On the Postback ASP.NET

He said Postback ASP.NET, you have to say Web Page of the life cycle, but the life cycle of Web Page is not a few words can say clearly, it is a programming perspective here purely station, leaving aside the life cycle of Web Page On Postback.

We know that both ASP.NET1.x, 2.0, or even later versions, ASP.NET Client-final Render to browse through the browser is the same: a simple HTML. Client way through the Submit Form Fill Form data submitted to the Server for processing. We now take a look at the entire process ASP.NET Postback program processing.

First, we look through a Sample ASP.NET how to handle a Postback Click a Button induced by. Here is the Web Page of HTML:

<% @ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

<! 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 runat ="server" >

< title > Test Page </ title >

</ head >

< body >

< form id ="form1" runat ="server" >

< div >

< asp:Label runat ="server" ID ="LabelMessage" ForeColor ="red" ></ asp:Label >

</ div >

< div >

< asp:Button runat ="server" ID ="Button1" Text ="Button1" OnClick ="Button1_Click" OnCommand ="Button_Command" CommandArgument ="Button1" />

< asp:Button runat ="server" ID ="Button2" Text ="Button2" OnClick ="Button2_Click" OnCommand ="Button_Command" CommandArgument ="Button2" UseSubmitBehavior ="false" />

< asp:Button runat ="server" ID ="Button3" Text ="Button3" OnClick ="Button3_Click" OnCommand ="Button_Command" CommandArgument ="Button3" UseSubmitBehavior ="false" />

</ div >

</ form >

</ body >

</ html >

Very simple, it defines three Button, respectively registered their two Event: Click and Command. 3 Button's Command Event Hander is the same: Button_Command, by specifying the Event Handler CommandArgument to make judgment in the end is what triggered the Button Command Event.

The following is a Code Behind:

using System;

using System.Data;

using System.Configuration;

using System.Web;

using System.Web.Security;

using System.Web.UI;

using System.Web.UI.WebControls;

using System.Web.UI.WebControls.WebParts;

using System.Web.UI.HtmlControls;

public partial class _Default : System.Web.UI.Page

{

protected void Page_Load(object sender, EventArgs e)

{

}

protected void Button1_Click(object sender, EventArgs e)

{

string message = string.Format("The {0} event of {1} is fired", "Click", "Button1");

this.LabelMessage.Text = message;

}

protected void Button2_Click(object sender, EventArgs e)

{

string message = string.Format("The {0} event of {1} is fired", "Click", "Button2");

this.LabelMessage.Text = message;

}

protected void Button3_Click(object sender, EventArgs e)

{

string message = string.Format("The {0} event of {1} is fired", "Click", "Button3");

this.LabelMessage.Text = message;

}

protected void Button_Command(object sender, CommandEventArgs e)

{

string message = string.Format("The {0} event of {1} is fired", "Command", e.CommandArgument);

this.LabelMessage.Text += "; " + message;

}

Let's run Page, and Click a button (such as Button2):

We can see through the top of the Message, Button2's Click Event and Command has triggered.

This Blog is told from the perspective of the subject method calls throughout the program running: Render is from HTML to Client end user Click a button input is Postback to the Server side, and two triggers Event, Event Handler print execution the relevant Message.

First we look at the design of ASP.NET Page Render to Client side HTML is what it looks like:

<! 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 >

Test Page

</ title >

</ head >

< body >

< form name ="form1" method ="post" action ="Default.aspx" id ="form1" >

< div >

< input type ="hidden" name ="__EVENTTARGET" id ="__EVENTTARGET" value ="" />

< input type ="hidden" name ="__EVENTARGUMENT" id ="__EVENTARGUMENT" value ="" />

< input type ="hidden" name ="__VIEWSTATE" id ="__VIEWSTATE" value ="/wEPDwUKMTA0NDQ2OTE5OWRk281L4eAk7iZT10hzg+BeOyoUWBQ=" />

</ div >

< script type ="text/javascript" >

<!--

var theForm = document.forms['form1'];

if (!theForm) {

theForm = document.form1;

}

function __doPostBack(eventTarget, eventArgument) {

if (!theForm.onsubmit || (theForm.onsubmit() != false)) {

theForm.__EVENTTARGET.value = eventTarget;

theForm.__EVENTARGUMENT.value = eventArgument;

theForm.submit();

}

// -->

</ script >

< div >

< span id ="LabelMessage" style ="color:Red;" ></ span >

</ div >

< div >

< input type ="submit" name ="Button1" value ="Button1" id ="Button1" />

< input type ="button" name ="Button2" value ="Button2" onclick ="javascript:__doPostBack('Button2','')" id ="Button2" />

< input type ="button" name ="Button3" value ="Button3" onclick ="javascript:__doPostBack('Button3','')" id ="Button3" />

</ div >

</ form >

</ body >

</ html >

上面的HTMLBody部分大体包括3个部分：

1．定义了3个hidden field：

< input type ="hidden" name ="__EVENTTARGET" id ="__EVENTTARGET" value ="" />

< input type ="hidden" name ="__EVENTARGUMENT" id ="__EVENTARGUMENT" value ="" />

< input type ="hidden" name ="__VIEWSTATE" id ="__VIEWSTATE" value ="/wEPDwUKMTA0NDQ2OTE5OWRk281L4eAk7iZT10hzg+BeOyoUWBQ=" />

从他们的命名可以看出他们分别代表的意思：__EVENTTARGET代表触发Event的Control的Unique name；__EVENTARGUMENT代表为Event Handler定义的额外的参数；__VIEWSTATE：代表的是Viewstate。

