Cacheable promises

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

router.js

import EmberRouter from '@ember/routing/router';
import config from './config/environment';

const Router = EmberRouter.extend({
  location: 'none',
  rootURL: config.rootURL
});

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

export default Router;

routes.index.js

import Ember from 'ember';
import { inject as service } from '@ember/service';
export default Ember.Route.extend({
  dateService: service(),
  setupController(controller, model){
    let dateService = this.get('dateService');
    return dateService.loadDates().then((dates) => {
      controller.set('dates', dates);    
    });
  }
});

routes.second-route.js

import Ember from 'ember';
import { inject as service } from '@ember/service';
export default Ember.Route.extend({
  dateService: service(),
  setupController(controller, model){
    let dateService = this.get('dateService');
    return dateService.loadDates().then((dates) => {
      controller.set('dates', dates);    
    });
  }
});

services.date-service.js

import Ember from 'ember';

export default Ember.Service.extend({
  loadDates(){
    if(this.dates){
      return Ember.RSVP.resolve(this.dates);
    }else{
      return this._loadAndCacheDatesFromApi(); 
    }
  },
  _loadAndCacheDatesFromApi(){
    // this is actually an api call
    return new Ember.RSVP.Promise((resolve) => {
      setTimeout(() => {
        this.set('dates', ['2019', '2020', '2021']);
        resolve(this.dates);
      }, 5000);
    });
  }
});

templates.application.hbs

{{outlet}}

templates.index.hbs

<h4>Index</h4>

{{#if dates}}
   {{#each dates as |date|}} {{date}} {{/each}}
   <br>
   {{#link-to 'second-route'}}Click here to see the cached dates resolve instantly{{/link-to}}
{{else}}
Loading...
{{/if}}

templates.second-route.hbs

<h4>second route</h4>

{{#if dates}}
   {{#each dates as |date|}} {{date}} {{/each}}
   <br>
   {{#link-to 'index'}}even the first route resolves instantly now{{/link-to}}
{{else}}
Loading...
{{/if}}

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}