SwipeDismissBehavior用法及實現原理
引文
無意間發現design兼容庫中有一個叫做SwipeDismissBehavior的類，顧名思義它就是用來實現滑動刪除的了。莫非現在滑動刪除又有更簡單的解決辦法了？鑒于之前RecyclerView中已經有ItemTouchHelper，而且也非常簡單，所以很好奇到底有何不同，于是決定研究研究，看看它的實現原理以及應用場景：真的能替代其他的（不管是第三方還是RecyclerView自帶的ItemTouchHelper）滑動刪除嗎？。
很不幸SwipeDismissBehavior現在的文檔還很少，只有stackoverfolw上有點價值的討論。
先來直接從API的角度使用SwipeDismissBehavior，然后再講解SwipeDismissBehavior的原理。從而說明為什么SwipeDismissBehavior只能和CoordinatorLayout一起使用？為什么SwipeDismissBehavior對CoordinatorLayout中RecyclerView的item不起作用。
SwipeDismissBehavior的用法
SwipeDismissBehavior的用法非常簡單。
第一步：引入design庫：
compile 'com.android.support:appcompat-v7:23.1.0' compile 'com.android.support:design:23+'
第二步：把要滑動刪除的View放在CoordinatorLayout中：
xml代碼：
<?xml version="1.0" encoding="utf-8"?> <android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools" android:id="@+id/coordinatorLayout" android:layout_width="match_parent" android:layout_height="match_parent" > <TextView android:id="@+id/swip" android:layout_width="match_parent" android:layout_height="200dip" android:background="#32CD32" android:text="別刪我" android:textSize="20dip" android:gravity="center" /> </android.support.design.widget.CoordinatorLayout>
第三步：在MainActivity中為View設置一個SwipeDismissBehavior對象：
package com.jcodecraeer.swipedismissbehaviordemo;
import android.os.Bundle;
import android.support.design.widget.CoordinatorLayout;
import android.support.design.widget.SwipeDismissBehavior;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
TextView swipeView = (TextView)findViewById(R.id.swip);
final SwipeDismissBehavior<View> swipe
= new SwipeDismissBehavior();
swipe.setSwipeDirection(
SwipeDismissBehavior.SWIPE_DIRECTION_ANY);
swipe.setListener(
new SwipeDismissBehavior.OnDismissListener() {
@Override public void onDismiss(View view) {
}
@Override
public void onDragStateChanged(int state) {}
});
CoordinatorLayout.LayoutParams coordinatorParams =
(CoordinatorLayout.LayoutParams) swipeView.getLayoutParams();
coordinatorParams.setBehavior(swipe);
}
}
然后運行就能得到如下效果：
就是這么簡單。
下面來講講SwipeDismissBehavior的原理。
要講SwipeDismissBehavior，得先講講CoordinatorLayout.Behavior。因為
SwipeDismissBehavior類是如下定義的。
public class SwipeDismissBehavior<V extends View> extends CoordinatorLayout.Behavior<V>
從這個定義可以看出SwipeDismissBehavior繼承自CoordinatorLayout.Behavior。
CoordinatorLayout與Behavior
Behavior是CoordinatorLayout的一個內部類
public static abstract class Behavior<V extends View>
它只定義了一些抽象方法，其中最主要的當屬下面兩個（與本文相關）：
public boolean onInterceptTouchEvent(CoordinatorLayout parent, V child, MotionEvent ev) {
return false;
}
public boolean onTouchEvent(CoordinatorLayout parent, V child, MotionEvent ev) {
return false;
} CoordinatorLayout會在自己的onInterceptTouchEvent()方法中調用Behavior的
onInterceptTouchEvent：
b.onInterceptTouchEvent(this, child, ev);
把自己(this)、與此Behavior對象相關的子view（child）以及MotionEvent ev傳遞過去。這三個參數對于實現一個Behavior都至關重要。
CoordinatorLayout遍歷子view，判斷子view的mLayoutParam變量中是否有Behavior成員，如果有則調用Behavior的onInterceptTouchEvent和onTouchEvent方法。上面的代碼中，swipeView通過
CoordinatorLayout.LayoutParams coordinatorParams = (CoordinatorLayout.LayoutParams) swipeView.getLayoutParams(); coordinatorParams.setBehavior(swipe);
