Page 的生命周期学习小结.doc

Page 的生命周期学习小结初始化(Initialization)页面被请求时，第一个被执行的总是构造函数(constructor). 你可以在这里初始化很多自定义属性或对象。不过这里有一些限制，因为 page 还没有被完全初始化。特别地，你必须使用 HttpContext.Current 来访问 QueryString, Form, Cookies 集合，以及 Cache 对象。而 Session 对象在 constructor 里是无法访问的。The first method that executes when a page is requested is always the constructor. You can initialize many of the custom properties or objects in the page at this time, although there are a few limitations since the page hasnt fully initialized yet. In particular, you will have to use the HttpContext.Current object to access the Request QueryString, Form, and Cookies collections, along with the Cache object. However, there is no way to access the Session collection within the constructor.下面接着执行的是 AddParsedSubObject 方法，这个方法把组成该 page 的所有子控件添加到控件集合树中。在很多高级的页面模板解决方案中，该方法通常被覆盖，以便把页面的控件添加到一个特殊的页面模板中去。该方法递归的被子控件调用，所有这些子控件都是这时候初始化的，从最里面的开始。The next method that executes is the AddParsedSubObject method, which actually adds all the individual controls that make up a page into the control collection tree. This method is typically overridden by some of the advanced page template solutions in order to add the page contents into a particular control in the page template. This method is recursively called on all children controls for each page control, and each of these controls is initialized at this time, starting with the innermost.接着是 DeterminePostBackMode 方法。该方法允许你影响 IsPostBack 的值，以及相关事件。如果你想从数据库中加载 ViewState 以便 redirect 时，这个可能对你有用。因为 ViewState 仅仅在 IsPostBack 为 true 的时候被恢复。你可以通过返回 null 来强制不 postback, 或者返回 Request.Form 来强制 postback. 这个方法是不推荐使用的，除非是在特殊的情况下，因为他还影响其他的事件。The next method that executes is the DeterminePostBackMode method of the page class. This method allows you to affect the value of IsPostBack and all the related events, which might be useful if you need to load ViewState from a database for a redirect, since we will see that ViewState is only restored when IsPostBack is actually true. Just return null to force non-PostBack and return Request.Form to force a PostBack. This is not recommended, except in special cases, since it affects other events too. 然后是 OnInit 方法。通常这是我们使用到的第一个方法。这时，所有控件已经被初始化，也就是说所有原始值都被设定了。而 ViewState 以及所有其他 post 的值还没有被应用到控件上。也就是说这时候所有通过代码或者用户操作做的更改还没有被恢复。这通常是创建或重新创建动态控件的最佳时机。The pages OnInit method is executed next, which is typically the first method used. By the time this occurs, all the controls defined in your page will be initialized, which means that all the original values specified in the aspx page will be applied. However, ViewState and any posted values will not have been applied to controls yet, so anything that changed as a result of the code or user will not yet be restored. This is usually the best place to create, and recreate, any dynamic controls used. 恢复和加载(Restore and Load)接下来的 LoadPageStateFromPersistenceMedium 方法，仅仅在 PostBack 时被执行。当你要改变保存 ViewState 的方法时(使用 Session 或其他自定义的储存方法)，覆盖这个方法，以及后面的 SavePageStateToPersistenceMedium 方法。注意：该方法并不真正加载 ViewState 到 page 及其子控件。The next method, LoadPageStateFromPersistenceMedium, only executes for PostBacks. This method is overridden when you have altered how the pages ViewState was saved, later in the SavePageStateToPersistenceMedium, when using Session or Custom Stores. The default implementation assumes ViewState is a Base64 encoded hidden form field, but this can be changed in these two methods using code found in this related article. Note that this method does not actually restore ViewState to the page or its controls.ViewState 被取回后，接着 LoadViewState 方法将它们恢复到 page， 并递归的恢复到每一个子控件(只有 PostBack 的那些).这时，每个控件已经被恢复到了它上次执行时的状态，但用户 post 的值还没有被应用。因为这属于 ViewState. 这个方法是恢复所有在事件中创建的动态控件的最好时机。After ViewState is retrieved, the next method, LoadViewState, restores it to the page, and then recursively to each control and their children, but again only on PostBacks. After this executes, each control will now be restored to the state it was last time, but user posted values will still not be applied, since this is not part of ViewState. The best use of this method is to restore any dynamic controls you created in events, based on values you must manually save in ViewState, which is now available to use.下一个是 ProcessPostData 方法。仅仅在 PostBack 时被执行。而且这个方法不能被覆盖，因为它是页面基类中实现的一个私有方法。这个方法最终将用户 post 的值，通过匹配控件的名称的方法，恢复到页面。这时，page 已经被完全恢复了。动态控件必须在这个方法之前被创建。这个方法同时也为稍后的 changed 事件记录控件值的改变。The next method, ProcessPostData, only executes for PostBacks, and it is not able to be overridden, since it is a private method implemented by the base page class. This method finally restores the users posted form values by matching up controls with the names of the values, which means your page will finally be fully restored. The only catch is that dynamic controls must be created prior to this page method. This is also the method that records changes in values for the changed events later.然后才是 OnLoad 方法。大部分的代码中都使用这个方法，因为这是在 page 的生命周期中，第一个所有的值都被恢复了的地方。我们可以通过检查 IsPostBack 属性来避免不必要的重设状态。同时也可以检查 IsValid 属性来进行验证。同时还可以在这里创建动态控件。所有这些控件的方法都会被执行并捕获，包括 ViewState. 但回发的值不可以。