Ext.extend()的说明

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>

<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>

<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>

<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