給自己設置了一個類型為SwipeDismissBehavior的Behavior，而且它又是CoordinatorLayout的子view，因此當CoordinatorLayout遍歷到了這個cardview的時候，會嘗試從這個swipeView獲得Behavior：
final LayoutParams lp = (LayoutParams) child.getLayoutParams(); final Behavior b = lp.getBehavior();
注：這里的LayoutParams是CoordinatorLayout.LayoutParams類，跟ViewGroup的還是有所區別，他是CoordinatorLayout的內部類。其實任何一個布局比如LinearLayout都有自己的LayoutParams類型，也都是定義在布局類的內部。
如果檢測到這個Behavior不為空，就調用它的onInterceptTouchEvent和onTouchEvent方法。
if (!intercepted && b != null) {
switch (type) {
case TYPE_ON_INTERCEPT:
intercepted = b.onInterceptTouchEvent(this, child, ev);
break;
case TYPE_ON_TOUCH:
intercepted = b.onTouchEvent(this, child, ev);
break;
}
if (intercepted) {
mBehaviorTouchView = child;
}
} 自此CoordinatorLayout的任務完成，因為我們設置的這個Behavior是SwipeDismissBehavior對象，所以接下來該怎么處理就交給SwipeDismissBehavior了。
以上過程基本都在CoordinatorLayout的performIntercept(MotionEvent ev, final int type)方法里。
SwipeDismissBehavior
那么我們看看SwipeDismissBehavior的onInterceptTouchEvent和onTouchEvent方法到底做了什么呢？
public boolean onInterceptTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) {
switch(MotionEventCompat.getActionMasked(event)) {
case 1:
case 3:
if(this.mIgnoreEvents) {
this.mIgnoreEvents = false;
return false;
}
break;
default:
this.mIgnoreEvents = !parent.isPointInChildBounds(child, (int)event.getX(), (int)event.getY());
}
if(this.mIgnoreEvents) {
return false;
} else {
this.ensureViewDragHelper(parent);
return this.mViewDragHelper.shouldInterceptTouchEvent(event);
}
}
public boolean onTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) {
if(this.mViewDragHelper != null) {
this.mViewDragHelper.processTouchEvent(event);
return true;
} else {
return false;
}
} 在onInterceptTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) 中，調用了ensureViewDragHelper(parent)：
private void ensureViewDragHelper(ViewGroup parent) {
if(this.mViewDragHelper == null) {
this.mViewDragHelper = this.mSensitivitySet?ViewDragHelper.create(parent, this.mSensitivity, this.mDragCallback):ViewDragHelper.create(parent, this.mDragCallback);
}
}
可以看到這里其實是用parent參數創建了一個ViewDragHelper，根據前面的分析，這里的parent其實就是CoordinatorLayout對象。如果你熟悉ViewDragHelper，那么基本上都能猜到SwipeDismissBehavior要做些什么了。
SwipeDismissBehavior就是根據 MotionEvent和parent創建了一個實現了滑動刪除的ViewDragHelper。
具體實現的代碼請看SwipeDismissBehavior的mDragCallback變量。
private final Callback mDragCallback = new Callback() {
private int mOriginalCapturedViewLeft;
public boolean tryCaptureView(View child, int pointerId) {
this.mOriginalCapturedViewLeft = child.getLeft();
return true;
}
public void onViewDragStateChanged(int state) {
if(SwipeDismissBehavior.this.mListener != null) {
SwipeDismissBehavior.this.mListener.onDragStateChanged(state);
}
}
public void onViewReleased(View child, float xvel, float yvel) {
int childWidth = child.getWidth();
boolean dismiss = false;
int targetLeft;
if(this.shouldDismiss(child, xvel)) {
targetLeft = child.getLeft() < this.mOriginalCapturedViewLeft?this.mOriginalCapturedViewLeft - childWidth:this.mOriginalCapturedViewLeft + childWidth;
dismiss = true;
} else {
targetLeft = this.mOriginalCapturedViewLeft;
}
if(SwipeDismissBehavior.this.mViewDragHelper.settleCapturedViewAt(targetLeft, child.getTop())) {
