使用AppBarLayout的小坑

AppBarLayout继承自 LinearLayout 。它的直接childView可以设置：scrollFlags,常用方式是：

app:layout_scrollFlags="scroll|enterAlways|snap|exitUntilCollapsed|enterAlwaysCollapsed"
scroll：当RecyclerView向上滚时，childView会跟着一起向上滚并实现隐藏
enterAlways：当RecyclerView向下滚时，childView会跟着一起向下滚并重新显示
snap：当childView还没有完全隐藏或显示时，会根据当前滚动的距离，自动选择是隐藏还是显示
exitUntilCollapsed：随之滚动完成折叠后就保留在界面上，不再移除屏幕

小坑就是：
当AppBarLayout有多个childView时，并且每个childView有不同的scrollFlags（不同或者没有）时，从第0~不能滑动的childView为止的childView会随着滚动而隐藏显示。
如下图（实际上能滑动的只有A、B）：

AppBarLayout的childView.png

因为计算可滑动范围时，getTotalScrollRange在AppBarLayout.Behavior中调用

    /**
     * Returns the scroll range of all children.
     *
     * @return the scroll range in px
     */
    public final int getTotalScrollRange() {
        if (mTotalScrollRange != INVALID_SCROLL_RANGE) {
            return mTotalScrollRange;
        }

        int range = 0;
        for (int i = 0, z = getChildCount(); i < z; i++) {
            final View child = getChildAt(i);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            final int childHeight = child.getMeasuredHeight();
            final int flags = lp.mScrollFlags;

            if ((flags & LayoutParams.SCROLL_FLAG_SCROLL) != 0) {
                // We're set to scroll so add the child's height
                range += childHeight + lp.topMargin + lp.bottomMargin;

                if ((flags & LayoutParams.SCROLL_FLAG_EXIT_UNTIL_COLLAPSED) != 0) {
                    // For a collapsing scroll, we to take the collapsed height into account.
                    // We also break straight away since later views can't scroll beneath
                    // us
                    range -= ViewCompat.getMinimumHeight(child);
                    break;
                }
            } else {
                // 当一个childView不能滑动之后，之后的childView都不能滑动
                // As soon as a view doesn't have the scroll flag, we end the range calculation.
                // This is because views below can not scroll under a fixed view.
                break;
            }
        }
        return mTotalScrollRange = Math.max(0, range - getTopInset());
    }