图形处理

 mFastDF = new PaintFlagsDrawFilter(Paint.FILTER_BITMAP_FLAG |
                                               Paint.DITHER_FLAG,
                                               0);

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

            mPaint.setShader(mShader1);
            canvas.drawPaint(mPaint);
            
            canvas.translate(mTouchCurrX - mTouchStartX,
                             mTouchCurrY - mTouchStartY);

            mPaint.setShader(mShader2);
            canvas.drawPaint(mPaint);
        }

        @Override
        public boolean onTouchEvent(MotionEvent event) {
            float x = event.getX();
            float y = event.getY();
            
            switch (event.getAction()) {
                case MotionEvent.ACTION_DOWN:
                    mTouchStartX = mTouchCurrX = x;
                    mTouchStartY = mTouchCurrY = y;
                    mDF = mFastDF;
                    invalidate();
                    break;
                case MotionEvent.ACTION_MOVE:
                    mTouchCurrX = x;
                    mTouchCurrY = y;
                    invalidate();
                    break;
                case MotionEvent.ACTION_UP:
                    mDF = null;
                    invalidate();
                    break;
            }
            return true;
        }

 在移动的时候使用

mFastDF = new PaintFlagsDrawFilter(Paint.FILTER_BITMAP_FLAG |
                                               Paint.DITHER_FLAG,
                                               0);

 /**
     * Subclass of DrawFilter that affects every paint by first clearing
     * the specified clearBits in the paint's flags, and then setting the
     * specified setBits in the paint's flags.
     *
     * @param clearBits These bits will be cleared in the paint's flags
     * @param setBits These bits will be set in the paint's flags
     */
    public PaintFlagsDrawFilter(int clearBits, int setBits) {
        // our native constructor can return 0, if the specified bits
        // are effectively a no-op
        mNativeInt = nativeConstructor(clearBits, setBits);
    }

 就是将位图抗锯齿等去掉，这样在移动时效率高。