Asynchronous Routing

Ember's approach to handling asynchronous logic in the router makes heavy use of the concept of Promises. In short, promises are objects that represent an eventual value. A promise can either fulfill (successfully resolve the value) or reject (fail to resolve the value). The way to retrieve this eventual value, or handle the cases when the promise rejects, is via the promise's then() method, which accepts two optional callbacks, one for fulfillment and one for rejection. If the promise fulfills, the fulfillment handler gets called with the fulfilled value as its sole argument, and if the promise rejects, the rejection handler gets called with a reason for the rejection as its sole argument. For example:

let promise = fetchTheAnswer();

promise.then(fulfillCallback, rejectCallback);

function fulfillCallback(answer) {
  console.log(`The answer is ${answer}`);
}

function rejectCallback(reason) {
  console.log(`Couldn't get the answer! Reason: ${reason}`);
}

Much of the power of promises comes from the fact that they can be chained together to perform sequential asynchronous operations:

import fetch from 'fetch';

let usernamesPromise = fetch('/usernames.json');

usernamesPromise.then(response => response.json())
                .then(fetchPhotosOfUsers)
                .then(applyInstagramFilters)
                .then(uploadTrendyPhotoAlbum)
                .then(displaySuccessMessage, handleErrors);

In the above example, if any of the methods fetchPhotosOfUsers, applyInstagramFilters, or uploadTrendyPhotoAlbum returns a promise that rejects, handleErrors will be called with the reason for the failure. In this manner, promises approximate an asynchronous form of try-catch statements that prevent the rightward flow of nested callback after nested callback and facilitate a saner approach to managing complex asynchronous logic in your applications.

When transitioning between routes, the Ember router collects all of the models (via the model hook) that will be passed to the route's controllers at the end of the transition. If the model hook (or the related beforeModel or afterModel hooks) return normal (non-promise) objects or arrays, the transition will complete immediately. But if the model hook (or the related beforeModel or afterModel hooks) returns a promise (or if a promise was provided as an argument to transitionTo), the transition will pause until that promise fulfills or rejects.

The router considers any object with a then() method defined on it to be a promise.

If the promise fulfills, the transition will pick up where it left off and begin resolving the next (child) route's model, pausing if it too is a promise, and so on, until all destination route models have been resolved. The values passed to the setupController() hook for each route will be the fulfilled values from the promises.

A basic example:

import Route from '@ember/routing/route';
import { later } from '@ember/runloop';

export default class TardyRoute extends Route {
  model() {
    return new Promise(function(resolve) {
      later(function() {
        resolve({ msg: 'Hold Your Horses' });
      }, 3000);
    });
  }

  setupController(controller, model) {
    console.log(model.msg); // "Hold Your Horses"
  }
}

When transitioning into route:tardy, the model() hook will be called and return a promise that won't resolve until 3 seconds later, during which time the router will be paused in mid-transition. When the promise eventually fulfills, the router will continue transitioning and eventually call route:tardy's setupController() hook with the resolved object.

This pause-on-promise behavior is extremely valuable for when you need to guarantee that a route's data has fully loaded before displaying a new template.

We've covered the case when a model promise fulfills, but what if it rejects?

By default, if a model promise rejects during a transition, the transition is aborted, no new destination route templates are rendered, and an error is logged to the console.

You can configure this error-handling logic via the error handler. When a promise rejects, an error event will be fired on that route and bubble up to route:application's default error handler unless it is handled by a custom error handler along the way, e.g.:

import Route from '@ember/routing/route';
import { action } from '@ember/object';

export default class GoodForNothingRoute extends Route {
  model() {
    return Promise.reject("FAIL");
  }

  @action
  error(reason) {
    alert(reason); // "FAIL"

    // Can transition to another route here, e.g.
    // this.transitionTo('index');

    // Uncomment the line below to bubble this error event:
    // return true;
  }
}

In the above example, the error event would stop right at route:good-for-nothing's error handler and not continue to bubble. To make the event continue bubbling up to route:application, you can return true; from the error handler.

Rejected model promises halt transitions, but because promises are chainable, you can catch promise rejects within the model hook itself and convert them into fulfills that won't halt the transition.

import Route from '@ember/routing/route';

export default class FunkyRoute extends Route {
  model() {
    return iHopeThisWorks().catch(function() {
      // Promise rejected, fulfill with some default value to
      // use as the route's model and continue on with the transition
      return { msg: 'Recovered from rejected promise' };
    });
  }
}