写在前面

　　好长时间没有写博客了，昨天花了些时间又整理了下之前发布过的《Ember.js之computed Property》文章，并创建了一个测试代码库，花了些时间，希望能使用测试代码的方式，来演示如何使用Ember.js同时能避免升级Ember版本后，一些功能上的变化所带来的隐含Bug。

　　如果大家对Ember.js有兴趣想一起研究的话，欢迎大家一起维护测试代码库  ：）

　　本文主要是针对Ember.Array中的[]和@each，数组方法进行详细分析。

文章索引

JS前端框架之Ember.js系列

一、如何使用[] & @each

　　Ember中二者均可用于计算属性的绑定，使用十分方便，可以像如下方式定义依赖关系：

1 totalArea: Ember.computed(‘sizeArray.[]‘, function () {
2       return this.get(‘sizeArray‘).reduce(function (prev, cur) {
3         return prev + Ember.get(cur, ‘height‘) * Ember.get(cur, ‘width‘);
4       }, 0);
5 }),

　　或者：

1 totalArea: Ember.computed(‘[email protected]‘, function () {
2       return this.get(‘sizeArray‘).reduce(function (prev, cur) {
3         return prev + Ember.get(cur, ‘height‘) * Ember.get(cur, ‘width‘);
4       }, 0);
5 }),

　　这样就定义了一个依赖于数组sizeArray的计算属性，建立一个绑定关系，只要sizeArray发生变化，totalArea就会被重新计算，使用十分简单，只要在function前面罗列出依赖的属性即可。虽然使用简单，但是在使用上还是有些细节的，下一节将用测试代码来讲解使用上的细节。

注：Ember计算属性有多种写法，这里给出了Ember推荐写法，更多具体细节请参考文章《Ember.js之computed Property-1. What is CP》

二、Array.[] & [email protected]特性总结

(Ember v1.13.7) 

1. [email protected] 特性

当CP属性依赖于.property(‘[email protected]‘)时：

　　- columns里面任何一个元素的isLoaded属性发生变化。

　　- columns增加元素或者删除子元素。

　　- columns数组本身被重新赋值。

　　- 不能依赖@[email protected] 。

 1 test(‘computed property depend on @each.height‘, function (assert) {
 2   var Size = Ember.Object.extend({height: 0, width: 0});
 3   var rectangle = Ember.Object.extend({
 4     totalArea: Ember.computed(‘[email protected]‘, function () {
 5       return this.get(‘sizeArray‘).reduce(function (prev, cur) {
 6         return prev + Ember.get(cur, ‘height‘) * Ember.get(cur, ‘width‘);
 7       }, 0);
 8     }),
 9     sizeArray: [
10       Size.create({height: 10, width: 10}),
11       Size.create({height: 10, width: 10})
12     ]
13   }).create();
14   var sizeArray = rectangle.get(‘sizeArray‘);
15
16   assert.equal(rectangle.get(‘totalArea‘), 200, "the total area should be 200");
17
18   sizeArray.pushObject(Size.create({height: 10, width: 10}));
19   assert.equal(rectangle.get(‘totalArea‘), 300, "the total area should be 300 after added");
20
21   sizeArray.removeAt(0);
22   assert.equal(rectangle.get(‘totalArea‘), 200, "the total area should be 200 after removed");
23
24   sizeArray[0].set(‘height‘, 20);
25   assert.equal(rectangle.get(‘totalArea‘), 300, "the total area should be 300 after ‘height‘ changed");
26
27   sizeArray[0].set(‘width‘, 20);
28   assert.equal(rectangle.get(‘totalArea‘), 300, "the total area should not be changed after ‘width‘ changed");
29
30   sizeArray.clear();
31   assert.equal(rectangle.get(‘totalArea‘), 0, "the total area should be 0 after reset rectangle.sizeArray");
32 });

[参考代码](https://github.com/Cuiyansong/ember-table-learnning/blob/master/ember-test/tests/unit/computed-property-test.js#L164-Lundefined)

2. [email protected]特性

当CP属性依赖于.property(‘[email protected]‘)时：

　　- columns增加或删除元素。

　　- columns自身被替换或重新赋值。

 1 test(‘computed property depend on @each‘, function (assert) {
 2   var Size = Ember.Object.extend({height: 0, width: 0});
 3   var rectangle = Ember.Object.extend({
 4     totalArea: Ember.computed(‘[email protected]‘, function () {
 5       return this.get(‘sizeArray‘).reduce(function (prev, cur) {
 6         return prev + Ember.get(cur, ‘height‘) * Ember.get(cur, ‘width‘);
 7       }, 0);
 8     }),
 9     sizeArray: [
10       Size.create({height: 10, width: 10}),
11       Size.create({height: 10, width: 10})
12     ]
13   }).create();
14   var sizeArray = rectangle.get(‘sizeArray‘);
15
16   assert.equal(rectangle.get(‘totalArea‘), 200, "the total area should be 200");
17
18   sizeArray.pushObject(Size.create({height: 10, width: 10}));
19   assert.equal(rectangle.get(‘totalArea‘), 300, "the total area should be 300 after added");
20
21   sizeArray.removeAt(0);
22   assert.equal(rectangle.get(‘totalArea‘), 200, "the total area should be 200 after removed");
23
24   sizeArray[0].set(‘height‘, 20);
25   assert.equal(rectangle.get(‘totalArea‘), 200, "the total area should not be changed");
26
27   sizeArray.clear();
28   assert.equal(rectangle.get(‘totalArea‘), 0, "the total area should be 0 after reset rectangle.sizeArray");
29 });

