 Loading / Error Substates  
The Ember Router allows you to provide feedback that a route is loading, as well as when an error occurs in loading a route. The error and loading substates exist as a part of each route, so they should not be added to your router.js file. To utilize a substate, the route, controller, and template may be optionally defined as desired. 
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: Router.map(function() {
  this.route('slow-model');
});
 import Route from '@ember/routing/route';
import { inject as service } from '@ember/service';

export default class SlowModelRoute extends Route {
  @service store;

  model() {
    return this.store.findAll('slow-model');
  }
}
 If you navigate to slow-model, in the model hook using Ember Data, 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: Router.map(function() {
  this.route('user', function() {
    this.route('about', function() {
      this.route('slow-model');
    });
  });
});
 Each of the following assumes a transition from the application or index route. When accessing user.about.slow-model route then Ember will alternate trying to find a routeName-loading or loading template in the hierarchy starting with user.about.slow-model-loading:  user.about.slow-model-loading 
user.about.loading or user.about-loading
 
user.loading or user-loading
 
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 user.about route then Ember will search for:  user.about-loading 
user.loading or user-loading
 
loading or application-loading
  It's important to note that user.about.loading template is not considered now. Ember will not look above the common parent in a transition between child routes. For example, if the user transitions from user.about.index to user.about.slow-model the following search for template will happen:  user.about.slow-model-loading 
user.about.loading or user.about-loading
  Notice that user.loading, user-loading, loading, and application-loading are not included here, Since about is the common parent for index and slow-model. This means we'll need to handle loading at every level within the route hierarchy where loading feedback is important.  The loading event  If the various beforeModel/model/afterModel hooks don't immediately resolve, a loading event will be fired on that route. import Route from '@ember/routing/route';
import { inject as service } from '@ember/service';
import { action } from '@ember/object';

export default class FooSlowModelRoute extends Route {
  @service store;

  model() {
    return this.store.findAll('slow-model');
  }

  @action
  loading(transition, originRoute) {
    let controller = this.controllerFor('foo');
    controller.set('currentlyLoading', true);
    return true; // allows the loading template to be shown
  }
}
 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: import Route from '@ember/routing/route';
import { action } from '@ember/object';

export default class FooSlowModelRoute extends Route {
  // ...
  @action
  async loading(transition, originRoute) {
    let controller = this.controllerFor('foo');
    controller.set('currentlyLoading', true);
    transition.promise.finally(function() {
        controller.set('currentlyLoading', false);
    });
  }
};
 In case we want both custom logic and the default behavior for the loading substate, we can implement the loading action and let it bubble by returning true. import Route from '@ember/routing/route';
import { action } from '@ember/object';

export default class FooSlowModelRoute extends Route {
  // ...
  @action
  loading(transition) {
    let start = new Date();
    transition.promise.finally(() => {
      this.notifier.notify(`Took ${new Date() - start}ms to load`);
    });

    return true;
  }
};
  
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. 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:  articles.overview-error 
articles.error or articles-error
 
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. The model hooks (beforeModel, model, and afterModel) of an error substate are not called. Only the setupController method of the error substate is called with the error as the model. See example below: setupController(controller, error) {
  console.log(error.message);
  super.setupController(...arguments)
}
 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. import Route from '@ember/routing/route';
import { inject as service } from '@ember/service';
import { action } from '@ember/object';

export default class ArticlesOverviewRoute extends Route {
  @service store;

  model(params) {
    return this.store.findAll('privileged-model');
  }

  @action
  error(error, transition) {
    if (error.status === '403') {
      this.replaceWith('login');
    } else {
      // Let the route above this handle the error.
      return true;
    }
  }
};
 Analogous to the loading event, you could manage the error event at the application level to avoid writing the same code for multiple routes. In case we want to run some custom logic and have the default behavior of rendering the error template, we can handle the error event and let it bubble by returning true. import Route from '@ember/routing/route';
import { inject as service } from '@ember/service';
import { action } from '@ember/object';

export default class ArticlesOverviewRoute extends Route {
  @service store;

  model(params) {
    return this.get('store').findAll('privileged-model');
  }

  @action
  error(error) {
    this.notifier.error(error);
    return true;
  }
};
   

    
{'path': 'public/docs/ember~3/routing/loading-and-error-substates.html', 'source': 'DevDocs.IO'}