1. requestLayout():
   重新佈局整個ViewTree，開始呼叫measure()方法。
   requestLayout()是view的方法，view也可以呼叫該方法。

   Call this when something has changed which has invalidated the layout of this view. 
   This will schedule a layout pass of the view tree. This should not be called while the view hierarchy is currently in a layout pass.
    
   

2. invalidate():
   如果當前View可見，onDraw()方法將會被呼叫。

   Invalidate the whole view. If the view is visible, onDraw() will be called at some point in the future.
   This must be called from a UI thread. To call from a non-UI thread, call postInvalidate().
   

在寫SimpleIndicator時，並不是一定不會走onMeasure()方法，在某種條件下會走該方法的。可以搜其他的原始碼分析文章，具體檢視。