AsyncCommand

Async delegate command

Class AsyncCommand is an implementation of asynchronous delegate command with handy bindabale properties.

Creating

The following example demonstrates how to create a command:

IAsyncCommand UpdateWeatherCommand = new AsyncCommand(
    async () =>
    {
        // Call async method of the external service.
        var weather = await SeatherService.GetCurrentWeatherAsync();

        // Update property bound to the UI control.
        Temperature = weather.Temperature;
        Wind = weather.Wind;
    },
    // Optinally it is possible to set a condition when the command can be executed.
    // In the example: forbid execution until loading of cities is completed.
    parameter => !LoadCitiesCommand.IsExecuting());

There are several constructor overloads to create the command that accepts a parameter and/or cancellation token or not. For example, the command with cancellation token:

IAsyncCommand UpdateWeatherCommand = new AsyncCommand(
    async (token) =>
    {
        // ...
    });

Binding

The command can be bound to the UI (e.g. Button control) as usual delegate command. Additionally, the Execution property can be bound to extend the reaction of UI to the command execution process. It is null before the first execution, and after it contains the execution details.

The following example demonstrates how to hide some control while command is executing:

Visibility="{Binding MyCommand.Execution.IsNotCompleted, Converter={StaticResource BoolToVisibilityConverter}}"

The Execution can be used to get to know is command completed ot not, was it cancelled or failed, the failure exception and its message. All properties can be bound to UI.

Also AsyncCommand has a property CancelCommand - a special ready for binding command that cancels the execution. In order to use it, the command should be created with the method that accepts a cancellation token.

Usage in the view model code

All properties of the AsyncCommand, of course, can be used in the code of the view model. Note, that command doesn't support returning result, because the main usage of it is binding to the UI. But it can be executed from code and contains two methods for this, Execute and ExecuteAsync, synchronous and asynchronous versions respectively.

Additional method IsExecuting() can be used to check the command state (e.g. in the other command predicate like in the example above). It takes into account that Execution can be null, so no need to check it.