How to allow users to cancel a time consuming WCF service call from Silverlight application? - wcf

Question

In a Silverlight 4.0 (with Prism) application I am maintaining, a page has a Search button which makes WCF service call asynchronously to fetch some data from database, based on the search criteria entered by the user. During the search operation (i.e. from the time Search button is clicked till the time search results are displayed on page), a progress bar window (Silverlight child window) is shown on top of the page (I know it blocks the user for the duration, but this is how all the pages of the application were designed and I cannot change it). Sometimes the Search operation takes long time (several minutes) to return and show the data, and sometimes it shows the results really fast, depending on the search criteria. Now the users want the flexibility to be able to cancel any Search operation if they feel it is taking too much time to show the results.
To address this requirement, I tried the following:
1) Added a Cancel Search button on the Progress-bar window.
2) Added a public static boolean variable (Cancel Search flag, with default value false) in the Progress bar's ViewModel.
3) In the Cancel Search button's click command handler in the same viewmodel, set the Cancel Search flag to true and then closed the Progress bar window.
4) In the asynchronous WCF Serach Service's callback method, first checked if the Progress bar's Cancel Search flag is true. If yes, ignore the results (don't do anything). Otherwise, bound the returned result to the UI as usual.
However, when testing the Cancel Search button, I found that the WCF callback method is getting called before the Cancel button's click command handler, hence the search results are getting displayed even when Cancel Search button is clicked. Obviously this will happen for short running WCF calls which gets executed and completed before the user can click the Cancel button. For long running WCF calls (which gets several minutes to get the results), the above solution may work fine. But while hitting the search button, I do not have any way to know if it is going to be a short-running async call or long running, do I? So is it at all possible to fulfill the requirement in this way or using some other way? Making synchronous WCF call perhaps (but again I gathered it's a big no-no for Silverlight?

Answer 1

When you subscribe to any asynchronous operation from the WCF the WCf sends back the response to the handler with the results.
In your case you have a search button which talks to WCF for result but in case where the wait time is more you want a mechanism to break the operation.
When you click on the cancel button you try to unsubscribe from the event so that you instruct not to listen the callback resulting no results. This can be achieve by using -= operator with yout callback handler.
Hope this helps.

Answer 2

Add a CanExecuteCancelCommand for the Cancel command that checks a boolean variable. When you initiate the call to the WCF service, set this variable to true and call the RaiseCanExecuteChanged method of the command. Equally, as soon as the WCF service returns, set the property to false and call the RaiseCanExecuteChanged method again.
This allows the user to click the button only while you are waiting for the service to return its result.
Something like this:
private bool _canCancelWCFService = false;
DelegateCommand _WCFCommand;
void Method()
{
_WCFCommand = new DelegateCommand(ExecuteWCFService, CanExecuteWCFService);
}
private bool CanExecuteWCFService()
{
return _canCancelWCFService;
}
private void ExecuteWCFService()
{
_canCancelWCFService = true;
_WCFCommand.RaiseCanExecuteChanged();
_wcfService.Completed += new EventHandler<CompletedEventArgs>(CompletedCallback);
//make WCF call here
_wcfService.DoSomething();
}
static void CompletedCallback(object sender, CompletedEventArgs e)
{
_canCancelWCFServiceCall = false;
_WCFCommand.RaiseCanExecuteChanged();
//Process results here...
}