title: Why won't Flutter mobX store code generate?
body: <p>I have a mobx store and it's really hit and miss getting my storename.g.dart generated. </p>
  
  <p>The first time, I copied an example from medium and just kept running the generator until eventually it gave code. </p>
  
  <p>The second time, I wrote my own store forgot the abstract keyword. Added it in and it worked. </p>
  
  <p>I thought that was the problem. </p>
  
  <p>Now, third project, I think I have all my code down but it's still not generating. Many actions but no output. </p>
  
  <p>What makes it work or fail? This is my current mobx</p>
  
  <pre><code>import 'package:mobx/mobx.dart';
  import 'package:firebase_auth/firebase_auth.dart';
  
  // have this line
  // then generate with
  // flutter packages pub run build_runner build
  // or
  // flutter packages pub run build_runner clean; flutter packages pub run build_runner build --delete-conflicting-outputs
  
  // todo edit this file name
  part 'usermodel.g.dart';
  
  class UserModel = UserModelBase with _$UserModel;
  
  abstract class UserModelBase implements Store {
    @observable
    FirebaseUser user;
  
    @action
    setUser(FirebaseUser u){
      user = u;
    }
  
    dispose() {}
  }
  </code></pre>