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
Related
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.
I have a page that creates a list, a button and a textfield dynamically in the Page_Load handler. When you click the button, the text from the textbox is added to the list and everything is re-created by clearing the controls collection of the page and then re-adding all required controls.
So basically, the button gets created two times for every Postback (first in the Page_Load, then in the Button_Click handler).
This only works on the first PostBack. If you try it a second time, the Button_Click handler never gets called (although the dynamic controls get re-created on PostBack).
What am I missing here? Is it possible that the Button_Click event is still wired-up to the first button (the one that was thrown away via Controls.Clear() )?
Page_Load is too late to create the button; it should be in Page init or preinit. If you readd the button, after clearing it, you have to readd the event handler too.
I have finally figured out what the cause of the problem is: when the controls are created more than once, the Event-Handlers (e.g. Button.Click) are also registered more than once. I don't know why, but this causes all of them to stop working. In other words, the events of the disposed Buttons somehow interfer with the events of the actual Buttons.
I solved this by ensuring that the handlers are only registered once (in the Page_Load).
I have two radio button on my asp.net page, with AutoPostBack = True
When I click on either of those, they each set a flag SaveData=False
However, when I click on them, the page_load event occurs first, so the page_load event saves the data, then the radiobutton_OnCheckedChanged event is triggered.
How can I trigger the OnCheckedChanged before the page_load event?
You can't.
You need to read up on the page lifecycle.
You can't trigger the OnCheckedChanged before radiobutton_OnCheckedChanged because that's the normal page life cycle.
You can check in Page_Load if Page.IsPostBack is true or not, and then don't save (or save) the data.
Troy - you will have to take care of this on the client side (javascript / jquery).
Your AutoPostBack = True causes the form to do a postback which calls page load.
Its all about how the page lifecycle and server side events work.
You can't the page always needs to load before events can be fired. This has to do with control creation, view state management, control state, etc.
What you want to do in your Page_Load is something like:
if(!this.IsPostBack)
{
// Do the stuff you want on the initial page load
}
Anything that you do not want to happen when the radio buttons are clicked, put it inside the if {} block. IsPostBack indicates that the page is processing a post-back event, which is what happens when you click one of your radio buttons.
You can't change the order/sequence of event handler execution. However you may move the code inside the page_load to another event handler.
My ASP.NET form contains a collection of dynamically-created radiobuttons that are created and configured in the Page_Load event handler.
Normally, I process postback data in the Page_Load handler, using the condition:
if (IsPostBack)
However, since the controls that I need to access are created IN the Page_Load handler, the postback data from the previous rendering of the page is lost. To better illustrate the problem, here is an outline of the events as they occur:
1-Page_Load is invoked for the first time
2-An unknown number of radiobuttons are created dynamically
3-The radiobuttons are configured, based on information present on the server
4-The radiobuttons are added to the page's content
5-The user selects an option, and clicks the submit button
6-The Page_Load handler is invoked for the second time
7-The radio-buttons are added dynamically, exactly as before
8-The radio-button that the user checked is seemingly non-existant for processing
It seems like I need to be processing different parts of this in different event handlers. Is there an event that occurs after postback, but while the original radiobuttons are still accessible?
Check out the "Init" events...
http://msdn.microsoft.com/en-us/library/ms178472.aspx
In my web application I load user controls in the page_loadComplete event. This works fine, however when a button is clicked in a user control, the click event is never fired. Does this has something to do with the page lifecycle? That button click (UI) events occur before the LoadComplete event?
You need to make sure the click event of the button is once again subscribed to, before event handlers fire. LoadComplete happens after control events. For a reference, the ASP.NET Page Life Cycle Overview gives a pretty nice summary.
Snippet:
...
Load
Control events
LoadComplete
PreRender
...
You also need to make sure that the controls that you dynamically load all end up in the same place, so viewstate and controlstate can be reapplied to the same hierarchy as before postback.
Basically, you need to load all dynamic controls on each postback.
Here's someone with the same problem, and solutions to some of them: ASP.NET dynamic controls
Actually what is happening to your situation is, when you click on the button, before event raising, LoadCoplete event fire first in the page lifecycle and same control is again created and here is your event is lost.
Event Handling in ASP.NET page happens after Validation and before Rendering phase. And Validation phase happens after Load.
LoadComplete happens after Control events and before RreRender event.