Mediator Pattern

Motivations

• lots of classes in the design of framework• If certain principles are not applied the final

framework will end in a total mess • to avoid tight coupled frameworks, we need a

mechanism to facilitate the interaction between objects in a manner in that objects are not aware of the existence of other objects.

Mediator

Mediator promotes loose coupling by keeping objects from referring to each other explicitly

Intent

• Define an object that encapsulates how a set of objects interact.

• Design an intermediary to decouple many peers

• Promote the many-to-many relationships between interacting peers to “full object status”.

Check list

• Identify a collection of interacting objects that would benefit from mutual decoupling.

• Encapsulate those interactions in the abstraction of a new class.

• Create an instance of that new class and rework all “peer” objects to interact with the Mediator only.

• Balance the principle of decoupling with the principle of distributing responsibility evenly.

• Be careful not to create a “controller” or “god” object.

Mediator design example

• Let us consider design of a class in which one object send messages to other objects by means of if then if condition explicitly

public class WorkerObject { private string _message; public WorkerObject(string message) { _message = message; } public string Message { get{return _message;} set{_message = value; } public SendMessage(string message) { Console.WriteLine("Message sent : " + message); } }

WorkerObject senderObject = new WorkerObject("message0"); WorkerObject workerObject1 = new WorkerObject("message1"); WorkerObject workerObject2 = new WorkerObject("message2"); WorkerObject workerObject3 = new WorkerObject("message3"); if(!workerObject1.Message.Equals(senderObject.Message) { workerObject1.SendMessage(senderObject.Message); } if(!workerObject2.Message.Equals(senderObject.Message) { workerObject2.SendMessage(senderObject.Message); } if(!workerObject3.Message.Equals(senderObject.Message) { workerObject3.SendMessage(senderObject.Message); }

Mediator

• we create a mediator class, DoSomeMediation.

• This class contains two methods Register and SendMessage

• The Register method catalogs all the classes we want to mediate between.

• The SendMessage method is where the functional code actually exists

public class DoSomeMediation { private static ArrayList _workerObjects = new ArrayList();

public static int Register(WorkerObject workerObject) {

return _workerObjects.Add(workerObject); } public static void SendMessage(WorkerObject senderObject) { if(senderObject == null) return; string messageToSend = senderObject.Message; foreach(WorkerObject workerObject in _workerObjects)

{

//send message to all other objects registered if(!workerObject.Message.Equals(senderObject.Message))

workerObject.SendMessage(messageToSend); }

} }

WorkerObject senderObject = new WorkerObject("message0"); WorkerObject workerObject1 = new WorkerObject("message1"); WorkerObject workerObject2 = new WorkerObject("message2");

WorkerObject workerObject3 = new WorkerObject("message3"); DoSomeMediation.Register(senderObject); DoSomeMediation.Register(workerObject1); DoSomeMediation.Register(workerObject2); DoSomeMediation.Register(workerObject3); DoSomeMediation.SendMessage(senderObject);

Flow diagram

WorkerObj1

Worker Obj2

SenderObj

CCS Model

• SenderObject(reg,sendmedmsg,sendobjmsg,printmsg)= reg.sendmedmsg’.SenderObject<reg,sendmedmsg,sendobjmsg,printmsg>

• Mediator(sendmedmsg,sendobj1msg,sendobj2msg)= sendmedmsg.sendobj1’msg.Mediator<sendmedmsg,sendobj1msg,sendobj2msg> + sendmedmsg.sendobj2msg.Mediator<sendmedmsg,sendobj1msg,sendobj2msg>

• Workerobj1(reg,sendmedmsg,sendobjmsg,printmsg)=sendobjmsg.printmsg’.Workerobj1<reg,sendmedmsg,sendobjmsg,printmsg>

• Workerobj2(reg,sendmedmsg,sendobjmsg,printmsg)=sendobjmsg.printmsg’.Workerobj2<reg,sendmedmsg,sendobjmsg,printmsg>

Simulation on MWB