namespace System
{

  /// <summary>
  /// Encapsulates an event publisher and a handler while still allowing the objects to be garbage collected.
  /// </summary>
  /// <typeparam name="TTarget">The type of the class that declares the handler.</typeparam>
  /// <typeparam name="TEventArgs">The type of <see cref="EventArgs"/> this event handler accepts as an argument.</typeparam>
  public class WeakHandler<TTarget, TEventArgs>
    where TTarget : class, IWeakHandler<TEventArgs>
    where TEventArgs : EventArgs
  {

    private WeakReference _target;

    /// <summary>
    /// Initializes a new object of <see cref="WeakHandler"/>.
    /// </summary>
    /// <param name="target">The object that handles the event.</param>
    public WeakHandler(TTarget target)
    {
      if (target == null) throw new ArgumentNullException("target");

      _target = new WeakReference(target);
    }

    /// <summary>
    /// The method that should actually be subscribed to the event.
    /// </summary>
    /// <param name="sender">The object that publishes the event.</param>
    /// <param name="e">The <see cref="EventArgs"/> to be passed to the event handler.</param>
    public virtual void OnEventRaised(object sender, TEventArgs e)
    {
      if (Target != null)
        Target.OnEventRaised(sender, e);
    }

    /// <summary>
    /// The object that handles the event.
    /// </summary>
    public TTarget Target
    {
      get
      {
        return (TTarget)_target.Target;
      }
    }
  }

  /// <summary>
  /// Defines an event handler of the specified <see cref="EventArgs"/>. 
  /// </summary>
  /// <typeparam name="TEventArgs">The <see cref="EventArgs"/> type of this event.</typeparam>
  public interface IWeakHandler<TEventArgs>
  where TEventArgs : EventArgs
  {
    /// <summary>
    /// The method to be raised when the event is fired.
    /// </summary>
    /// <param name="sender">The object that raised the event.</param>
    /// <param name="e">The <see cref="EventArgs"/> of the event.</param>
    void OnEventRaised(object sender, TEventArgs e);
  }
}