title: componentWillUpdate read the old state
body: <p>I thought that when <code>componentWillUpdate</code> was called, I could read the latest state i.e. states from <code>this.state</code>. However, I realized I was wrong after I checked the document more closely. The fact is that even if <code>componentWillUpdate</code> is invoked after <code>this.state</code>, it still reads the 'Old' state. What is the point of this design? How is <code>componentWillUpdate</code> useful?</p>