Question

I have a specialized UserControl to play media content called PlayerView.
The control has its own commands (readonly, not provided by client).

public partial class PlayerView
{
    public PlayerView()
    {
        InitializeComponent();
        PlayCommand = new RelayCommand(() =>
            {
                // Play some media: audio/video.
            });
    }

    ...

    #region PlayCommand property

    private static readonly DependencyPropertyKey PlayCommandPropertyKey = DependencyProperty.RegisterReadOnly(
      "PlayCommand",
      typeof(ICommand),
      typeof(PlayerView),
      new PropertyMetadata());

    public static readonly DependencyProperty PlayCommandProperty = PlayCommandPropertyKey.DependencyProperty;

    public ICommand PlayCommand
    {
        get { return (ICommand)GetValue(PlayCommandProperty); }
        private set { SetValue(PlayCommandPropertyKey, value); }
    }

    #endregion

    ...
}

The play command of the control works fine from XAML:

<Controls:PlayerView x:Name="PlayerView" />
<Button Command="{Binding ElementName=PlayerView, Path=PlayCommand, Mode=OneWay}" Content="Play" />

But currently, I am implemeting slideshow feature and I would like to execute the play command of the control from the ViewModel.

public class SlideshowViewModel : ViewModelBase
{
    // Stores collection of audio/video clips to be played by the PlayerView.

    // Assume that this ViewModel should invoke PlayerView PlayCommand.
}

public class MainViewModel : ViewModelBase
{
    // Stores a lot of stuff.

    public SlideshowViewModel Slideshow { get; }
}

The question is: how the SlideshowViewModel can execute the PlayCommand of this control? Is there a best practice?

Answer 1

If I am understanding your issue correctly, the ViewModel should contain the implementation of the Command, not the View. This would be a truer MVVM implementation, and then the VM can call that command from within itself, if necessary.

edit:

to answer your question,

public partial class PlayerView : IHaveAPlayCommand
{
public PlayerView()
{
      this.DataContext = new ViewModel(this);
}
}


public class ViewModel
{
      IHaveAPlayCommand view;
      public ViewModel(IHaveAPlayCommand view)
      {
           this.view = view
      }

}