Ext中extend

http://www.cnblogs.com/cdts_change/archive/2009/09/14/1566172.html

http://wangyu.iteye.com/blog/210849

Ext.extend方法是用来实现类的继承。 
extend(Object subclass,Object superclass,[Object overrides] : Object 
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。