and last views are
    if (firstPosition == 0) {
        mFirstPositionDistanceGuess = firstTop - listPadding.top;
    } else {
        mFirstPositionDistanceGuess += incrementalDeltaY;
    }
    if (firstPosition + childCount == mItemCount) {
        mLastPositionDistanceGuess = lastBottom + listPadding.bottom;
    } else {
        mLastPositionDistanceGuess += incrementalDeltaY;
    }
    // 判斷是否在最頂部且手指向下滑動,是的話即不能向下滑動了
    final boolean cannotScrollDown = (firstPosition == 0 &&
            firstTop >= listPadding.top && incrementalDeltaY >= 0);
    // 判斷是否在最底部且手指向上滑動,是的話即不能向上滑動了
    final boolean cannotScrollUp = (firstPosition + childCount == mItemCount &&
            lastBottom <= getHeight() - listPadding.bottom && incrementalDeltaY <= 0);
    // listview無法滾動即返回
    if (cannotScrollDown || cannotScrollUp) {
        return incrementalDeltaY != 0;
    }
    // incrementalDeltaY<0說明手指是向上滑動的,即listview內容視圖是向下移動顯示的
    final boolean down = incrementalDeltaY < 0;

    final boolean inTouchMode = isInTouchMode();
    if (inTouchMode) {
        hideSelector();
    }

    final int headerViewsCount = getHeaderViewsCount();
    final int footerViewsStart = mItemCount - getFooterViewsCount();

    int start = 0;
    int count = 0;

    if (down) {
        // 手指是向上滑動 incrementalDeltaY < 0
        int top = -incrementalDeltaY;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            top += listPadding.top;
        }
        for (int i = 0; i < childCount; i++) {
            final View child = getChildAt(i);
            if (child.getBottom() >= top) {
                break;
            } else {
                // 最top的子view已經滑出listview
                count++;
                int position = firstPosition + i;
                if (position >= headerViewsCount && position < footerViewsStart) {
                    // The view will be rebound to new data, clear any
                    // system-managed transient state.
                    child.clearAccessibilityFocus();
                    // 將最頂部滑出的子view 加入到mRecycler的mScrapViews中保存
                    mRecycler.addScrapView(child, position);
                }
            }
        }
    } else {
        // 手指是向下滑動 incrementalDeltaY > 0
        int bottom = getHeight() - incrementalDeltaY;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            bottom -= listPadding.bottom;
        }
        for (int i = childCount - 1; i >= 0; i--) {
            final View child = getChildAt(i);
            if (child.getTop() <= bottom) {
                break;
            } else {
                // 最底部的子view已經滑出listview
                start = i;
                count++;
                int position = firstPosition + i;
                if (position >= headerViewsCount && position < footerViewsStart) {
                    // The view will be rebound to new data, clear any
                    // system-managed transient state.
                    child.clearAccessibilityFocus();
                    // 將最底部滑出的子view 加入到mRecycler的mScrapViews中保存
                    mRecycler.addScrapView(child, position);
                }
            }
        }
    }

    mMotionViewNewTop = mMotionViewOriginalTop + deltaY;

    mBlockLayoutRequests = true;

    if (count > 0) {
        // 將上面滑出的子view 從listview中detach掉
        detachViewsFromParent(start, count);
        mRecycler.removeSkippedScrap();
    }

    // invalidate before moving the children to avoid unnecessary invalidate
    // calls to bubble up from the children all the way to the top
    if (!awakenScrollBars()) {
       invalidate();
    }

    // 核心滾動便宜代碼,根據incrementalDeltaY同步偏移所有的子view
    offsetChildrenTopAndBottom(incrementalDeltaY);

    if (down) {
        mFirstPosition += count;
    }

    // 根據條件判斷是否填充滑動進入listview的子view
    final int absIncrementalDeltaY = Math.abs(incrementalDeltaY);
    if (spaceAbove < absIncrementalDeltaY || spaceBelow < absIncrementalDeltaY) {
        fillGap(down);
    }

    if (!inTouchMode && mSelectedPosition != INVALID_POSITION) {
        final int childIndex = mSelectedPosition - mFirstPosition;
        if (childIndex >= 0 && childIndex < getChildCount()) {
            positionSelector(mSelectedPosition, getChildAt(childIndex));
        }
    } else if (mSelectorPosition != INVALID_POSITION) {
        final int childIndex = mSelectorPosition - mFirstPosition;
        if (childIndex >= 0 && childIndex < getChildCount()) {
            positionSelector(INVALID_POSITION, getChildAt(childIndex));
        }
    } else {
        mSelectorRect.setEmpty();
    }

