Implement NotifierService

This service provides an easy-to-use and safe (regarding to memory
leaks) way to emit data from services to controllers.

This component will be used in `ImageWriterService` to emit the progress
state instead of accepting an `onProgress` callback.

lib/browser/modules/notifier.js

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+/*
+ * Copyright 2016 Resin.io
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+'use strict';
+
+/**
+ * @module Etcher.notifier
+ */
+
+const angular = require('angular');
+const notifier = angular.module('Etcher.notifier', []);
+
+/*
+ * Based on:
+ * http://www.codelord.net/2015/05/04/angularjs-notifying-about-changes-from-services-to-controllers/
+ */
+
+notifier.service('NotifierService', function($rootScope) {
+
+  /**
+   * @summary Safely subscribe to an event
+   * @function
+   * @public
+   *
+   * @description
+   * We say "safely" since this subscribe function will listen
+   * to the scope's `$destroy` event and unbind itself automatically.
+   *
+   * @param {Object} scope - angular scope
+   * @param {String} name - event name
+   * @param {Function} callback - callback
+   *
+   * @example
+   * NotifierService.subscribe($scope, 'my-event', function() {
+   *   console.log('Event received!');
+   * });
+   */
+  this.subscribe = function(scope, name, callback) {
+    const handler = $rootScope.$on(name, function(event, data) {
+      return callback(data);
+    });
+
+    scope.$on('$destroy', handler);
+  };
+
+  /**
+   * @summary Emit an event
+   * @function
+   * @public
+   *
+   * @param {String} name - event name
+   * @param {*} data - event data
+   *
+   * @example
+   * NotifierService.emit('my-event', 'Foo');
+   */
+  this.emit = function(name, data) {
+    $rootScope.$emit(name, data);
+  };
+
+});

tests/browser/modules/notifier.spec.js

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+'use strict';
+
+const m = require('mochainon');
+const angular = require('angular');
+require('angular-mocks');
+require('../../../lib/browser/modules/notifier');
+
+describe('Browser: Notifier', function() {
+
+  beforeEach(angular.mock.module('Etcher.notifier'));
+
+  describe('NotifierService', function() {
+
+    let $rootScope;
+    let NotifierService;
+
+    beforeEach(angular.mock.inject(function(_$rootScope_, _NotifierService_) {
+      $rootScope = _$rootScope_;
+      NotifierService = _NotifierService_;
+    }));
+
+    it('should be able to emit an event without data', function() {
+      let spy = m.sinon.spy();
+      NotifierService.subscribe($rootScope, 'foobar', spy);
+      NotifierService.emit('foobar');
+      m.chai.expect(spy).to.have.been.calledOnce;
+      m.chai.expect(spy).to.have.been.calledWith(undefined);
+    });
+
+    it('should be able to emit an event with data', function() {
+      let spy = m.sinon.spy();
+      NotifierService.subscribe($rootScope, 'foobar', spy);
+      NotifierService.emit('foobar', 'Hello');
+      m.chai.expect(spy).to.have.been.calledOnce;
+      m.chai.expect(spy).to.have.been.calledWith('Hello');
+    });
+
+    it('should emit the correct event', function() {
+      let spy1 = m.sinon.spy();
+      let spy2 = m.sinon.spy();
+      NotifierService.subscribe($rootScope, 'foobar', spy1);
+      NotifierService.subscribe($rootScope, 'foobaz', spy2);
+      NotifierService.emit('foobar');
+      m.chai.expect(spy1).to.have.been.calledOnce;
+      m.chai.expect(spy2).to.not.have.been.called;
+    });
+
+  });
+
+});