让我们先实现组件化的AutoFlex脚本:
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{
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{
this._element.style.height = this._element.scrollHeight
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{
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上面这段代码我们遵循这样的代码规范:
(1)类名使用Pascal命名法,函数使用骆驼法,这一点很重要,因为JavaScript本没有类这个概念,我们使用Function模拟类,为了区别function定义的是类的概念还是函数的概念,我们使用不同的命名法。
(2)类的属性在构造函数中定义和初始化,类的函数中通过prototype添加。
(3)非对外的成员名使用_开头。
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这段代码需要注意以下几点:
(1)在JavaScript中,0相当于false,null也相当于false,所以我们只需判断if(this.maxHeight)就能覆盖没有传maxHeight参数和maxHeight参数为0的两种情况。
(2)在附加事件响应程序时,我们不是直接写this._element.onpropertychange = this._onPropertyChange而是this._element.onpropertychange = this._getPropertyChangeHandler()。这是为什么呢?原来如果我们使用前一种写法,当_onPropertyChange方法被事件触发时,它的执行上下文,也就是this指针是指向触发事件的对象或window对象,这是JavaScript中事件的一个特别需要重视的特点,如果是这样的话,_onPropertyChange()方法中的代码将不能正确执行。在后一种写法中,我们通过JavaScript的闭包特性和Function.call()方法使得_onPropertyChange()方法在执行时能得到期待的this指针。
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注:闭包——函数始终可以使用定义它时在它外围的(即它可访问到的)变量,而无论它在何时执行。如上例的_getPropertyChangeHandler()方法中的代码,我们用obj变量保存this的指针,那么接下来定义的函数始终可以访问到obj变量,即使是在被事件触发时也是如此。call——Javascript中的Function原型上有两个功能类似的方法——call和apply,它们都用来执行当前函数,但被执行函数中的this指针指向call或apply方法的第一个参数,如上例中的obj._onPropertyChange.call(obj),这个语句被事件触发时指针this本来是指向window对象的,然而我们使用call方法使得_onPropertyChange方法内部的语句访问的this指针指向obj对象。
call方法和apply方法的区别在于传递参数的方式不同,call方法把第一个参数作为被调用方法的this指针,其余参数作为被调用方法的参数,apply也把第一个参数作为被调用方法的this指针,然后用一个数组把参数打包传给被调函数。如,myFunction.call(context,arg0,arg1,arg2)和myFunction.apply(context,[arg0,arg1,arg2]);。
准备好了组件化的脚本后,控件的实现变得非常简单:
<%@ Register TagPrefix="cc1" Namespace="ClassLibrary1" Assembly="ClassLibrary1" %> 该指令注册自定义控件
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get{
if (ViewState["MaxHeight"] == null)
{ return 0; }
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ViewState["MaxHeight"] = value;
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此外新增了MaxHeight属性,用来指定最大扩展高度。
然后重写OnPreRender()方法,检测访问浏览器对脚本的支持能力,并输出AutoFlex组件脚本,通过IsClientScriptBlockRegistered()方法保证在同样的页面中,这段脚本只会输出一次。
最后重写Render()方法,在页面表单的最后注册初始化客户端组件所需的脚本.
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生成控件,放入页面中进行测试:
<cc1:autoflextextarea id="AutoFlexTextArea1" runat="server"></cc1:autoflextextarea>
<cc1:autoflextextarea id="AutoFlexTextArea2"
runat="server" MaxHeight="200"></cc1:autoflextextarea>
最终呈现的页面内容如下:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head><title>
无标题页
</title></head>
<body>
<form name="form1" method="post" action="TestAutoFlexTextArea.aspx" id="form1">
<div>
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="/wEPDwUKMTEyMj U4MzUwMGRkE8+2KGaUuMKOsu+evZ76vSgAYMU=" />
</div>
<script type="text/javascript">
<!--
function AutoFlex(id,maxHeight){
//省略AutoFlex组件代码;
}
// -->
</script>
<div>
<textarea name="AutoFlexTextArea1" rows="2" cols="20" id="AutoFlexTextArea1"></textarea>
<textarea name="AutoFlexTextArea2" rows="2" cols="20" id="AutoFlexTextArea2"></textarea>
</div>
<div>
<input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION" value="/wEWAwL/88OJCQKmoPGEDAKloPGEDNkP9kNJ1Pg3kQ7qvP+7/kqXZYV0" />
</div>
<script type="text/javascript">
<!--
var AutoFlexTextArea1_AutoFlex = new AutoFlex('AutoFlexTextArea1',0);
AutoFlexTextArea1_AutoFlex.initiate();
var AutoFlexTextArea2_AutoFlex = new AutoFlex('AutoFlexTextArea2',200);
AutoFlexTextArea2_AutoFlex.initiate();
// -->
</script>
</form>
</body>
</html>