I've seen that you can do something like this:

```
// App.js
class App extends Component {
  @observable a = 'something'

  @action change() {
    this.a = 'else'
  }

  render() {
    return (
      ...
    )
  }
}

```

And I've also seen that you can write your store in a separate file (which is what I do):

```
// StoreA.js
class StoreA {
  @observable a = 'something'

  @action change() {
    this.a = 'else'
  }
}

// App.js
@observer
class App extends Component {
  constructor() {
    this.store = new StoreA()
  }

  ...
}

```

Is there any difference between these approaches? Is one better than the other?