自定义带阴影的ImageView

假设设定一个ShadowImageView宽度为200dp，高度为158dp。

图片真正展示区域是 200dp * 140dp

阴影区域为 200dp * 158dp，虽然真正展示的部分只有底部，上面被覆盖掉了

重点在于设置paddingBottom

也就是说drawable的真正绘制区域不包含padding

public class ShadowImageView extends android.support.v7.widget.AppCompatImageView {

    private Paint mPaint = new Paint();
    Rect mRectF = new Rect();
    Bitmap bitmap;
    private int _18DP;

    public ShadowImageView(Context context) {
        this(context, null);
    }

    public ShadowImageView(Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public ShadowImageView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        _18DP = DensityUtil.dp2px(context, 18f); //dp转px
        bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.pic_shadow_banner); 
        //解析阴影的图片资源
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        mRectF.left = 0;
        mRectF.top = 0;
        mRectF.right = w;
        mRectF.bottom = h; //mRecF就是 200dp * 158dp
        this.setPadding(0, 0, 0, _18DP); //指定shadowHeight;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawBitmap(bitmap, null, mRectF, mPaint); //阴影蒙层置于底部，先绘制
        super.onDraw(canvas);
    }
}