What is a Partial Postback?

问题:

I'm an experienced web developer in several different web technology stacks. I currently work in .Net and I've become curious about the Partial Postback.

I understand what the Partial Postback's function is and how to work with it programatically, but I don't like using technology that I don't understand, how is a Partial Postback implemented.

I understand HTTP Requests and Asynchronous Requests, the thing that bugs me about the Partial Postback is that it seems to be both.

Maybe I'm just missing something but it appears to me that the Partial Request does both, first firing off an Asynchronous POST Request but the Browser seems to be aware and the activity indicator starts to spin, which normally only occurs during an HTTP Request Page Render.

So can anyone shed some light on how Microsoft implemented, at the HTTP Request level, the Partial Postback?

回答1:

From How to write your own partial postback in ASP.NET 2.0:

At the heart of the partial post back construction is the XMLHttpRequest, a DOM API. It can be used inside a web browser scripting language, such as JavaScript, to send an HTTP request directly to a web server without having to reload the entire page and handling the response from the server again within the scripting language. This data, in the form of XML, can then be used to manipulate the page elements on the client side.

When the button on the ASPX page has been clicked, a client side HTTP request is made using the XMLHttpRequest API. This request is handled by an HttpHandler on the web server. The HttpHandler receives the request, processes it, and sends back the response to the XMLHttp object on the ASPX page. The XMLHttp object in turn consumes the response and renders the appropriate UI changes without the browser having to do a full refresh of the page.

An UpdatePanel not only refreshes the controls which it contains, it also updates the ViewState value with the one obtained from the server, after processing.

回答2:

I know this question has already been answered, but I disagree with the answer...

In my humble opinion, the term 'partial postback' is being misused in the above-mentioned article "How to write your own partial postback in ASP.NET 2.0". In this article, the author shows you how to make an AJAX call to an HttpHandler. This is a very different process than making PartialPostback calls in ASP.NET.

I beleive this 'difference' is eluded to in the (above answers) after-thought comment which states:

"An UpdatePanel not only refreshes the controls which it contains, it also updates the ViewState value with the one obtained from the server, after processing."

While this final comment somewhat elusively-illustrates the definition of a 'Partial Postback' using an ASP.NET UpdatePanel...it doesn't explain what a partial-postback is (which, once again, is a very different process than that of a normal AJAX call).

To elaborate...

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MICROSOFT ASP.NET AJAX Using UpdatePanel's:
At a basic level, Microsoft AJAX supports asynchronous requests via a partial-page-postback. Partial postbacks iterate through the same page lifecycle as a synchronous full page postback, but only specific regions or controls on the page are refreshed - thus achieving partial page rendering. MICROSOFT ASP.NET AJAX is dependent on the interceptor pattern to generate and handle a partial-postback. Upon initialization, MICROSOFT ASP.NET AJAX JavaScript libraries add a set of client event handlers to intercept calls that would normally initiate a full page postback. The handler functions intercept postback calls from registered controls, generate a partial postback, process the response content and then update page content asynchronously. Since MICROSOFT ASP.NET AJAX is built on the existing ASP.NET postback architecture it utilizes event-validation and maintains the view state throughout the partial-postback process. Your standard 'normal' AJAX call doesn't do these things!

To put it shortly...

MICROSOFT ASP.NET AJAX uses 'normal' AJAX to 'ajax-ify' it's pages and achieve partial-updates...and in doing so, it trades & manages view-state MULTIPLE times throughout a single call. This is WHY it is called a 'partial-postback'. Subsequently, this is also why they originally described the UpdatePanel as a means to make your pages 'ajaxy' (because it is not the same thing as using AJAX).

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NORMAL AJAX Calls:
Asynchronous requests initiated using JavaScript in the browser creates a new connection to a server. Yes...this may include stateful postbacks to a page, but also stateless requests to resources apart from the current page. However, in the case of an asynchronous postback , only the information that needs to be processed by the current page on the server is passed INTO the request (and you can control this). Meaning, the contents of the entire page do not need to be submitted, no view-state needs to be managed & the (native) overhead embedded into the page-lifecycle can be bypassed. Meanwhile, an asynchronous postback only calls server-events associated with processing the CURRENT REQUEST. This is why normal AJAX is so much faster than PARTIAL POSTBACKS!

These Points Illustrate...

Not only WHAT a 'partial postback' is...but WHY there is a difference between a 'partial postback' and an 'ajax' call. Which is why this is a better answer.

回答3:

Take a look at:

http://www.codeproject.com/KB/aspnet/PartialPostback.aspx