SeniorUI0503_Xfermode刮刮卡（橡皮檫）效果

高级UI汇总​​​​​​​
eniorUI05_Paint_Xfermode模式
##1 Effect Picture

##2 Demo
SeniorUI05_GuaGuaCardActivity
##3 Requirement
刮刮卡效果：显示一张图片，手势触摸滑动露出图片后面覆盖的内容
##4 Theory

• 自定义View，同一位置画两张图片，上层位置手指划过路径实现透明，从而实现“刮”的效果
• 利用xfermode合成上层图片
• 上层图片为src，创建一个与src等宽高无内容透明图片dst
• 手指触摸的路径画到dst上
• 利用xfermode的SRC_OUT将src和dst合成，结果是src中去掉dst的图片内容，最终生成我们想要的刮刮卡效果

##5 Core Code

public class GuaGuaCardView_SRCOUT extends View {
    private Paint mBitPaint;
    private Bitmap BmpDST,BmpSRC,BmpText;
    private Path mPath;
    private float mPreX,mPreY;
    public GuaGuaCardView_SRCOUT(Context context, AttributeSet attrs) {
        super(context, attrs);

        setLayerType(View.LAYER_TYPE_SOFTWARE, null);
        mBitPaint = new Paint();
        mBitPaint.setColor(Color.RED);
        mBitPaint.setStyle(Paint.Style.STROKE);
        mBitPaint.setStrokeWidth(45);

        BmpText = BitmapFactory.decodeResource(getResources(),R.drawable.guaguaka_text1,null);
        BmpSRC = BitmapFactory.decodeResource(getResources(), R.drawable.guaguaka,null);
        BmpDST = Bitmap.createBitmap(BmpSRC.getWidth(), BmpSRC.getHeight(), Bitmap.Config.ARGB_8888);
        mPath = new Path();
    }

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

        canvas.drawBitmap(BmpText,0,0,mBitPaint);

        int layerId = canvas.saveLayer(0, 0, getWidth(), getHeight(), null, Canvas.ALL_SAVE_FLAG);

        //先把手指轨迹画到目标Bitmap上
        Canvas c = new Canvas(BmpDST);
        c.drawPath(mPath,mBitPaint);

        //然后把目标图像画到画布上
        canvas.drawBitmap(BmpDST,0,0,mBitPaint);

        //计算源图像区域
        mBitPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT));
        canvas.drawBitmap(BmpSRC,0,0,mBitPaint);

        mBitPaint.setXfermode(null);
        canvas.restoreToCount(layerId);
    }

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