[javascript]图解+注释版 Ext.extend()
Ext.extend() 体现了程序员非凡的制造轮子的能力。它基于 javascript 古老的对象模型，最大程度地模拟出现代面向对象语言的类型继承的语意。但是在程序界有太多的“与XXX很像”，但是实际上又有很多差别。要想最彻底、最精确地理解 Ext.extend()，最直接（往往也是最有效）的方法就是去读它的源代码。为了达到更好的可读性，我更改了部分变量名称，加上了详细的注释。
1 /**
2 * <p>Extends one class to create a subclass and optionally overrides members with the passed literal. This method
3 * also adds the function "override()" to the subclass that can be used to override members of the class.</p>
4 * For example, to create a subclass of Ext GridPanel:
5 * <pre><code>
6 MyGridPanel = Ext.extend(Ext.grid.GridPanel, {
7 constructor: function(config) {
8
9 // Create configuration for this Grid.
10 var store = new Ext.data.Store({...});
11 var colModel = new Ext.grid.ColumnModel({...});
12
13 // Create a new config object containing our computed properties
14 // *plus* whatever was in the config parameter.
15 config = Ext.apply({
16 store: store,
17 colModel: colModel
18 }, config);
19
20 MyGridPanel.superclass.constructor.call(this, config);
21
22 // Your postprocessing here
23 },
24
25 yourMethod: function() {
26 // etc.
27 }
28 });
29 </code></pre>
30 *
31 * <p>This function also supports a 3-argument call in which the subclass's constructor is
32 * passed as an argument. In this form, the parameters are as follows:</p>
33 * <div class="mdetail-params"><ul>
34 * <li><code>subclass</code> : Function <div class="sub-desc">The subclass constructor.</div></li>
35 * <li><code>superclass</code> : Function <div class="sub-desc">The constructor of class being extended</div></li>
36 * <li><code>overrides</code> : Object <div class="sub-desc">A literal with members which are copied into the subclass's
37 * prototype, and are therefore shared among all instances of the new class.</div></li>
38 * </ul></div>
39 *
40 * @param {Function} superclass The constructor of class being extended.
41 * @param {Object} overrides <p>A literal with members which are copied into the subclass's
42 * prototype, and are therefore shared between all instances of the new class.</p>
43 * <p>This may contain a special member named <tt><b>constructor</b></tt>. This is used
44 * to define the constructor of the new class, and is returned. If this property is
45 * <i>not</i> specified, a constructor is generated and returned which just calls the
46 * superclass's constructor passing on its parameters.</p>
47 * <p><b>It is essential that you call the superclass constructor in any provided constructor. See example code.</b></p>
48 * @return {Function} The subclass constructor from the <code>overrides</code> parameter, or a generated one if not provided.
49 */
50 Ext.extend = function(){
51 // inline overrides
52 var io = function(o){
53 for(var m in o){
54 this[m] = o[m];
55 }
56 };
57 var oc = Object.prototype.constructor; // 如果一个对象的 constructor == oc，说明它是使用类似 { age:22 } 这种语法创建的字面量对象，
58 // 而且没有对constructor属性赋值
59
60 return function(subClass, superClass, overrides){
61 if(typeof superClass == 'object'){
62 // 如果 superClass 是对象而不是构造函数，就说明是使用的是
63 // var Cat = Ext.extend(Animal, {
64 // say : function() {
65 // document.writeln("I'm a cat name " + this.name);
66 // }
67 // });
68 // 这种方式调用的。也就是说返回值是subClass, subClass 参数其实是 superClass，superClass参数其实是overrides，overrides参数应该被忽略 www.zzzyk.com
69 overrides = superClass; // 忽略 overrides 参数
70 superClass = subClass; // subClass 参数其实是 superClass
71 // subClass 参数将作为将来的返回值。
72 // 如果 overrides 对象没有自定义构造函数，为其定义一个，并且里面调用父类构造函数；
73 // 如果 overrides 对象含有自定义构造函数，把overrides的构造函数赋给子类
74 subClass = overrides.constructor != oc ? overrides.constructor : function(){
75 superClass.apply(this, arguments); // 调用父类构造函数（前提是子类构造函数的参数可以比父类少，但是
补充：web前端 , JavaScript ,
