/// <summary>
/// Subscribes the given callback on the event. The callback will be invoked every time the event
/// is raised. This method establishes a weak reference between the event source and the object
/// holding the supplied callback, aka subscriber. That means that the subscription is kept alive
/// only as long as both event source and subscriber are kept in memory via strong references from
/// other objects. The event source alone doesn't keep the subscriber in memory. You have to keep a
/// strong reference to the returned subscription object to achieve this. The subscription can be
/// canceled at any time by disposing the returned subscription object. Otherwise, the subscription
/// is automatically canceled if the subscriber is being garbage collected. For this to happen no
/// other strong reference to the returned subscription must exist.
/// </summary>
///
/// <param name="action">
/// The callback to subscribe on the event.
/// </param>
///
/// <returns>
/// An object that represents the newly created subscription. Disposing this object will cancel the
/// subscription and remove the callback from the event source's subscription list.
/// </returns>
///
/// <exception cref="ArgumentNullException">
/// A null reference was passed to a method that did not accept it as a valid argument.
/// </exception>
public readonly IDisposable SubscribeWeak(Action <T> action)
{
if (action == null)
{
throw new ArgumentNullException(nameof(action));
}
if (mSource == null)
{
return(NullSubscription.Instance);
}
return(mSource.AddWeak(action));
}