Android利用SRC_OUT模式下的Xfermode实现橡皮

SRC_OUT模式和SRC_IN相反，相对位置，目标像素的透明度越高，原像素的透明度越低，目标像素不透明，则原像素透明，利用这个可以实现橡皮擦效果

定义一些变量

    private Bitmap bitmapSrc;
    //path的paint
    private Paint mPathPaint = new Paint();
    private Paint mPaint = new Paint();
    private Bitmap bitmapDst;
    //手势轨迹
    private Path path = new Path();
    //目标画布
    private  Canvas canvasDst;

初始化参数，创建和原图片相同宽高的透明bitmap

    public RubberView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        setLayerType(LAYER_TYPE_SOFTWARE,null);
        
        bitmapSrc = BitmapFactory.decodeResource(getResources(), R.mipmap.xyjy6);
        
        //利用SRC_OUT使用透明的白底图片
        Bitmap.Config config = Bitmap.Config.ARGB_8888;
        bitmapDst = Bitmap.createBitmap(bitmapSrc.getWidth(), bitmapSrc.getHeight(), config);
        canvasDst = new Canvas(bitmapDst);
        
        //利用SRC_OUT使用不透明的颜色来描绘手势移动
        mPathPaint.setColor(Color.WHITE);
        //橡皮擦大小
        mPathPaint.setStrokeWidth(30);
        mPathPaint.setStyle(Paint.Style.STROKE);
        mPathPaint.setStrokeCap(Paint.Cap.ROUND);
    }

在onTouchEvent事件中记录手指移动轨迹

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        switch (event.getAction()){
            case MotionEvent.ACTION_DOWN:
                path.moveTo(event.getX(),event.getY());
                postInvalidate();
                return true;
            case MotionEvent.ACTION_MOVE:
                path.lineTo(event.getX(),event.getY());
                postInvalidate();
                break;
        }
        
        return false;
    }

在onDraw方法中进行处理

    @Override
    protected void onDraw(Canvas canvas) {
        //在目标画布上画手势轨迹
        canvasDst.drawPath(path,mPathPaint);
        canvas.drawBitmap(bitmapDst,0,0, mPaint);
        
        Xfermode xfermode = new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT);
        mPaint.setXfermode(xfermode);
        canvas.drawBitmap(bitmapSrc, 0, 0, mPaint);
        mPaint.setXfermode(null);
    }

项目地址：https://gitee.com/aruba/Xfermode.git