ViewCompat.postOnAnimation(child, SwipeDismissBehavior.this.new SettleRunnable(child, dismiss));
} else if(dismiss && SwipeDismissBehavior.this.mListener != null) {
SwipeDismissBehavior.this.mListener.onDismiss(child);
}
}
private boolean shouldDismiss(View child, float xvel) {
if(xvel != 0.0F) {
boolean distance1 = ViewCompat.getLayoutDirection(child) == 1;
return SwipeDismissBehavior.this.mSwipeDirection == 2?true:(SwipeDismissBehavior.this.mSwipeDirection == 0?(distance1?xvel < 0.0F:xvel > 0.0F):(SwipeDismissBehavior.this.mSwipeDirection == 1?(distance1?xvel > 0.0F:xvel < 0.0F):false));
} else {
int distance = child.getLeft() - this.mOriginalCapturedViewLeft;
int thresholdDistance = Math.round((float)child.getWidth() * SwipeDismissBehavior.this.mDragDismissThreshold);
return Math.abs(distance) >= thresholdDistance;
}
}
public int getViewHorizontalDragRange(View child) {
return child.getWidth();
}
public int clampViewPositionHorizontal(View child, int left, int dx) {
boolean isRtl = ViewCompat.getLayoutDirection(child) == 1;
int min;
int max;
if(SwipeDismissBehavior.this.mSwipeDirection == 0) {
if(isRtl) {
min = this.mOriginalCapturedViewLeft - child.getWidth();
max = this.mOriginalCapturedViewLeft;
} else {
min = this.mOriginalCapturedViewLeft;
max = this.mOriginalCapturedViewLeft + child.getWidth();
}
} else if(SwipeDismissBehavior.this.mSwipeDirection == 1) {
if(isRtl) {
min = this.mOriginalCapturedViewLeft;
max = this.mOriginalCapturedViewLeft + child.getWidth();
} else {
min = this.mOriginalCapturedViewLeft - child.getWidth();
max = this.mOriginalCapturedViewLeft;
}
} else {
min = this.mOriginalCapturedViewLeft - child.getWidth();
max = this.mOriginalCapturedViewLeft + child.getWidth();
}
return SwipeDismissBehavior.clamp(min, left, max);
}
public int clampViewPositionVertical(View child, int top, int dy) {
return child.getTop();
}
public void onViewPositionChanged(View child, int left, int top, int dx, int dy) {
float startAlphaDistance = (float)this.mOriginalCapturedViewLeft + (float)child.getWidth() * SwipeDismissBehavior.this.mAlphaStartSwipeDistance;
float endAlphaDistance = (float)this.mOriginalCapturedViewLeft + (float)child.getWidth() * SwipeDismissBehavior.this.mAlphaEndSwipeDistance;
if((float)left <= startAlphaDistance) {
ViewCompat.setAlpha(child, 1.0F);
} else if((float)left >= endAlphaDistance) {
ViewCompat.setAlpha(child, 0.0F);
} else {
float distance = SwipeDismissBehavior.fraction(startAlphaDistance, endAlphaDistance, (float)left);
ViewCompat.setAlpha(child, SwipeDismissBehavior.clamp(0.0F, 1.0F - distance, 1.0F));
}
}
}; 問題
那么我們的問題來了？SwipeDismissBehavior可以替代RecyclerView的ItemTouchHelper或者其他列表滑動刪除庫嗎？
答案是不能。
因為CoordinatorLayout遍歷子View的時候，只遍歷了第一層view，而列表的滑動刪除對象是在RecyclerView的里面，不是CoordinatorLayout的直接子view。
再者，既然是RecyclerView的item，那么它的LayoutParams就是RecyclerView.LayoutParams 它無法強制轉換成CoordinatorLayout.LayoutParams，所以運行的時候會報錯：
java.lang.ClassCastException: android.support.v7.widget.RecyclerView$LayoutParams cannot be cast to android.support.design.widget.CoordinatorLayout$LayoutParams
因此SwipeDismissBehavior只適合本文開始的那種用法。
來自： http://www.jcodecraeer.com//a/anzhuokaifa/androidkaifa/2015/1103/3650.html