    mBlockLayoutRequests = false;

    invokeOnItemScrollListener();

    return false;
} 
  fillGap(): 
  滾動過程判斷需要加載填充滑動進的子view的處理部分 
  @Override
void fillGap(boolean down) {
    final int count = getChildCount();
    if (down) {
        // 手指是向上滑動,需要填充最底部滑動進的子view
        int paddingTop = 0;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            paddingTop = getListPaddingTop();
        }
        final int startOffset = count > 0 ? getChildAt(count - 1).getBottom() + mDividerHeight :
                paddingTop;
        fillDown(mFirstPosition + count, startOffset);
        correctTooHigh(getChildCount());
    } else {
        // 手指是向下滑動,需要填充最頂部滑動進的子view
        int paddingBottom = 0;
        if ((mGroupFlags & CLIP_TO_PADDING_MASK) == CLIP_TO_PADDING_MASK) {
            paddingBottom = getListPaddingBottom();
        }
        final int startOffset = count > 0 ? getChildAt(0).getTop() - mDividerHeight :
                getHeight() - paddingBottom;
        fillUp(mFirstPosition - 1, startOffset);
        correctTooLow(getChildCount());
    }
} 
  onTouchEvent()之onTouchUp(): 
  private void onTouchUp(MotionEvent ev) {
    switch (mTouchMode) {

    ....

    case TOUCH_MODE_SCROLL:
        final int childCount = getChildCount();
        if (childCount > 0) {
            final int firstChildTop = getChildAt(0).getTop();
            final int lastChildBottom = getChildAt(childCount - 1).getBottom();
            final int contentTop = mListPadding.top;
            final int contentBottom = getHeight() - mListPadding.bottom;
            if (mFirstPosition == 0 && firstChildTop >= contentTop &&
                    mFirstPosition + childCount < mItemCount &&
                    lastChildBottom <= getHeight() - contentBottom) {
                mTouchMode = TOUCH_MODE_REST;
                reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
            } else {
                final VelocityTracker velocityTracker = mVelocityTracker;
                velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);

                final int initialVelocity = (int)
                        (velocityTracker.getYVelocity(mActivePointerId) * mVelocityScale);
                // Fling if we have enough velocity and we aren't at a boundary.
                // Since we can potentially overfling more than we can overscroll, don't
                // allow the weird behavior where you can scroll to a boundary then
                // fling further.
                boolean flingVelocity = Math.abs(initialVelocity) > mMinimumVelocity;
                // 判斷滿足fling條件則進入
                if (flingVelocity &&
                        !((mFirstPosition == 0 &&
                                firstChildTop == contentTop - mOverscrollDistance) ||
                          (mFirstPosition + childCount == mItemCount &&
                                lastChildBottom == contentBottom + mOverscrollDistance))) {
                    if (!dispatchNestedPreFling(0, -initialVelocity)) {
                        if (mFlingRunnable == null) {
                            mFlingRunnable = new FlingRunnable();
                        }
                        reportScrollStateChange(OnScrollListener.SCROLL_STATE_FLING);
                        // fling走起
                        mFlingRunnable.start(-initialVelocity);
                        dispatchNestedFling(0, -initialVelocity, true);
                    } else {
                        mTouchMode = TOUCH_MODE_REST;
                        reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
                    }
                } else {
                    // 不滿足fling條件 
                    mTouchMode = TOUCH_MODE_REST;
                    reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
                    if (mFlingRunnable != null) {
                        mFlingRunnable.endFling();
                    }
                    if (mPositionScroller != null) {
                        mPositionScroller.stop();
                    }
                    if (flingVelocity && !dispatchNestedPreFling(0, -initialVelocity)) {
                        dispatchNestedFling(0, -initialVelocity, false);
                    }
                }
            }
        } else {
            mTouchMode = TOUCH_MODE_REST;
            reportScrollStateChange(OnScrollListener.SCROLL_STATE_IDLE);
        }
        break;

    ....

