Asynchronous commands

Commands in MVVM are a very precious good. Actually, MVVM can’t exist without them because they allow a developer to bind to a method (that’s actually all an ICommand implementation is). However, sometimes it is required to create asynchronous commands. Starting with Catel 3.1, the AsynchronousCommand is introduced.

With the AsynchronousCommand, it is possible to create a command that executes a method in the background without blocking the UI thread. It is possible to report progress to the UI thread.

Want to use async methods? Use the TaskCommand

Creating the command

The AsynchronousCommand can be used the same as the the regular Command class. The command must be constructed like this:

CancelableAsyncCommand = new AsynchronousCommand(OnCancelableAsyncCommand, () => !CancelableAsyncCommand.IsExecuting);

The second parameter is the CanExecute delegate, and this example does not allow multiple executions of the same command at the same time.

Running the command

Running the command is the same as the regular Command class. The view can simply bind to the command like the code below:

<Button Command="{Binding CancelableAsyncCommand}" Content="Execute Command" />

Canceling the command

Last but not least, the cool thing about the AsynchronousCommand is that it can complete in two ways. It either completes the method by itself, or it is canceled from the outside. To cancel a running command, two steps must be accomplished:

1. Bind to the CancelCommand property of the AsynchronousCommand:

<Button Command="{Binding CancelableAsyncCommand.CancelCommand}" Content="Cancel Command" />

2. Make sure the execute action checks the ShouldCancel property like in the example below:

private void OnCancelableAsyncCommand()
{
    for (var i = 1; i <= 100; i++)
    {
        if (CancelableAsyncCommand.ShouldCancel)
        {
            // If we should cancel, break out of the loop
            break;
        }

        Thread.Sleep(100);
    }
}

Note that this example looks stupid and you should not use Thread.Sleep, but this is just for the sake of simplicity and the example

Reporting progress

During the execution of a command, it is possible to report progress back to the main thread. This can done by using the ReportProgress method. All code inside the ReportProgress will be executed in the UI thread:

private void OnCancelableAsyncCommand()
{
    for (var i = 1; i <= 100; i++)
    {
        if (CancelableAsyncCommand.ShouldCancel)
        {
            // If we should cancel, break out of the loop
            break;
        }

        var i1 = i;
        CancelableAsyncCommand.ReportProgress(() => Messages.Add(i1.ToString(CultureInfo.InvariantCulture)));

        Thread.Sleep(100);
    }
}

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