can you define autopostback and postback and it's differences and which web server control support autopostback and postback and also the difference between postback and callback?
postback: the page performs a POST request to itself
autopostback: a postback is performed automatically, for example when the selected value of a dropdown changes
callback: a method which is invoked when a certain event occurs such as clicking on a button. It also could have another meaning that a page performs a request back to the server (such as an AJAX request).
A postback is initiated by the browser, and reloads the whole page, usually when a control on the page (e.g. a button) is changed.
With some controls (e.g. Checkboxes), you choose if changing the control should result in a postback. This property is called AutoPostback.
A callback is initiated by java script in the page, and can load/update parts of the page, e.g. by manipulating the DOM.
Controls that support AutoPostback includes:
List item
ListControl
BulletedList
CheckBoxList
DropDownList
ListBox
RadioButtonList
CheckBox
TextBox
PostBack happens when data is send to the server (he page performs a POST request to itself)
IsPostBack helps you to identify if postback happen or not
Autopostback if this property of control is true helps to you post as soon as data change on the contol or some event occur
callback is a special postback, so a round-trip always occurs; however, unlike the classic postback, the script callback doesn't redraw the whole page. ViewState is not updated during a callback, it is for postback.
Related
I have a webforms page with a gridview and several other buttons. In debugging, I've noticed all the binding to gridview executes during postback in the Page_Load sub. Only after this is all done do the click handler(s) get invoked and in my case the handler does a Response.Redirect to another page.
So, I found this on SO: Run the button event handler before page_load.
It suggests the possibility of detecting the target of the postback during page_load and I was thinking of exiting the page_load in this case and letting the button click handler load a new page (i.e. avoiding all the wasted building and formatting of the gridview.
Here is a snippet from my Page_Load:
If IsPostBack Then
Dim targetOfPostBack As String = Request.Params("__EVENTTARGET").ToString()
End If
During PostBack the __EVENTTARGET is always an empty string rather than something suggesting the "add" button I clicked (i.e. ctl00$MainContentPlaceHolder$btnAddEvent). Trapped here:
The OP on SO above is expressing this same approach in C# while my implementation is VB.Net. What am I doing wrong?
On your asp button try setting UseSubmitBehavior="False"
Use the UseSubmitBehavior property to specify whether a Button control uses the client browser's submit mechanism or the ASP.NET postback mechanism. By default the value of this property is true, causing the Button control to use the browser's submit mechanism. If you specify false, the ASP.NET page framework adds client-side script to the page to post the form to the server.
When the UseSubmitBehavior property is false, control developers can use the GetPostBackEventReference method to return the client postback event for the Button. The string returned by the GetPostBackEventReference method contains the text of the client-side function call and can be inserted into a client-side event handler.
http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.button.usesubmitbehavior(v=vs.110).aspx
You can write a client side script to redirect the page thus avoiding the postback all together. Set "OnClientClick" (Asp.NET Button) or "OnClick" (HTML Buttom) property and then simply return false.
Eg:
Script:
function RedirectToURL()
try {
window.open("<Your URL Here>", "_self", "menubar=0,resizable=
} catch (e) {
}
return flase;
}
Button :
<input type="button" id="btnToggleConfig" onclick="RedirectToURL()" runat="server"
value="Click here to Goto Link" />
This is the ASP.NET page life cycle at work: http://msdn.microsoft.com/en-us/library/ms178472(v=vs.85).aspx
A few options:
check out this post for _EVENTTARGET being empty:
__EVENTTARGET is empty on postback of button click
Handle the redirect on the client side (if possible)
Only fill the gridview only once on page_load (If Not IsPostback) if that's an option
If you're only worried about the gridview being recreated and eating up some 'processing power' or time, depending on size, users, etc, may not be a big issue, so could leave as-is.
I am reading the textbook "Sams ASP.NET 4 unleashed" and I am confused about the Difference between TextChanged property and AutoPostBack property with respect to ASP.NET TextBox control.
The book explains the two properties as :
AutoPostBack—Enables you to post the form containing the TextBox back
to the server automatically when the contents of the TextBox is
changed.
then,
TextChanged—Raised on the server when the contents of the text box are
changed.
Then it continues to explain :
When the AutoPostBack property has the value True, the form containing
the TextBox is automatically posted back to the server when the
contents of the TextBox changes. If you modify the contents of the
text box and tab out of the TextBox control, the form is automatically
posted back to the server, and the contents of the TextBox display.
The line that confused me is this one :
You can handle TextChanged event even when you don’t use the
AutoPostBack property.
Well, what is the difference between Autopostback property and textchanged property of both cause the page to be posted again to the server? Or is it that the working of both the methods differ from each other? I am confused..please help me understand it better.
You can handle TextChanged event even when you don’t use the AutoPostBack property.
Yes, that's true. You can handle the TextChanged event on the next roundtrip to the server even if it doesn't post back immediately.
So when you handle the event and AutoPostBack is set to true, the postback will happen immediately and the TextChanged-event is raised.
When AutoPostBack is set to false, you can handle the event anyway on the next postback(f.e. a button-click)
what is the difference between Autopostback property and textchanged property of both cause the page to be posted again to the server?
So the bold printed is simply wrong.
The difference is that the one controls the postback-behaviour and the other registers an event handler.
TextChanged will be raised when postback is completed and directed to self page. After Load cycle this event is raised. Which means that TextChanged is raising between two postbacks.
AutoPostBack property is activating postback, when content of textbox is changed and focused out from this textbox. After that postback occurs, page is directed to self. Then again after Load cycle TextChanged event is raised.
Also TextChanged is an 'event' not property.
I have a dropdownlist, in the client side, i have it's on change event. If validation is passed, it's selected inded changed event should be fired(server side). My side, the server event is not getting fired. Autopostback is also set is true.
Any suggestion ?
If you are adding the code from the code-behind (as opposed to in the ASPX or ASCX markup), make sure you add it in the Page_Init event or override CreateChildControls. If you want until the Page_Load event to add it, the ASP.NET has already initialized the control state and view state and then will not be aware that the dropdown exists, so when the postback comes it won't know what control to route it to.
Also, in this case, make sure you are always adding the control to the page, not just when Page.IsPostback==false
Make sure your page code is in autoeventwireup = true.
you can check it in your page design page on top.
if autoeventwireup is set to false your event doesn't fire. auto event will set automatically event so it is necessary to write else you have to set event on initialize state.
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