Android橡皮擦效果

public class ActionView extends View {
    private Paint mPaint = new Paint();
    private Path mPath = new Path();//手指滑动路径
    private Canvas mCanvas;//缓存画布
    private Bitmap mBitmap;//缓存图片
    private float pointX,pointY;//触点坐标

    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();//初始化画笔

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

    }

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

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

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

        canvas.drawBitmap(mBitmap,null);
    }


    @Override
    public boolean onTouchEvent(MotionEvent event) {
        pointX = event.getX();
        pointY = event.getY();
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                mPath.moveTo(pointX,pointY);//将路径移动到点(pointX,pointY)，不绘制
                break;
            case MotionEvent.ACTION_MOVE:
                mPath.lineTo(pointX,pointY);//绘制一条从上个触点到点(pointX,pointY)的线条
                break;
        }
        invalidate();//重新绘图
        return true;
    }

    private void initPaint() {//初始化画笔
        mPaint.setDither(true);//设定是否使用图像抖动处理，会使绘制出来的图片颜色更加平滑和饱满，图像更加清晰
        mPaint.setAntiAlias(true);//设置抗锯齿
        mPaint.setStrokeWidth(30);
        mPaint.setColor(Color.RED);//设置画笔颜色
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPaint.setStrokeJoin(Paint.Join.ROUND);//圆角
        mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST));
    }

}