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CHAPTER 10 FUNCTIONAL ROLES
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BuilderBinder
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+Build() +Bind()
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coordinator
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worker1
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worker2
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Coordinator +DoSomething()
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Figure 10-14. A coordinator that synchronizes two workers
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Coordinator DoSomething()
Create()
Thread
Worker1
Worker2
Start(worker1.Method1)
Create()
Method1()
Start(worker2.Method1)
Method1()
Wait for Workers() Message1()
Figure 10-15. Using a coordinator to run workers concurrently The Coordinator manages the workers using event notifications, so the Coordinator class is not coupled to the Worker classes in any way. At run time, the Binder gives the Coordinator references to the Worker methods that need to be run. The Coordinator then calls these methods directly or indirectly, with a Thread.Start call. The Coordinator doesn t know what objects or methods are being invoked. Listing 10-9 and Listing 10-10 show C# and VB .NET implementations of the complete system. Listing 10-9. A C# Implementation of the Coordinator Team public class BuilderBinder { public Coordinator coordinator; public Worker1 worker1; public Worker2 worker2; public void Build() { coordinator = new Coordinator(); worker1 = new Worker1(); worker2 = new Worker2(); }
CHAPTER 10 FUNCTIONAL ROLES
public void Bind() { coordinator.OnWorker1Method1 = new Coordinator.Method1Handler(worker1.Method1); coordinator.OnWorker2Method1 = new Coordinator.Method1Handler(worker2.Method1); coordinator.OnWorker2Method2 += new Coordinator.Method2Handler(worker2.Method2); } } public class Coordinator { Thread threadWorker1, threadWorker2; public void DoSomething() { threadWorker1 = new Thread(new ThreadStart(OnWorker1Method1) ); threadWorker2 = new Thread(new ThreadStart(OnWorker2Method1) ); threadWorker1.Start(); threadWorker2.Start(); threadWorker1.Join(); threadWorker2.Join(); FireWorker2Method2(); } public delegate void Method1Handler(); public Method1Handler OnWorker1Method1; public Method1Handler OnWorker2Method1; public delegate void Method2Handler(); public event Method2Handler OnWorker2Method2; void FireWorker2Method2() { if (OnWorker2Method2 != null) OnWorker2Method2(); } } public class Worker1 { public void Method1() {/*...*/} } public class Worker2 { public void Method1() {/*...*/} public void Method2() {/*...*/} }
CHAPTER 10 FUNCTIONAL ROLES
Listing 10-10. A VB .NET Implementation of the Coordinator Team Public Class BuilderBinder Public _coordinator As Coordinator Public _worker1 As Worker1 Public _worker2 As Worker2 Public Sub Build() _coordinator = New Coordinator _worker1 = New Worker1 _worker2 = New Worker2 End Sub Public Sub Bind() _coordinator.OnWorker1Method1 = AddressOf _worker1.Method1 _coordinator.OnWorker2Method1 = AddressOf _worker2.Method1 AddHandler _coordinator.OnWorker2Method2, AddressOf _worker2.Method2 End Sub End Class Public Class Coordinator Private threadWorker1, threadWorker2 As Thread Public Sub DoSomething() threadWorker1 = New Thread(OnWorker1Method1) threadWorker2 = New Thread(OnWorker2Method1) threadWorker1.Start() threadWorker2.Start() threadWorker1.Join() threadWorker2.Join() FireWorker2Method2() End Sub Public OnWorker1Method1 As ThreadStart Public OnWorker2Method1 As ThreadStart Public Event OnWorker2Method2() Sub FireWorker2Method2() RaiseEvent OnWorker2Method2() End Sub End Class Public Class Worker1 Public Sub Method1() '... End Sub End Class
CHAPTER 10 FUNCTIONAL ROLES
Public Class Worker2 Public Sub Method1() '... End Sub Public Sub Method2() '... End Sub End Class By relegating all the worker-synchronization logic in the Coordinator, the workers are simple, with a very high degree of coherence. The design introduces no coupling between the Coordinator and Worker classes. At run time, there is signature coupling between the Coordinator instance and the two Worker instances. You could use the Coordinator, without changes, to control entirely different kinds of workers. Figure 10-16 shows the coupling diagram of the system.
Coordinator
BuilderBinder
Worker1
Worker2
Figure 10-16. The coupling diagram of the system
Leveraging Multiple Workers to Achieve an Objective
Using a coordinator to manage the various workers makes it possible to achieve a collaborative effort by workers without the individual workers knowing anything about each other. Splitting a task into subtasks, handled by different workers, is often an effective design strategy, because each worker is simpler and easier to develop and test. When a Coordinator uses multiple workers as a team, a high-level command to the Coordinator generally results in a series of lower-level commands to the workers, as shown in Figure 10-17. Any number of workers can make up a team. The Coordinator is responsible for calling the right Worker method at the right time. The Coordinator also has full control over which thread a Worker call is made on.
CHAPTER 10 FUNCTIONAL ROLES
Coordinator
High-Level Command()
Worker1
Worker2
Command1()
Command2()
Command3()
Figure 10-17. Workers collaborating to support a complex task
Mediating Communication Between Workers
When workers collaborate to achieve an objective, some workers may need to fire events to other workers directly. For example, assume a system has two workers, A and B, and that A needs a service provided by B. A problem arises when B provides the service not as a single operation, but rather with a series of smaller-grained operations M1, M2, and M3. To achieve the desired result, A would have to call those three methods, and in a specific order. Assuming Worker A uses a signal N1 to request B s service, you have a problem: You can t bind N1 to B directly, because B doesn t expose a single method that provides the entire service needed, as shown in Figure 10-18.
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