理解测量

构造先执行,此时onMeasur,onLayout,onDraw都没有创建headerViewHeight = headerView.getMeasuredHeight();在onMeasur时调用headerView.getHeight();在onLayout时调用所以 headerViewHeight 为 0 ,所以要主动调用headerView.measure(0, 0);   // 主动触发测量，mesure内部会调用onMeasureheaderViewHeight = headerView.getMeasuredHeight();//测量@Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {    super.onMeasure(widthMeasureSpec, heightMeasureSpec);}//布局@Overrideprotected void onLayout(boolean changed, int left, int top, int right,        int bottom) {    super.onLayout(changed, left, top, right, bottom);}//绘制@Overrideprotected void onDraw(Canvas canvas) {    super.onDraw(canvas);}三个方法的生命周期