title: MobX autorun not working class component constructor
body: <p>React is actually unbelievably hard T_T....</p>
  <p>I just want to <strong>invoke a component method</strong> when there's a state change. I can do this easily with a watcher in Vue. But what am I supposed to do in React class component and MobX's autorun? Would this work in a functional component instead?</p>
  <pre><code>import someStore 
  
  @observer
  class MyComponent {
      constructor(){
         autorun( 
           // references someStore.someObservable
           // call component methods 
           // but this isn't hit when there's change
          )
      }
  }
  </code></pre>