RelayCommand and RelayCommand<T>

The RelayCommand and RelayCommand<T> are ICommand implementations that can expose a method or delegate to the view. These types act as a way to bind commands between the viewmodel and UI elements.

Platform APIs: RelayCommand, RelayCommand<T>, IRelayCommand, IRelayCommand<T>

How they work

RelayCommand and RelayCommand<T> have the following main features:

• They provide a base implementation of the ICommand interface.
• They also implement the IRelayCommand (and IRelayCommand<T>) interface, which exposes a NotifyCanExecuteChanged method to raise the CanExecuteChanged event.
• They expose constructors taking delegates like Action and Func<T>, which allow the wrapping of standard methods and lambda expressions.

Working with ICommand

The following shows how to set up a simple command:

public class MyViewModel : ObservableObject
{
    public MyViewModel()
    {
        IncrementCounterCommand = new RelayCommand(IncrementCounter);
    }

    private int counter;

    public int Counter
    {
        get => counter;
        private set => SetProperty(ref counter, value);
    }

    public ICommand IncrementCounterCommand { get; }

    private void IncrementCounter() => Counter++;
}

And the relative UI could then be (using WinUI XAML):

<Page
    x:Class="MyApp.Views.MyPage"
    xmlns:viewModels="using:MyApp.ViewModels">
    <Page.DataContext>
        <viewModels:MyViewModel x:Name="ViewModel"/>
    </Page.DataContext>

    <StackPanel Spacing="8">
        <TextBlock Text="{x:Bind ViewModel.Counter, Mode=OneWay}"/>
        <Button
            Content="Click me!"
            Command="{x:Bind ViewModel.IncrementCounterCommand}"/>
    </StackPanel>
</Page>

The Button binds to the ICommand in the viewmodel, which wraps the private IncrementCounter method. The TextBlock displays the value of the Counter property and is updated every time the property value changes.

Examples

• Check out the sample app (for multiple UI frameworks) to see the MVVM Toolkit in action.
• You can also find more examples in the unit tests.