一 显示函数：

showAsDropDown(View anchor)

相对于某个控件无平移

showAsDropDown(View anchor, int xoff, int yoff)

相对与某个控件的位置有偏移，
Xoff:正值表示向左，负值右
yoff:正值表示向下，负值上

showAsDropDown(View anchor, int xoff, int yoff, int gravity)

api:19引入的
多了相对于anchor控件的位置（例如正中央Gravity.CENTER，下方Gravity.BOTTOM等）

showAtLocation(View parent, int gravity, int x, int y)

相对于父控件的位置，可偏移，也可不偏移

二 背景

PopupWindow重写以下几个方法

    private ViewGroup mView;
    @Override
    public void showAsDropDown(View anchor) {
        showAsDropDown(anchor, 0, 0);
    }

    @Override
    public void showAsDropDown(View anchor, int xoff, int yoff) {
        super.showAsDropDown(anchor, xoff, yoff);
        mView = (ViewGroup) anchor.getRootView();
        setShowBackground(anchor);
    }

    @Override
    public void showAsDropDown(View anchor, int xoff, int yoff, int gravity) {
        super.showAsDropDown(anchor, xoff, yoff, gravity);
        mView = (ViewGroup) anchor.getRootView();
        setShowBackground(anchor);
    }

    @Override
    public void showAtLocation(View parent, int gravity, int x, int y) {
        super.showAtLocation(parent, gravity, x, y);
        mView = (ViewGroup) parent.getRootView();
        setShowBackground(parent);
    }

    protected void setShowBackground(View view) {
        mView.addView(mBackground);
        AlphaAnimation alpha = new AlphaAnimation(0f, 1f);
        alpha.setDuration(500);
        mBackground.setAnimation(alpha);
    }

    @Override
    public void dismiss() {
        //关闭移除背景
        mView.removeView(mBackground);
        super.dismiss();
    }