2．一段script：

< script type ="text/javascript" >

<!--

var theForm = document.forms['form1'];

if (!theForm) {

theForm = document.form1;

}

function __doPostBack(eventTarget, eventArgument) {

if (!theForm.onsubmit || (theForm.onsubmit() != false)) {

theForm.__EVENTTARGET.value = eventTarget;

theForm.__EVENTARGUMENT.value = eventArgument;

theForm.submit();

}

// -->

</ script >

定义了一个__doPostBack function完成Postback的操作，该function只有区区3行代码，前两行通过参数对上面定义的两个hidden field赋值，然后向Server端提交表单。

3．一段HTML对应通过ASP.NET定义的Web Control。

< div >

< span id ="LabelMessage" style ="color:Red;" ></ span >

</ div >

< div >

< input type ="submit" name ="Button1" value ="Button1" id ="Button1" />

< input type ="button" name ="Button2" value ="Button2" onclick ="javascript:__doPostBack('Button2','')" id ="Button2" />

< input type ="button" name ="Button3" value ="Button3" onclick ="javascript:__doPostBack('Button3','')" id ="Button3" />

div>

我们定义的3个Button被转化成3个能向Server端提交表单的<input > Tag, 但是他们提交表的方式却不一样，第一个以<input type="submit">的方式提交，后面两个通过调用javascript的方式提交表单（<input type="button">）。对于一个System.Web.UI.WebControls.Button，默认采用第一种提交方式，但是我们通过设置UseSubmitBehavior属性（这个属性时ASP.NET 2.0新加的，1x没有相应的设置），改变其表单提交的行为。

当用户Click Button2的时候，调用__doPostBack，并传入两个参数：一个代表出发Event的对象的Unique name，也就是Button2的名称，另一个描述Event的额外信息的参数，这里不需要，所以这里是空字符串。在__doPostBack中把这两个参数赋值给两个Hidden Field：__EVENTTARGET，__EVENTARGUMENT。然后向Server端提交表单，完成Postback。

然后我们来看看Server如何处理这个Postback，关于Web Page的生命周期在这里就不详细介绍了。Server端通过__EVENTTARGET这个hidden field的值找到对应的Server端的Control，通过Reflection确定该Control是否实现了System.Web.UI.IPostBackEventHandler Interface。如果该Control确实实现了该Interface，那么调用Page的RaisePostBackEvent方法，这是一个Virtual的方法，可以被Override。我们来看该方法的定义。

[EditorBrowsable(EditorBrowsableState.Advanced)]

protected virtual void RaisePostBackEvent(IPostBackEventHandler sourceControl, string eventArgument)

{

sourceControl.RaisePostBackEvent(eventArgument);

}

我们可以看到该方法直接调用该sourceControl的RaisePostBackEvent，并传入一个eventArgument参数，在这个例子中sourceControl就是__EVENTTARGET对应的Web Control：Button2，eventArgument就是__EVENTTARGET对应的值：一个空字符串。Button2的类型是System.Web.UI.WebControls.Button。我们来看看System.Web.UI.WebControls.Button中的RaisePostBackEvent方法是如何定义的：

protected virtual void RaisePostBackEvent( string eventArgument)

{

base.ValidateEvent(this.UniqueID, eventArgument);

if (this.CausesValidation)

{

this.Page.Validate(this.ValidationGroup);

}

this.OnClick(EventArgs.Empty);

this.OnCommand(new CommandEventArgs(this.CommandName, this.CommandArgument));

}

这个方法也很简单，先进行Validation，然后先后出发两个Event:OnClick 和OnCommand，随后调用对应的Event handler，这和我们的输出结果是吻合的。

这基本上就是整个Postback的整个程序执行的过程，现在我们对我们的Page作一些小的有趣的改动，来验证一下：

Client端和Server端进行交互的途径就是提交表单（Form Submitting）,而我们现在有两种方式来提交表单：通过<input type="submit">控件；通过调用javascript:__doPostBack。基于这一点我们在Html中加了下面一段javascript：

< script type ="text/javascript" >

function postback()

{

__doPostBack('Button1','');

}

document.getElementById("Button2").onclick = postback;

document.getElementById("Button3").onclick = postback;

</ script >

我们override Button2和Button3的onclick event，把'Button1作为参数传入__doPostBack方法，可以想象，现在无论Click那个Button，程序都将认为之Click Button1。有兴趣的可以亲自试试，无论Click那个Button，显示的效果都将是下面的样子：

接下来我们取消上面的改动，在Server对Code作另一方面的尝试。我们前面说过，Server接受到Client的Postback，对于事件的Web Control（或者Html Server Control），如果实现了System.Web.UI.IPostBackEventHandler接口，会调用Page的virtual方法：RaisePostbackEvent，我们现在来Override这个方法：

protected override void RaisePostBackEvent(IPostBackEventHandler sourceControl, string eventArgument)

{

sourceControl = this.Button1;

base.RaisePostBackEvent(sourceControl, eventArgument);

}

在上面的Code中，我们把sourceControl设为Button1，这样无论在client端Click的那个Button，现在都将认为是对Button的Click。运行的结果和上面一样。

通过上面的介绍，我们知道了Page的RaisePostBackEvent会调用Source Control的RaisePostBackEvent方法，这个方法是定义在IPostBackEventHandler接口中，很多Control都实现了这个方法，对于Button来说，这个方法是Virtual的，它可以被你Override，如果感兴趣的话，可以自己写一个Custom Button，并Override该方法，看看执行的情况，相信会使加深你对Postback的理解。

转载于:https://www.cnblogs.com/zhangchenliang/p/3467363.html

On the Postback ASP.NET

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