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9: Behavioral Patterns: Iterator, Mediator, and Observer
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Figure 9-5 shows an example of a moderation panel from a mailing list program. The program has trapped a message as potential spam and is asking for confirmation and further action. One of the allowable actions is to automatically reject any future emails from that address.
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Clearly, the moderation process here is performing part of the role of the Mediator pattern: establishing a communication protocol and content filter. The other part keeping members unaware of each other is inherently part of the mailing list concept of mailing to a central address and then broadcasting from there.
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Now, let s consider the Mediator pattern at the UML diagram level. As illustrated in Figure 9-6, it consists of only two classes that use messages to communicate:
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Registers with a Mediator by supplying a Receive method; issues Send requests to other colleagues via the Mediator
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Broadcasts sent messages to all signed-on Colleagues using the Respond delegate
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Colleague mediator +Receive( ) +Send( ) Mediator Respond( ) : Callback +Send( ) +SignOn(Callback)
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send calls mediator Send
Respond calls Receive back
QUIZ
Match the Mediator Pattern Players with the Mailing List Illustration
To test whether you understand the Mediator pattern, cover the lefthand column of the table below and see if you can identify its players and methods among the items from the illustrative example (Figure 9-5), as shown in the righthand column. Then check your answers against the lefthand column.
Colleague Mediator Respond Callback Send Receive
A user subscribed to a mailing list The mailing list moderator Calls back the signed-on users A means of contacting all users on the list Mailing a message to the list Receiving a message from the list
The Mediator pattern makes provisions for more than one mediator. For example, there may be many different mailing lists created under one mailing system. Each list may have a different moderator, different rules of engagement, and a different list of users, but the structure of the lists is identical. Therefore, creating a new Mediator is merely an instantiation operation and does not require subclassing or an interface.
Implementation
Key points of the Mediator pattern are: Each Colleague is passed a Mediator at instantiation and keeps it as a private reference. Each Mediator keeps a list of signed-on Colleagues as a private reference. The callback to the colleagues is a very thin interface just a method. Therefore, a dictionary list (names with callbacks) of Colleagues in the Mediator would be overkill: we can just use the delegate mechanism to chain the Receive methods. On the other hand, within one mediator system, there could be Colleague types that implement the Receive method differently. This variation is shown in the theory code in Example 9-3.
1 2 3 4 5 using System; using System.Collections.Generic; class MediatorPattern {
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9: Behavioral Patterns: Iterator, Mediator, and Observer
6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 // Mediator Pattern Judith Bishop Sept 2007 /* The Mediator maintains a list of colleagues and specifies the communication methods that it can mediate - in this case, Send. Receive is implemented at Colleague level and called via a delegate supplied by the colleagues to the mediator on sign-on. */ class Mediator { public delegate void Callback (string message, string from); Callback respond; public void SignOn (Callback method) { respond += method; } public void Block (Callback method) { respond -=method; } public void Unblock (Callback method) { respond +=method; } // Send is implemented as a broadcast public void Send(string message, string from) { respond(message, from); Console.WriteLine( ); } } class Colleague { Mediator mediator; protected string name; public Colleague(Mediator mediator,string name) { this.mediator = mediator; mediator.SignOn(Receive); this.name = name; } public virtual void Receive(string message, string from) { Console.WriteLine(name +" received from "+from+": " + message); } public void Send(string message) { Console.WriteLine("Send (From "+name+ "): "+message); mediator.Send(message, name); } } class ColleagueB : Colleague { public ColleagueB(Mediator mediator,string name) : base (mediator, name) {
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