I'm a newbie to ASP.Net, so my question might be a little bit dumb, I don't quite understand how each stage works in ASP.Net page life cycle, I wrote some code includes a simple page with a label control and a button control on it and found the sequence is :
1.Page initialization event handled.
2.Page load event handled.
3.Page prerender event handled.
4.Page load event handled.
5.Page postback event handled.
6.Button click event handled.
7.Page prerender event handled.
My questions are:
Why some events like page load are raised twice?
People usually said page_load event happens before button click event. I don't quite get it, do people mean page_load event is handled before button click event? if yes then I understand, so is it just something like when we click a submit button, we actually fire 2 events, one for button click event, and one for page load event and page load event is handled first?
Can anyone put these stages in a simple way to explain how each stage steps in? like when a user clicks a button, what's happening behind the scene
Why some events like page load are raised twice?
The PageLoad event is raised once per page request from the server. When an asp.net page is posted back to the server, it's PageLoad event will be called again. If you check your IIS logs, you should see two requests for the page from your browser/client. The first will be the original request for the page, and the second one will be after your button press (which I assume is causing the post back).
People usually said page_load event happens before button click event. I don't quite get it, do people mean page_load event is handled before button click event? if yes then I understand, so is it just something like when we click a submit button, we actually fire 2 events, one for button click event, and one for page load event and page load event is handled first?
In the lifetime of you an asp.net page, it begins when a client requests the page, and ends after the page is rendered to the client. The link sent to you by Andrew Shepherd provides a very good tutorial on the asp.net page life cycle. Be sure you read this and understand it.
Can anyone put these stages in a simple way to explain how each stage steps in? like when a user clicks a button, what's happening behind the scene?
It really depends on what you've got wired up to the button. The button could cause client-side scripting to execute, or it could cause some action to take place on the web server.
Related
I have the following scenario:
UserControlA contains a <asp:Button id="bSomeid" onClick="AddItem" /> with some code to an item to a shopping basket in AddItem.
UserControlB contains some LinkButton's that dynamically add a selection of UserControlA to the page in the OnClick event.
This is all done in an UpdatePanel. It is a little more complicated but I have pruned the information to what I believe is causing the problem, I will add more information if necessary.
The problem I have is that it takes 2 clicks for the AddItem event to trigger after I have added the items to the page after clicking the LinkButton.
I understand why this is happening - it is to late in the page cycle to register events for the next post back in the onclick - but can anyone think of a way around this? Can I force an event to be triggered on the next postback? I have tried to think of a way to run my code in page_load but I requuire access to the sender in the onClick.
Using .NET 4.0.
EDIT
I managed to find a way to get the link button sending the request in the Page_Load (using Request.Form["__EVENTTARGET"];) so I moved my code to the Page_load event. It still requires 2 clicks so I am assuming it isn't something to do with the onClick being registered to late.
Are there any other general things to check that could cause a button to require 2 clicks to post an event properly?
If your suspicion about being late in page life cycle is true then you can try using ScriptManager.RegisterAsyncPostBackControl method to register dynamically added controls in the link button click - considering that your button is within user control, you need to add public method into UserControlA that would actually register the button bSomeid1 and link button click from UserControlB would actually call the A control's method.
EDIT :
Another cause for button click not happening can be that button being dynamic control is not added in the page hierarchy when post-back happens (or it gets added very late in the page life cycle when the post back data is already processed). A really full-proof solution should add dynamic controls back to the page hierarchy in page_load it-self (and strictly maintaining same controls ids within hierarchy). If that's not possible then you can sniff the request (Request.Form) to detect the post-back.
In your case, you should ascertain if the button is indeed causing the post-back on each click. If yes, what is the POST data (Request.Form) for the first request - what is the __EVENTTARGET value on the first click (and post-back)? That should start your trouble-shooting.
On the other hand, a simple work-around could be to use html anchor element (you can still use link button) and have a javascript handler in the click event that would set some hidden variable and then submit the form (you can simulate the click on hidden button to trigger ASP.NET client side submit pipeline) . Now the hidden variable value can be used on the post-back to determine which link button has been clicked.
"Are there any other general things to check that could cause a button to require 2 clicks to post an event properly?"
Does it require two clicks on the control, or does it take accept a single click elsewhere on the screen, and then fire first time with a single click on the control?
I have my own (similar) issue with the Updatepanel where the first (expected) trigger does not fire and it seems that a single click elsewhere, and then the subsequent triggers fires first time (which totals 2 clicks)
[edit] Since you are working on this ATM, it may help me as well. Do you have a textbox with a trigger event on it? I do, and if I leave this blank (so that it does not fire) then there is no need for a second click.
I have page that uses a multiview. Each view contains a separate user control. One of these user controls has a list view with an image button that causes the loading of a different view in the multiview. All is fine up until this point. When the user hits the back button, they are taken back to the user control that contains the list view. The user then clicks on another image button to view different data and it returns to the detail user control using the same data as before. While debugging, I have seen that the item command event does not fire after hitting the back button.
I have tried replacing the multiview and putting each user control into separate panels. This did not change the outcome at all.
I have tried setting a cookie that expires 5 seconds after page load. When the user continues to the next page, then clicks back (and it has been longer than 5 seconds), I force the form to submit again. This loads the next control again instead of reloading the page.
I have tried setting the cacheability to no cache. This causes a "page expired" message and the user has to refresh the page. This is ugly for the user and definitely takes away from the user experience.
