Postback handlers
A postback handler is a mechanism which allows to intercept commands or static commands on any control, and execute any custom logic before the command or static command is actually performed. A postback handler can decide to cancel the action completely, it can access the response from the server, and it can handle any errors that may occur.
ConfirmPostBackHandler
Imagine you have a button in your page which removes some data. You will probably want to display a confirmation dialog before the data are actually deleted. This is exactly the use case for postback handlers.
You can attach a postback handler to any control using the PostBack.Handlers property. You have to declare it as an inner element, because it is a collection of objects.
<dot:Button Text="Submit" Click="{command: Submit()}">
<PostBack.Handlers>
<dot:ConfirmPostBackHandler Message="Do you really want to submit the form?" />
</PostBack.Handlers>
</dot:Button>
The ConfirmPostBackHandler is a built-in handler which displays the default JavaScript confirmation box. In the example, this handler is applied to all events on the Button control. If the user declines the confirmation, the postback won't be initiated.
SuppressPostBackHandler
<dot:SuppressPostBackHandler Suppress={value: !CommandAllowed} /> will cancel any postback on the component if the Suppress condition is true.
Note that the condition is only evaluated client-side, so this handler cannot be used as a security mechanism.
Restrict postback handlers to specific events
If the control has multiple events with command bindings and you need to apply the handler only on a specific event, you can use the EventName property.
If the EventName property is not specified, the handler is applied to all events the control can make.
<dot:GridView ... SortChanged="{command: SortChanged}">
<PostBack.Handlers>
<dot:ConfirmPostBackHandler EventName="SortChanged"
Message="Do you really want to sort the table?" />
</PostBack.Handlers>
...
</dot:GridView>
Combine postback handlers
You can add multiple handlers in the collection. They will chain in the exact same order they were specified.
Write custom postback handlers
The postback handler mechanism is extensible. You can find more information in the custom postback handlers chapter.