Question

Abstracting commands into the View Model is a valuable practice with XAML/MVVM projects. I get that. And, I see ICommand in in WinRT; but, how do we implement it? I haven’t found a sample that actually works. Anyone know?

Answer 1

My all time favorite has to be the DelegateCommand provided by the Microsoft Patterns and Practices team. It allows you to create a typed command:

MyCommand = new DelegateCommand<MyEntity>(OnExecute);
...
private void OnExecute(MyEntity entity)
{...}

It also provides a way to raise the CanExecuteChanged event (to disable/enable the command)

MyCommand.RaiseCanExecuteChanged();

Here’s the code:

public class DelegateCommand<T> : ICommand
{
    private readonly Func<T, bool> _canExecuteMethod;
    private readonly Action<T> _executeMethod;

    #region Constructors

    public DelegateCommand(Action<T> executeMethod)
        : this(executeMethod, null)
    {
    }

    public DelegateCommand(Action<T> executeMethod, Func<T, bool> canExecuteMethod)
    {
        _executeMethod = executeMethod;
        _canExecuteMethod = canExecuteMethod;
    }

    #endregion Constructors

    #region ICommand Members

    public event EventHandler CanExecuteChanged;

    bool ICommand.CanExecute(object parameter)
    {
        try
        {
            return CanExecute((T)parameter);
        }
        catch { return false; }
    }

    void ICommand.Execute(object parameter)
    {
        Execute((T)parameter);
    }

    #endregion ICommand Members

    #region Public Methods

    public bool CanExecute(T parameter)
    {
        return ((_canExecuteMethod == null) || _canExecuteMethod(parameter));
    }

    public void Execute(T parameter)
    {
        if (_executeMethod != null)
        {
            _executeMethod(parameter);
        }
    }

    public void RaiseCanExecuteChanged()
    {
        OnCanExecuteChanged(EventArgs.Empty);
    }

    #endregion Public Methods

    #region Protected Methods

    protected virtual void OnCanExecuteChanged(EventArgs e)
    {
        var handler = CanExecuteChanged;
        if (handler != null)
        {
            handler(this, e);
        }
    }

    #endregion Protected Methods
}