What is the difference between a callback and a postback?
A 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.
How to make a callback?
In the client side JavaScript code, if GetCallbackEventReference() method is reference, then when the JavaScript code is executed, a channel to the server is opened and an HTTP request is sent to the remote ASP.NET page.
How does the ASP.NET runtime know this HTTP request is a Callback rather than a Postback?
After the ASP.NET runtime get a HTTP request, it looks for a __CALLBACKID entry in the Request collection. If such an entry is found, the runtime concludes that a callback invocation is being made.
GetCallbackEventReference() syntax
public string GetCallbackEventReference (
string target,
string argument,
string clientCallback,
string context,
string clientErrorCallback,
bool useAsync
)
there are overloads such as
ClientScriptManager.GetCallbackEventReference (Control, String, String, String)
ClientScriptManager.GetCallbackEventReference (Control, String, String, String, Boolean)
GetCallbackEventReference() Parameters
target
The name of a server Control that handles the client callback. The control must implement the ICallbackEventHandler interface and provide a RaiseCallbackEvent method (which could be the page itself).
argument
An argument passed from the client script to the server RaiseCallbackEvent method.
clientCallback
The name of the client event handler that receives the result of the successful server event.
context
Client script that is evaluated on the client prior to initiating the callback. The result of the script is passed back to the client event handler.
clientErrorCallback
The name of the client event handler that receives the result when an error occurs in the server event handler.
useAsync
true to perform the callback asynchronously; false to perform the callback synchronously.