I am looking for the cleanest way for a user to click back and have the page reloaded so that the item command event fires correctly again.
The reason is that Back doesn't affect the Page Life Cycle. It's definitely because the page is cached and cached page doesn't execute on server. You can try this code to get rid of this issue.
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.Cache.SetNoStore();
It took a lot of research to find this answer, so hopefully other people stumble upon this question and find my answer. I was astonished that I was actually able to find this. Ok, enough gloating.
Because the page does not postback when the user clicks the back button, the events are not fired correctly causing problems with the next page. What has to happen is you need to be able to handle the browser's navigation buttons (i.e. back and forward). To do this you have to set EnableHistory to true within the script manager and handle the Navigate event from the script manager. You can then reload the controls using the information you save in the state object.
I used these articles from Dino Esposito on DotNetSlackers.com as a reference. Server Side History Management and Client Side History Management
I have a ListView that contains a LinkButton. When I start this webapp and click the LinkButton, the response (call a web service, send data back) is back within a second or so. It's very quick. If I click the link again, or a different link (the listview has a few), it takes almost a minute to get the response back.
I set a breakpoint on the ItemCommand handler and the various methods downstream up to the point I get a response. Best I can tell, I can't set any breakpoints upstream from the ItemCommand, closer to LinkButton being clicked.
It turns out the service and associated method calls function just fine, no performance issues at all. The delay is actually occurring somewhere between clicking the LinkButton and the ItemCommand event handler being called. When I click the LinkButton, it is almost a minute before the breakpoint on the ItemCommand handler is hit. Now that I know WHERE the problem is, how do I find out WHAT is causing the problem?
Is there any way to look at what's going on to cause the delay? I tried using ASP.NET tracing but that doesn't seem to show anything as no postback actually occurs.
UPDATE: If I wait approximately 1 minute between clicking LinkButtons, there is no performance issue. Of course, the downside is I'm waiting a minute between clicks... doesn't really address the issue.
I was going through an article on event bubbling in asp.net and came to know that although it is possible to subscribe to the click event of a user control's button from the containing page, "doing so would break some of the object oriented rules of encapsulation". A better idea is to publish an event in the user control to allow any interested parties to handle the event.
My question is that exactly how does a direct subscription to the button's click event from a containing page would break the object oriented rules of encapsulation?
Apologies if its a dumb question. :|
Thanks!
The Button is supposed to be encapsulated by the UserControl.
If the Page binds directly to events on the button, then the page is now dependent on the inner workings of the UserControl.
The Page should be consuming the UserControl, not the UserControl's button. If the author of the UserControl later wants to remove the button and use some fancy new method of firing its "Submit" event, your page could be broken because the button may no longer exist.
For that matter, if the owner of the UserControl decides in v1.1 to rename the button from btnSubmit to SubmissionButton, it could break your page, as well.
Better to consume the UserControl and let it be concerned with its own inner workings.
The idea is that the button of the control is an implementation detail of the UI of the control. If you republish the click event you could reimplement that button as an ImageButton, LinkButton, etc.
I think it's OK to attach an event handler at the page level to the button if the button is a permanent fixture of the UI. It saves a lot of event code, especially with a lot of buttons.
When loading a page for the first time (!IsPostback), I am creating a button in code and adding it to my page, then adding an event handler to the click event.
However, when clicking the button, after the page reloads, my event handler does not fire.
Can anyone explain why?
#Brad: Your answer isn't complete; he's most likely doing it too late in the page lifecycle, during the Page_Load event.
Okay, here's what you're missing.
ASP.NET is stateless. That means, after your page is rendered and sent to the browser, the page object and everything on it is destroyed. There is no link that remains on the server between that page and what is on the user's browser.
When the user clicks a button, that event is sent back to the server, along with other information, like the hidden viewstate field.
On the server side, ASP.NET determines what page handles the request, and rebuilds the page from scratch. New instances of server controls are created and linked together according to the .aspx page. Once it is reassembled, the postback data is evaluated. The viewstate is used to populate controls, and events are fired.
This all happens in a specific order, called the Page Lifecycle. In order to do more complex things in ASP.NET, such as creating dynamic controls and adding them to the web page at runtime, you MUST understand the page lifecycle.
With your issue, you must create that button every single time that page loads. In addition, you must create that button BEFORE events are fired on the page. Control events fire between Page_Load and Page_LoadComplete.
You want your controls loaded before ViewState information is parsed and added to controls, and before control events fire, so you need to handle the PreInit event and add your button at that point. Again, you must do this EVERY TIME the page is loaded.
One last note; page event handling is a bit odd in ASP.NET because the events are autowired up. Note the Load event handler is called Page_Load...
You need to add the button always not just for non-postbacks.
If you are not reattaching the event handler on every postback, then the event will not exist for the button. You need top make sure the event handler is attached every time the page is refreshed. So, here is the order of events for your page:
Page is created with button and event handler is attached
Button is clicked, causing a postback
On postback, the page_load event skips the attaching of the event handler becaue of your !IsPostback statement
At this point, there is no event handler for the button, so clicking it will not fire your event
That is because the event binding that happens needs to be translated in to HTML. This postback that happens if bound to the page between OnInit and OnLoad. So if you want the button to bind events correclty make sure you do the work in OnInit.
See the Page Life Cycle explaination.
http://msdn.microsoft.com/en-us/library/ms178472.aspx