Listener design pattern (also known as Subscriber/Observer) is a 'well-proven' technique for asynchronous message passing. It is used when the client needs to be notified when 'something interesting' happens to another object (of class X) in a passive manner (i.e. the client is not checking every so often (pulling) in the state has changed).
The Recipe
There are 4 simple steps in order to implement the listener design pattern:
- Create a listener (to observer) interface (
IXListener) which contains appropriate notification method (e.g.Notify(),setText) - Add a list/array that holds references to attached listeners to class
X - Add Methods to
Xso that clients can attach/detach listeners (objects that confirm to theIXListenerinterface) - When the interesting event happens in
X, notify all attached listeners
Code
There are various ways to implement the functionality. This is a .NET (a'la) Java design pattern. In .NET you can also use Events.
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interface IXListener { void Notify(); } class X { List<IXListener> Clients = new List<IXListener>(); public void Attach(IXListener client) { this.Clients.Add(client); } public void Detach(IXListener client) { this.Clients.Remove(client); } public void InterestingEvent() { Clients.ForEach(x => x.Notify()); } } class Client1 : IXListener { public void Notify() { // TO DO } } class Client2 : IXListener { public void Notify() { // TO DO } } |