Writing an asynchronous RelayCommand implementing ICommand

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If you are familiar with MVVM, then you are already familiar with relay commands. We use relay commands to map a method with the ICommand raised from a control. Below is the typical code snippet you may have seen a lot of time.

The class below is pretty simple; it accepts a method and allows you to associate it with a command. Whenever the command is invoked, it is relayed to the method we have passed on as “execute” param. It acts like a disconnected event, if I may say.

public class RelayCommand : ICommand { private readonly Action executedMethod; private readonly Func<T, bool> canExecuteMethod;  public event EventHandler CanExecuteChanged; public RelayCommand(Action execute) : this(execute, null) { }  public RelayCommand(Action execute, Func<T, bool> canExecute) { this.executedMethod = execute ?? throw new ArgumentNullException("execute"); this.canExecuteMethod = canExecute; }  public bool CanExecute(object parameter) => this.canExecuteMethod == null || this.canExecuteMethod((T)parameter); public void Execute(object parameter) => this.executedMethod((T)parameter); public void OnCanExecuteChanged() => CanExecuteChanged?.Invoke(this, EventArgs.Empty); }

Since Action deals with void functions, we cannot get the async methods working. So I resorted for Func instead of Action and rewrote my RelayCommand to below.

public class RelayCommandAsync : ICommand { private readonly Func<T, Task> executedMethod; private readonly Func<T, bool> canExecuteMethod;  public event EventHandler CanExecuteChanged; public RelayCommandAsync(Func<T, Task> execute) : this(execute, null) { }  public RelayCommandAsync(Func<T, Task> execute, Func<T, bool> canExecute) { this.executedMethod = execute ?? throw new ArgumentNullException("execute"); this.canExecuteMethod = canExecute; }  public bool CanExecute(object parameter) => this.canExecuteMethod == null || this.canExecuteMethod((T)parameter); public async void Execute(object parameter) => await this.executedMethod((T)parameter); public void OnCanExecuteChanged() => CanExecuteChanged?.Invoke(this, EventArgs.Empty); }

Comments

• Anonymous
  July 28, 2017
  In our product, we had to have a less naive implementation : - as it is asynchronous, if the user spams the button, you don't want to trigger multiple request... So you need to monitor when the async function is started and when it is done.- as now the async function can do something long in the background, the user need to be notified about this.We've upgrade the ICommand to IAsyncCommand :public interface IAsyncCommand : ICommand, INotifyPropertyChanged { bool BackgroundOperationRunning { get; set; }}in that case, we can use the BackgroundOperationRunning to theme the button to have some animation while the async callback is running. The RelayCommandAsync is more complicated but it allows us a nice user experience