简单实现Android刮刮卡效果

public class PorterDuff {

  // these value must match their native equivalents. See SkPorterDuff.h
  public enum Mode {
    /** [0, 0] */
    CLEAR    (0),
    /** [Sa, Sc] */
    SRC     (1),
    /** [Da, Dc] */
    DST     (2),
    /** [Sa + (1 - Sa)*Da, Rc = Sc + (1 - Sa)*Dc] */
    SRC_OVER  (3),
    /** [Sa + (1 - Sa)*Da, Rc = Dc + (1 - Da)*Sc] */
    DST_OVER  (4),
    /** [Sa * Da, Sc * Da] */
    SRC_IN   (5),
    /** [Sa * Da, Sa * Dc] */
    DST_IN   (6),
    /** [Sa * (1 - Da), Sc * (1 - Da)] */
    SRC_OUT   (7),
    /** [Da * (1 - Sa), Dc * (1 - Sa)] */
    DST_OUT   (8),
    /** [Da, Sc * Da + (1 - Sa) * Dc] */
    SRC_ATOP  (9),
    /** [Sa, Sa * Dc + Sc * (1 - Da)] */
    DST_ATOP  (10),
    /** [Sa + Da - 2 * Sa * Da, Sc * (1 - Da) + (1 - Sa) * Dc] */
    XOR     (11),
    /** [Sa + Da - Sa*Da,
       Sc*(1 - Da) + Dc*(1 - Sa) + min(Sc, Dc)] */
    DARKEN   (12),
    /** [Sa + Da - Sa*Da,
       Sc*(1 - Da) + Dc*(1 - Sa) + max(Sc, Dc)] */
    LIGHTEN   (13),
    /** [Sa * Da, Sc * Dc] */
    MULTIPLY  (14),
    /** [Sa + Da - Sa * Da, Sc + Dc - Sc * Dc] */
    SCREEN   (15),
    /** Saturate(S + D) */
    ADD     (16),
    OVERLAY   (17);

    Mode(int nativeInt) {
      this.nativeInt = nativeInt;
    }

    /**
     * @hide
     */
    public final int nativeInt;
  }

public class GuaCardView extends View {

  private Bitmap mSrcText;
  private Bitmap mSrcBitmap;
  private Bitmap mDstBitmap;
  private Paint mPaint;
  private Path mPath;
  private float mStartX,mStartY;

  public GuaCardView(Context context, AttributeSet attrs) {
    super(context, attrs);
    setLayerType(LAYER_TYPE_SOFTWARE,null);
    mPaint = new Paint();
    mPaint.setColor(Color.RED);
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setStrokeWidth(45);
    mSrcText = BitmapFactory.decodeResource(getResources(), R.drawable.guaguaka_text1);
    mSrcBitmap = BitmapFactory.decodeResource(getResources(),R.drawable.guaguaka);
    mDstBitmap = Bitmap.createBitmap(mSrcBitmap.getWidth(),mSrcBitmap.getHeight(), Bitmap.Config.ARGB_8888);
    mPath = new Path();
  }

  @Override
  protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    //画最终呈现的图
    canvas.drawBitmap(mSrcText,0,0,mPaint);
    int layerId = canvas.saveLayer(0,0,getWidth(),getHeight(),null,Canvas.ALL_SAVE_FLAG);
    //把手指轨迹画到画布上
    Canvas c = new Canvas(mDstBitmap);
    c.drawPath(mPath,mPaint);
    //利用SRC_OUT绘制原图
    canvas.drawBitmap(mDstBitmap,0,0,mPaint);
    mPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT));
    canvas.drawBitmap(mSrcBitmap,0,0,mPaint);
    mPaint.setXfermode(null);
    canvas.restoreToCount(layerId);
  }

  @Override
  public boolean onTouchEvent(MotionEvent event) {
    switch(event.getAction()){
      case MotionEvent.ACTION_DOWN:
        mPath.moveTo(event.getX(),event.getY());
        mStartX = event.getX();
        mStartY = event.getY();
        return true;
      case MotionEvent.ACTION_MOVE:
        float endX = (mStartX+event.getX())/2;
        float endY = (mStartY+event.getY())/2;
        mPath.quadTo(mStartX,mStartY,endX,endY);
        mStartX = event.getX();
        mStartY = event.getY();
        break;
      default:
        break;
    }
    postInvalidate();
    return super.onTouchEvent(event);
  }
}

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