    }
     setPressed(false);
    // over_scroll_mode 時候資源回收處理
    if (mEdgeGlowTop != null) {
        mEdgeGlowTop.onRelease();
        mEdgeGlowBottom.onRelease();
    }

    // Need to redraw since we probably aren't drawing the selector anymore
    invalidate();
    removeCallbacks(mPendingCheckForLongPress);
    recycleVelocityTracker();

    mActivePointerId = INVALID_POINTER;

    if (PROFILE_SCROLLING) {
        if (mScrollProfilingStarted) {
            Debug.stopMethodTracing();
            mScrollProfilingStarted = false;
        }
    }

    if (mScrollStrictSpan != null) {
        mScrollStrictSpan.finish();
        mScrollStrictSpan = null;
    }
} 
  Up事件的fling處理主要在AbsListView的內部類FlingRunnable中: 
  FlingRunnable.start(): 
  void start(int initialVelocity) {
        int initialY = initialVelocity < 0 ? Integer.MAX_VALUE : 0;
        mLastFlingY = initialY;
        mScroller.setInterpolator(null);
        // 設置scroller做fling動作
        mScroller.fling(0, initialY, 0, initialVelocity,
                0, Integer.MAX_VALUE, 0, Integer.MAX_VALUE);
        mTouchMode = TOUCH_MODE_FLING;
        // 在下一幀調用run()方法
        postOnAnimation(this);

        if (PROFILE_FLINGING) {
            if (!mFlingProfilingStarted) {
                Debug.startMethodTracing("AbsListViewFling");
                mFlingProfilingStarted = true;
            }
        }

        if (mFlingStrictSpan == null) {
            mFlingStrictSpan = StrictMode.enterCriticalSpan("AbsListView-fling");
        }
    } 
  FlingRunnable實現了Runnable,復寫的run(): 
  @Override
    public void run() {
        switch (mTouchMode) {
        default:
            endFling();
            return;

        case TOUCH_MODE_SCROLL:
            if (mScroller.isFinished()) {
                return;
            }
            // Fall through
        case TOUCH_MODE_FLING: {
            if (mDataChanged) {
                layoutChildren();
            }

            if (mItemCount == 0 || getChildCount() == 0) {
                endFling();
                return;
            }

            final OverScroller scroller = mScroller;
            boolean more = scroller.computeScrollOffset();
            final int y = scroller.getCurrY();

            // Flip sign to convert finger direction to list items direction
            // (e.g. finger moving down means list is moving towards the top)
            int delta = mLastFlingY - y;

            // Pretend that each frame of a fling scroll is a touch scroll
            if (delta > 0) {
                // List is moving towards the top. Use first view as mMotionPosition
                mMotionPosition = mFirstPosition;
                final View firstView = getChildAt(0);
                mMotionViewOriginalTop = firstView.getTop();

                // Don't fling more than 1 screen
                delta = Math.min(getHeight() - mPaddingBottom - mPaddingTop - 1, delta);
            } else {
                // List is moving towards the bottom. Use last view as mMotionPosition
                int offsetToLast = getChildCount() - 1;
                mMotionPosition = mFirstPosition + offsetToLast;

                final View lastView = getChildAt(offsetToLast);
                mMotionViewOriginalTop = lastView.getTop();

                // Don't fling more than 1 screen
                delta = Math.max(-(getHeight() - mPaddingBottom - mPaddingTop - 1), delta);
            }

            // Check to see if we have bumped into the scroll limit
            View motionView = getChildAt(mMotionPosition - mFirstPosition);
            int oldTop = 0;
            if (motionView != null) {
                oldTop = motionView.getTop();
            }

            // Don't stop just because delta is zero (it could have been rounded)
            // fling滾動過程模擬的滑動處理
            final boolean atEdge = trackMotionScroll(delta, delta);
            final boolean atEnd = atEdge && (delta != 0);
            if (atEnd) {
                if (motionView != null) {
                    // Tweak the scroll for how far we overshot
                    int overshoot = -(delta - (motionView.getTop() - oldTop));
                    overScrollBy(0, overshoot, 0, mScrollY, 0, 0,
                            0, mOverflingDistance, false);
                }
                if (more) {
                    edgeReached(delta);
                }
                break;
            }

            if (more && !atEnd) {
                // fling沒有結束 繼續遞歸調用run()方法
                if (atEdge) invalidate();
                mLastFlingY = y;
                postOnAnimation(this);
            } else {    
                // fling已經結束
                endFling();

                if (PROFILE_FLINGING) {
                    if (mFlingProfilingStarted) {
                        Debug.stopMethodTracing();
                        mFlingProfilingStarted = false;
                    }

                    if (mFlingStrictSpan != null) {
                        mFlingStrictSpan.finish();
                        mFlingStrictSpan = null;
                    }
                }
            }
            break;
        }

        ....

        }
        }
    } 
  至此,ListView源碼分析完... 
    
    
    
  來自：http://www.jianshu.com/p/7f95297b6271


                    
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                         ListView Android開發 移動開發

Android ListView源碼學習

ListView源碼分析

主要從以下幾點進行源碼分析

簡要

構造函數初始化過程

onMeasure()

onLayout()

ListView.setAdapter()

onInterceptTouchEvent():

相關經驗

相關資訊

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目錄