Loading / Error Substates Edit Page

The Ember Router allows you to provide feedback that a route is loading, as well as when an error occurs in loading a route.

loading substates

During the beforeModel, model, and afterModel hooks, data may take some time to load. Technically, the router pauses the transition until the promises returned from each hook fulfill.

Consider the following:

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Router.map(function() {
  this.route('slow-model');
});

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export default Ember.Route.extend({
  model() {
    return this.store.findAll('slowModel');
  }
});

If you navigate to slow-model, in the model hook, the query may take a long time to complete. During this time, your UI isn't really giving you any feedback as to what's happening. If you're entering this route after a full page refresh, your UI will be entirely blank, as you have not actually finished fully entering any route and haven't yet displayed any templates. If you're navigating to slow-model from another route, you'll continue to see the templates from the previous route until the model finish loading, and then, boom, suddenly all the templates for slow-model load.

So, how can we provide some visual feedback during the transition?

Simply define a template called loading (and optionally a corresponding route) that Ember will transition to. The intermediate transition into the loading substate happens immediately (synchronously), the URL won't be updated, and, unlike other transitions, the currently active transition won't be aborted.

Once the main transition into slow-model completes, the loading route will be exited and the transition to slow-model will continue.

For nested routes, like:

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Router.map(function() {
  this.route('foo', function() {
    this.route('bar', function() {
      this.route('slow-model');
    });
  });
});

When accessing foo.bar.slow-model route then Ember will alternate trying to find a routeName-loading or loading template in the hierarchy starting with foo.bar.slow-model-loading:

1. foo.bar.slow-model-loading
2. foo.bar.loading or foo.bar-loading
3. foo.loading or foo-loading
4. loading or application-loading

It's important to note that for slow-model itself, Ember will not try to find a slow-model.loading template but for the rest of the hierarchy either syntax is acceptable. This can be useful for creating a custom loading screen for a leaf route like slow-model.

When accessing foo.bar route then Ember will search for:

1. foo.bar-loading
2. foo.loading or foo-loading
3. loading or application-loading

It's important to note that foo.bar.loading is not considered now.

The loading event

If the various beforeModel/model/afterModel hooks don't immediately resolve, a loading event will be fired on that route.

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export default Ember.Route.extend({
  model() {
    return this.store.findAll('slowModel');
  },
  actions: {
    loading(transition, originRoute) {
      let controller = this.controllerFor('foo');
      controller.set('currentlyLoading', true);
    }
  }
});

If the loading handler is not defined at the specific route, the event will continue to bubble above a transition's parent route, providing the application route the opportunity to manage it.

When using the loading handler, we can make use of the transition promise to know when the loading event is over:

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export default Ember.Route.extend({
  ...
  actions: {
    loading(transition, originRoute) {
      let controller = this.controllerFor('foo');
      controller.set('currentlyLoading', true);
      transition.promise.finally(function() {
          controller.set('currentlyLoading', false);
      });
    }
  }
});

error substates

Ember provides an analogous approach to loading substates in the case of errors encountered during a transition.

Similar to how the default loading event handlers are implemented, the default error handlers will look for an appropriate error substate to enter, if one can be found.

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Router.map(function() {
  this.route('articles', function() {
    this.route('overview');
  });
});

As with the loading substate, on a thrown error or rejected promise returned from the articles.overview route's model hook (or beforeModel or afterModel) Ember will look for an error template or route in the following order:

1. articles.overview-error
2. articles.error or articles-error
3. error or application-error

If one of the above is found, the router will immediately transition into that substate (without updating the URL). The "reason" for the error (i.e. the exception thrown or the promise reject value) will be passed to that error state as its model.

If no viable error substates can be found, an error message will be logged.

The error event

If the articles.overview route's model hook returns a promise that rejects (for instance the server returned an error, the user isn't logged in, etc.), an error event will fire from that route and bubble upward. This error event can be handled and used to display an error message, redirect to a login page, etc.

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export default Ember.Route.extend({
  model(params) {
    return this.store.findAll('problematicModel');
  },
  actions: {
    error(error, transition) {
      if (error) {
        return this.transitionTo('errorPage');
      }
    }
  }
});

Analogous to the loading event, you could manage the error event at the application level to avoid writing the same code for multiple routes.