add level button with initial behavior

app/controllers/application.js

@@ -1,5 +1,6 @@
 import Ember from 'ember';
 
 export default Ember.Controller.extend({
-  isOn: null
+  isOn: null,
+  level: null
 });

app/routes/application.js

@@ -1,16 +1,32 @@
 import Ember from 'ember';
 
 export default Ember.Route.extend({
+  levels: [1,2,3,4],
+
   setupController(controller) {
     controller.set('isOn', false);
   },
+
   actions: {
     turnOn() {
-      this.controllerFor('application').set('isOn', true);
-      this.set('isOn', true);
+      const c = this.controllerFor('application');
+      c.set('isOn', true);
+      c.set('level', this.get('levels').objectAt(0));
     },
     turnOff() {
-      this.controllerFor('application').set('isOn', false);
+      const c = this.controllerFor('application');
+      c.set('isOn', false);
+    },
+    nextLevel() {
+      if (this.controllerFor('application').get('isOn')) {
+        const c = this.controllerFor('application');
+        const level = c.get('level');
+        const levelIndex = this.get('levels').indexOf(level);
+        const nextLevel = this.get('levels').objectAt(
+          (levelIndex + 1) % this.get('levels').length
+        );
+        c.set('level', nextLevel);
+      }
     }
   }
 });

app/templates/application.hbs

@@ -1,6 +1,7 @@
 <div id="display">
   {{#if isOn}}
     ON
+    {{level}}
   {{else}}
     OFF
   {{/if}}
@@ -13,3 +14,7 @@
 <button {{action 'turnOn'}}>
   ON
 </button>
+
+<button {{action 'nextLevel'}}>
+  LEVEL
+</button>