MutablePromiseMixin

application.controller.js

import Ember from 'ember';

let i = 0;

export default Ember.Controller.extend({
  appName:'Ember Twiddle',
  myService: Ember.inject.service(),
  
  actions: {
    foo() {
      this.set("myService.promise", new Ember.RSVP.Promise(function(resolve) {
        resolve(i++);
      }));
    }
  }
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<button {{action "foo"}}>Foo</button>
{{myService.content}}
<br>
<br>

my-service.service.js

import Ember from 'ember';

let PromiseObjectProxy = Ember.ObjectProxy.extend(Ember.PromiseProxyMixin, {
  setPromise: Ember.on("init", function() {
    this.set("promise", this.get("deferred.promise"));
  })
});

PromiseObjectProxy.defer = function(promise) {
  return PromiseObjectProxy.create({ deferred: defer(promise) });
}

function defer(promise) {
  let deferred = Ember.RSVP.defer();
  
  promise.then((result) => deferred.resolve(result),
              (error) => deferred.reject(error));
  
  return deferred;
}

export default Ember.Service.extend({
  promise: Ember.computed({
    get() {
      return {};
    },
    set(key, promise, oldValue) {
      return PromiseObjectProxy.defer(promise);
    }
  }),
  
  content: Ember.computed.reads("promise.content")
});

twiddle.json

{
  "version": "0.4.17",
  "EmberENV": {
    "FEATURES": {}
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js"
  }
}