[参考代码](https://github.com/Cuiyansong/ember-table-learnning/blob/master/ember-test/tests/unit/computed-property-test.js#L104)

3. Array.[]特性

当CP属性依赖于.property(‘columns.[]‘)时：

　　- 与绑定.property(‘[email protected]‘) 行为相同。

 1 test(‘computed property depend on []‘, function (assert) {
 2   var Size = Ember.Object.extend({height: 0, width: 0});
 3   var rectangle = Ember.Object.extend({
 4     totalArea: Ember.computed(‘sizeArray.[]‘, function () {
 5       return this.get(‘sizeArray‘).reduce(function (prev, cur) {
 6         return prev + Ember.get(cur, ‘height‘) * Ember.get(cur, ‘width‘);
 7       }, 0);
 8     }),
 9     sizeArray: [
10       Size.create({height: 10, width: 10}),
11       Size.create({height: 10, width: 10})
12     ]
13   }).create();
14   var sizeArray = rectangle.get(‘sizeArray‘);
15
16   assert.equal(rectangle.get(‘totalArea‘), 200, "the total area should be 200");
17
18   sizeArray.pushObject(Size.create({height: 10, width: 10}));
19   assert.equal(rectangle.get(‘totalArea‘), 300, "the total area should be 300 after added");
20
21   sizeArray.removeAt(0);
22   assert.equal(rectangle.get(‘totalArea‘), 200, "the total area should be 200 after removed");
23
24   sizeArray[0].set(‘height‘, 20);
25   assert.equal(rectangle.get(‘totalArea‘), 200, "the total area should not be changed");
26
27   sizeArray.clear();
28   assert.equal(rectangle.get(‘totalArea‘), 0, "the total area should be 0 after reset rectangle.sizeArray");
29 });

[参考代码](https://github.com/Cuiyansong/ember-table-learnning/blob/master/ember-test/tests/unit/computed-property-test.js#L134)

(Ember v2.0.0)

2015-08-17 除建议用[]代替@each，暂未发现其他变化。

[参考代码](https://github.com/emberjs/ember.js/blob/v2.0.0/packages/ember-metal/lib/computed.js#L224)

注： 更多关于计算属性问题，请参考文章《Ember.js之computed Property-3.CP重要原则》

三、源码分析Array.[] & [email protected]

[email protected]返回的是一个特殊类型，这个类型便于实现绑定。

 1   ‘@each‘: computed(function() {
 2     if (!this.__each) {
 3       // ES6TODO: GRRRRR
 4       var EachProxy = requireModule(‘ember-runtime/system/each_proxy‘)[‘EachProxy‘];
 5
 6       this.__each = new EachProxy(this);
 7     }
 8
 9     return this.__each;
10   })

[参考这里](https://github.com/emberjs/ember.js/blob/stable-1-13/packages/ember-runtime/lib/mixins/array.js#L516)

Array.[]继承自Ember.Enumerable.[]并且返回this。

1   ‘[]‘: computed({
2     get(key) {
3       return this;
4     },
5     set(key, value) {
6       this.replace(0, get(this, ‘length‘), value);
7       return this;
8     }
9   }),

[参考这里](https://github.com/emberjs/ember.js/blob/stable-1-13/packages/ember-runtime/lib/mixins/array.js#L164)

二者之间唯一的区别是Array.[]返回的是一对象自身（普通对象组成的数组）, 而[email protected]返回的是EachProxy对象, 针对普通对象组成的数组而言，仅仅能检测到数组长度的变化和对象本身的变化，对数组内容发生变化则不得而知，而EachProxy对每一个元素增加observerListener监听器，当数组内容发生变化时，通知数组发生变更，实现了数组元素这一级别的监听。

 1 var EachProxy = EmberObject.extend({
 2
 3   init(content) {
 4     this._super(...arguments);
 5     this._content = content;
 6     content.addArrayObserver(this);
 7
 8     // in case someone is already observing some keys make sure they are
 9     // added
10     forEach(watchedEvents(this), function(eventName) {
11       this.didAddListener(eventName);
12     }, this);
13   },
14
15 ...

[参考这里](https://github.com/emberjs/ember.js/blob/stable-1-13/packages/ember-runtime/lib/system/each_proxy.js#L109)

四、相关引用

[[email protected]](http://emberjs.com/api/classes/Ember.Array.html#property__each)

[Emer-EachProxy](https://github.com/emberjs/ember.js/blob/stable-1-13/packages/ember-runtime/lib/system/each_proxy.js)

[Ember-study](https://github.com/Cuiyansong/ember-table-learnning)