用LayerDrawable实现两个图片的叠加效果

        		Drawable[] layers = new Drawable[2];
        		layers[0] = new ColorDrawable(primaryColor);
        		layers[1] = new ColorDrawable(0x7f000000);
        		LayerDrawable layerDrawable = new LayerDrawable(layers);
        		layerDrawable.setLayerInset(0, 0, 0, 0, 0);
        		layerDrawable.setLayerInset(1, 0, 0, 0, 0);
        		setBackground(layerDrawable);

也可以用 layer-list在xml中实现