Android橡皮擦效果

public class ActionView extends View {
    private Paint mPaint = new Paint();
    private Path mPath = new Path();//手指滑動路徑
    private Canvas mCanvas;//緩存畫布
    private Bitmap mBitmap;//緩存圖片
    private float pointX, pointY;//觸點坐標


public ActionView(Context context, AttributeSet attrs) {
    super(context, attrs);
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);

    int width = getMeasuredWidth();
    int height = getMeasuredHeight();

    initPaint();//初始化畫筆

    mBitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888); //初始化Bitmap
    mCanvas = new Canvas(mBitmap);
    mCanvas.drawColor(Color.parseColor("#c0c0c0"));//設置畫板背景

}

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    Bitmap mBackBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.bg);
    canvas.drawBitmap(mBackBitmap, 0, 0, null);

    mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));
    mCanvas.drawPath(mPath, mPaint);

    canvas.drawBitmap(mBitmap, 0, 0, null);
}


@Override
public boolean onTouchEvent(MotionEvent event) {
    pointX = event.getX();
    pointY = event.getY();
    switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
            mPath.moveTo(pointX, pointY);//將路徑移動到點(pointX, pointY)，不繪制
            break;
        case MotionEvent.ACTION_MOVE:
            mPath.lineTo(pointX, pointY);//繪制一條從上個觸點到點(pointX, pointY)的線條
            break;
    }
    invalidate();//重新繪圖
    return true;
}

private void initPaint() {//初始化畫筆
    mPaint.setDither(true);//設定是否使用圖像抖動處理，會使繪制出來的圖片顏色更加平滑和飽滿，圖像更加清晰
    mPaint.setAntiAlias(true);//設置抗鋸齒
    mPaint.setStrokeWidth(30);
    mPaint.setColor(Color.RED);//設置畫筆顏色
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setStrokeCap(Paint.Cap.ROUND);
    mPaint.setStrokeJoin(Paint.Join.ROUND);//圓角
    mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST));
}


}</pre>