Ext.extend方法是用来实现类的继承

Ext.extend方法是用来实现类的继承。

extend
public function extend( Object subclass, Object superclass, [Object overrides] )
Extends one class with another class and optionally overrides members with the passed literal. This class also adds the function "override()" to the class that can be used to override members on an instance.
Parameters:

    * subclass : Object
      The class inheriting the functionality
    * superclass : Object
      The class being extended
    * overrides : Object
      (optional) A literal with members

Returns:

    * void

This method is defined by Ext.

第一个参数：子类
第二个参数：父类
第三个参数：要覆盖的属性。
这里需要强调一下，子类继承下来的是父类中通过superclass.prototype方式定义的属性（包括用此方法定义的函数）。
例子如下：

<script type="text/javascript">    
    function S(){   
    }   
    S.prototype.s = "s";   
    S.prototype.s1 = "s1";   
    function C(){   
        this.c = "c";   
        this.c1 = "c1";   
    }   
    Ext.extend(C,S,{s1:"by c overload"});   
    var c = new C();   
    alert(c.s); //s   
    alert(c.s1); //by c overload   
</script> 

如果按下面这个方式写就会提示c.s没有定义(undefind)：

<script type="text/javascript">    
   function S(){   
      this.s = "s";   
      this.s1 = "s1";   
   }   
   function C(){   
      this.c = "c";   
      this.c1 = "c1";   
   }   
   Ext.extend(C,S,{s1:"by c overload"});   
   var c = new C();   
   alert(c.s); //undefind   
   alert(c.s1); //by c overload   
</script> 

也可以通过如下方式来实现类的继承：

<script type="text/javascript">    
   function S(){   
   }   
   S.prototype.s = "s";   
   S.prototype.s1 = "s1";   
   C = Ext.extend(S,{s1:"by c overload"});   
   var c = new C();   
   alert(c.s); //s   
   alert(c.s1); //by c overload   
</script>

此时，C是子类，S是父类