自定义ViewGroup显示scrollbar(滚动条)

大家在自定义Android ViewGroup的时候默认是不会draw滚动条的，但是网上这方面的资料比较少。
当我们想要显示滚动条时需要调用：

awakenScrollBars();

但是，你以为这就完了？
其实这样做并无卵用。
然后就开始百度。。
百度了一圈也没结果。
关键词换成英文终于搜到一篇7年前的stackoverflow
https://stackoverflow.com/questions/9515461/scroll-bar-to-custom-viewgroup
这里是说：

you have to call `awakenScrollBars()` ,which will trigger the scrollbars to draw 
on your custom viewGroup.
([more details](http://developer.android.com/reference/android/view/View.html#awakenScrollBars%28%29)) and the vertical scrollbar enabled has to be true `setVerticalScrollBarEnabled()`

Also you have to override functions `computeVerticalScrollExtent()` and `computeVerticalScrollRange` to set thumb's size and scrollbar scroll range .

要覆写这俩方法。
然后我打开ScrollView源码，直接复制过来。。简单修改了下。

@Override
    protected int computeVerticalScrollOffset() {
        return Math.max(0, super.computeVerticalScrollOffset());
    }
    @Override
    protected int computeVerticalScrollRange() {
        final int count = getChildCount();
        final int contentHeight = getHeight() ;
        if (count == 0) {
            return contentHeight;
        }

        int scrollRange = child.getBottom();
        final int scrollY = getScrollY();
        final int overscrollBottom = Math.max(0, scrollRange - contentHeight);
        if (scrollY < 0) {
            scrollRange -= scrollY;
        } else if (scrollY > overscrollBottom) {
            scrollRange += scrollY - overscrollBottom;
        }

        return scrollRange;
    }

这样就完美的显示了滚动条。