MobX – Why should I use `observer` when I could use `inject` when injecting data into a React component

MobX documentation suggests I should use observer on all my components. However by using inject I get more fine grained control over what data causes a re-rendering of my components.

My understanding is I that with observer, a change in all accessed observables in the last render will cause a re-render, even if the observable is nested deep in the data store, while inject only re-renders when observables accessed in the injector function change.

For example:

class Store{
  @observable data = {
    nestedData: {
      deepData: 'my_data'
    }
  }
}

const store = new Store();

... Assume the store is injected using <Provider /> component

// This will cause re-render when the data object changes
// for example: store.data = { new_data: 'new_data' }
@inject(stores => {
  return { data: stores.dataStore.data }; 
})
class MyComponent extends Component {  }

// This will re-render on change of the data object, but also
// on change of nestedData and deepData properties
@inject(stores => {
  return { data: stores.dataStore.data }; 
})
@observer
class MyComponent extends Component {  }

Could someone confirm my understanding of this?

In my opinion, it’s better to use only inject as it gives you more control, and can prevent unnecessary re-renders. If the data is deeply nested you could create a computed property that gets and prepares the data from the deep structure and then inject that property in the component.

Are there other benefits/drawbacks when using one over the other

Answer

I believe you are correct in your assessment. Let me try to rephrase for clarity:

@observer tracks which observables are used by render and automatically re-renders the component when one of these values changes.

We should note that @observable values used by render might be deeply nested within a given prop, per your example:

class Store{
  @observable data = {
    nestedData: {
      // changes to `deepData` would theoretically re-render any observer
      deepData: 'my_data' 
    }
  }
}

with observer, a change in all accessed observables in the last render will cause a re-render, even if the observable is nested deep in the data store

Bingo!

Although there’s a quirk with observable, as you’ll see in a moment…

---

On the other hand you have @inject which makes available to a component (via props) specific data structures defined by a Provider.

For example:

@inject('title')
class MyComponent extends React.Component {
    render() {
        return (<div>{this.props.title}</div>);
    }
}

const Container = () => (
    <Provider title="This value is passed as a prop using `inject`">
        <MyComponent />
    </Provider>
);

inject only re-renders when observables accessed in the injector function change.

Bingo!

inject will only spawn a re-render if the prop itself has recognized changes.

---

This is effectively the same issue with shouldComponentUpdate() and a deep-comparison of props — though observer seems to be far more efficient than shouldComponentUpdate.

In my opinion, it’s better to use only inject as it gives you more control, and can prevent unnecessary re-renders.

I wouldn’t necessarily go that far… it all depends on how you have your code structured.

If I modify your original example as so:

class Store{
    @observable data = {
        nestedData: {}
    };

    constructor() {
        this.data.nestedData.deepData = 'my_data';
    }
}

…the addition of deepData won’t actually get picked up as an observable change (i.e. re-render) because that property didn’t exist when we originally tagged data as an observable value. So that’s one problem.

A different approach could be to do something like this:

class Person {
    @observable name = 'John Doe';
}

class Store{
    @observable data = null;

    constructor() {
        this.data = new Person();
    }
}

This allows you to spread the observable values out across classes — so you might still want to inject Store into a component (to access Store.data but ultimately any observable changes come from updating Person.