Android刮刮乐效果-proterDuffXfermode的示例代码

public class PorterDuffXfermode extends Xfermode

public class ProterDuffXfermodeView extends AppCompatImageView {
 private Bitmap mBgBitmap,mFgBitmap;
 private Paint mPaint;
 private Canvas mCanvas;
 private Path mPath;

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

  private void init() {
  //创建Paint
  mPaint=new Paint();
  //设置透明度
  mPaint.setAlpha(0);
  //设置Xfermode模式
  mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));
  //Paint类型
  mPaint.setStyle(Paint.Style.STROKE);
  //Paint.Join.MITER-锐角，ROUND-圆弧，BEVEL-直线
  mPaint.setStrokeJoin(Paint.Join.ROUND);
  mPaint.setStrokeWidth(50);
  //线帽
  mPaint.setStrokeCap(Paint.Cap.ROUND);
  mPath=new Path();
  Drawable drawable =getDrawable();
  mBgBitmap=((BitmapDrawable)drawable).getBitmap();
  mFgBitmap=Bitmap.createBitmap(mBgBitmap.getWidth(),mBgBitmap.getHeight(),Bitmap.Config.ARGB_8888);
  mCanvas=new Canvas(mFgBitmap);
  mCanvas.drawColor(Color.GRAY);
 }
 @Override
 public boolean onTouchEvent(MotionEvent event) {
  switch (event.getAction()) {
   case MotionEvent.ACTION_DOWN:
    mPath.reset();
    mPath.moveTo(event.getX(), event.getY());
    break;
   case MotionEvent.ACTION_MOVE:
    mPath.lineTo(event.getX(), event.getY());
    break;
  }
  mCanvas.drawPath(mPath, mPaint);
  invalidate();
  return true;
 }
 @Override
 protected void onDraw(Canvas canvas) {
  canvas.drawBitmap(mBgBitmap, 0, 0,null);
  canvas.drawBitmap(mFgBitmap, 0, 0,null);
 }
}

 //设置画笔的样式
  mPaint.setStyle(Paint.Style.FILL);//填充内容
  mPaint.setStyle(Paint.Style.FILL_AND_STROKE);
  mPaint.setStyle(Paint.Style.STROKE);//描边
  //线帽
  mPaint.setStrokeCap(Paint.Cap.BUTT);//没有
  mPaint.setStrokeCap(Paint.Cap.ROUND);//圆的
  mPaint.setStrokeCap(Paint.Cap.SQUARE);//方形
	mPaint.setStrokeJoin(Paint.Join.MITER);//锐角
	mPaint.setStrokeJoin(Paint.Join.ROUND);//圆弧
	mPaint.setStrokeJoin(Paint.Join.BEVEL);//直线