The pages OnLoad method is executed next, which is typically used for most code, since this is the first place in the page lifecycle that all values are restored. Most code checks the value of IsPostBack to avoid unnecessarily resetting state. You may also wish to call Validate and check the value of IsValid in this method. You can also create dynamic controls in this method, and all the controls methods will be executed to catch up, including ViewState, but posted values would not be.Raised Events下一个方法，ProcessPostData 方法， 实际上是前面那个方法的第二个入口(second pass)。它仅仅处理回发，而且由于是私有方法，所以不能被覆盖。这个方法显得有些奇怪，但又是必要的。因为在 OnLoad 方法中重建的动态控件需要他们回发的值。所有在这个方法之后创建的动态控件，将只能恢复 ViewState, 而不能恢复回发的值，并且不能触发任何更改事件。The next method, ProcessPostData, is actually a second pass of the earlier method, which still only executes for PostBacks and is not overridable since private method. This second pass always seems a little odd when you first notice it in the page trace, but it occurs because dynamic controls recreated in OnLoad need their posted values. Any controls that are dynamically created after this method, while having ViewState restored, will not get their posted values nor be able to trigger any changed events.下一个方法， RaiseChangedEvents， 同样仅仅用于回发时。它是一个基类实现的私有方法。这时 changed 事件被真正触发。这基于前面 ProcessPostData 方法中标注出回发的值的差异。当有多个 changed 事件被触发时，其先后顺序是没有保证的。The next method, RaiseChangedEvents, also occurs only for PostBacks, and it too is a private method that is implemented in the base page class that cannot be overridden. This is the time in the page lifecycle that the changed events are actually raised, based on the differences that ProcessPostData noted before and after posted values. You may also want to call Validate and check the value of IsValid, if not already. There is no guarantee about the order that multiple changed events will be raised. 下面是 RaisePostBackEvent 方法。仅用于回发，而且是基类实现的私有方法。这是真正提交 form 的方法，除非是 postback。比如按钮，或者其他通过 javascript 提交的控件被触发。如果使用了 Validators, 如没有手动调用 Validate 方法，这时也已经被调用了。有时候 ie 的 bug 会使得表单被提交，而不引发事件。The next method, RaisePostBackEvent, also occurs only for PostBacks, and it too is a private method that is implemented in the base page class that cannot be overridden. This is where the event that actually submitted the form, unless using AutoPostBack, is actually raised, typically a button, or a control that uses javascript to submit. If not manually called already, Validate is also called, when using some Validators. Note a bug in IE sometimes allows the Enter key to submit without triggering an event. 接着是 OnPreRender 方法。这通常是在被绘制到浏览器之前，要更改 page 及其子控件的最后机会。你也可以在这里创建动态控件。但这时只能捕获 ViewState, 而不能接受 posted values， 而且没有事件。因为上面提到的 ie 的 bug, 这里可以用来捕获没有触发事件的 post back.The pages OnPreRender method is executed next, which is typically the last chance to affect the page and its controls before it is rendered to the clients browser. You can also create dynamic controls in this method, and all the controls methods will be executed to catch up, including ViewState, but the earlier private methods will not receive another pass, meaning no posted values are restored and no events. This is a good place to catch a PostBack without an event due to the bug noted in IE. 保存和绘制(Save and Render)下一个是 SaveViewState 方法。 不管是否 post back. 递归的应用到每一个子控件。ViewState 一般保存所有和 aspx 页面里不一样的属性，不管是被代码还是用户更改的。注意，由于控件的值是通过他们在控件树中的位置来保存的，所以如果在这之后添加动态控件到错误的位置， ViewState 可能会崩溃。The next method that executes, regardless whether PostBack or not, is SaveViewState which recursively runs for each control and their children, to create the ViewState. ViewState basically stores any property values that are different from the original values that are defined in the aspx page, whether changed in the code or by the user. Note that control values are saved according to their location in the control tree, so ViewState can get corrupted if dynamic controls are later added in wrong positions.下面是 SavePageStateToPersistenceMedium 方法。它真正的保存 page 的 ViewState. 这个方法可覆盖。如果重写的话，注意这里由于 的 bug, 需要手工设定一下 _VIEWSTATE，哪怕是空值。The next method, SavePageStateToPersistenceMedium, actually saves the page ViewState. This method is overridden, along with LoadPageStateFromPersistenceMedium, to save the ViewState either to Session or a Custom Store, instead of using the hidden field. This is useful for low bandwidth cases, and Session is the default for Mobile Pages. See the code found in this related article for details of using these two methods, and note that a bug in ASP.NET requires a _VIEWSTATE field to be sent, even if empty.接着是 Render 方法。它递归的调用到每个子控件，真正的绘制各自的 html, 发送到浏览器。在一些页面模板方案中，常常在这里添加通用的 header 和 footer. 而不用使用服务器控件。 注意在这里能作的更改必须是纯的 html. 因为这时候控件都已经绘制完了。The pages Render method is executed next, which recursively runs for each control and their children, to actually create and send the resulting html to the browser. This method is used in some page template solutions to add common headers and footers to the page without using server controls, which always have a little extra overhead. Note that changes made here must be pure html, since controls are done at this time. You can capture the html output using StringBuilder, StringWriter, and HtmlTextWriter.