Code example for LinearGradient

        canvas.drawRect(0, height, width, height + reflectionGap, deafalutPaint);
        canvas.drawBitmap(reflectionImage, 0, height + reflectionGap, null);
        Paint paint = new Paint();
        LinearGradient shader = new LinearGradient(0, bitmap.getHeight(), 0,
                bitmapWithReflection.getHeight() + reflectionGap, 0x70ffffff,
                0x00ffffff, TileMode.CLAMP);
        // Set the Transfer mode to be porter duff and destination in 
        paint.setXfermode(new PorterDuffXfermode(Mode.DST_IN));
        // Draw a rectangle using the paint with our linear gradient 
        canvas.drawRect(0, height, width, bitmapWithReflection.getHeight()
                + reflectionGap, paint);
        return bitmapWithReflection;
     * 将bitmap转化为drawable 
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