onMeasure()会调用measureVertical(int widthMeasureSpec, int heightMeasureSpec)
mTotalLength保存已经measure过的child所占用的高度
void measureChildBeforeLayout(View child, int childIndex,
int widthMeasureSpec, int totalWidth, int heightMeasureSpec,
int totalHeight) //对每一个child进行测量， heightMeasureSpec为LinearLayout本身的measureSpec，totalHeight为
LinearLayout已经被其他子试图所占用的高度
measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed)
child.getMeasuredHeight()获取该子试图的最终高度
ViewGroup.getChildMeasureSpec(int spec, int padding, int childDimension)
View.resolveSizeAndState(int size, int measureSpec, int childMeasuredState)