大家在自定義Android ViewGroup的時(shí)候默認(rèn)是不會(huì)draw滾動(dòng)條的，但是網(wǎng)上這方面的資料比較少。
當(dāng)我們想要顯示滾動(dòng)條時(shí)需要調(diào)用：

awakenScrollBars();

但是，你以為這就完了？
其實(shí)這樣做并無(wú)卵用。
然后就開(kāi)始百度。。
百度了一圈也沒(méi)結(jié)果。
關(guān)鍵詞換成英文終于搜到一篇7年前的stackoverflow
https://stackoverflow.com/questions/9515461/scroll-bar-to-custom-viewgroup
這里是說(shuō)：

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 .

要覆寫(xiě)這倆方法。
然后我打開(kāi)ScrollView源碼，直接復(fù)制過(guò)來(lái)。。簡(jiǎn)單修改了下。

@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;
    }

這樣就完美的顯示了滾動(dòng)條。