onWindowFocusChanged (boolean hasWindowFocus) 一个很重要的回调函数，执行后才能获取到view的width等属性

在实现一个LinearLayout的隐藏和显示的时候，需要根据其状态来改变另一个布局的位置，这需要获取到这个LinearLayout的宽度。刚开始觉得很简单，使用LinearLayout的getMeasuredWidth就可以获取到，不过后来试了几次，获取到的值都是0。

然后我为LinearLayout注册了各种监听来执行，也都是0，然后又在Activity的onCreate、onResume等周期函数里执行，依然为0！后来上网查找相关资料，看到有人提了一个onWindowFocusChanged，去api里查了下，里面是这么解释的：

Called when the window containing this view gains or loses focus. Note that this is
separate from view focus: to receive key events, both your view and its window must have focus. If a window is displayed on top of yours that takes input focus, then your own window will lose focus but the view focus will remain unchanged.

老实说，也就看懂个大概意思，感觉没有明确提到这个特性，还希望英语好的能给个合理的翻译。后来我就去重写了这个函数，在里面获取width，然后一执行，宽度不在为0！！下面是测试的时候打印的日志~