Dr. Dobb's Journal - July 2008 - (Page 23)

Figure 1: Two classes that interact using notifications. to a typed object, and calls one of the object’s methods. • Untyped calls. An untyped call is a call to a method of an object whose type is unknown. The caller knows only the signature of the method to be called, and knows nothing about the object to which the method belongs. Languages such as Java and C++ natively support typed calls. Languages such as C#, VB.NET, and Delphi support both typed and untyped calls. Java programmers can also use untyped calls, but need to resort to reflection and Method.invoke() to do so. In Figure 1, assume you have two classes A and B, which interact using notifications. When some event occurs in A, you want B.DoSomething() to be executed. To achieve this, A will have to send a notification to B. To send notifications using typed calls, class A needs to contain a field referencing type B. At runtime, this reference is somehow set; for this example, it doesn’t matter how or when. When firing the event, class A invokes the method B.DoSomething() on the referenced object using code of the form: if (referenceToB != null) referenceToB.DoSomething(); DoSomething of some instance of class B. Class A would then use this code to fire an event: if (myListener != null) myListener(); To send notifications using untyped calls, class A needs to have a field referencing not type B, but a method with the signature of B.DoSomething(). In .NET languages, delegates are language-defined entities that can point to methods with a given signature. In C#, class A might have a delegate defined like this: public delegate void SomeHandler(); In Java, the same thing can be accomplished using reflection with Method.invoke(). Class A would need to have a field of type Method. At runtime, class A would need to initialize the Method field to point to method DoSomething() of some instance of B. Class A would then use the Method field to invoke B.DoSomething(). Typed and untyped calls achieve the same net result (invoking DoSomething() on an instance of class B), but with important differences—with typed calls, class A must know about type B, so A must be created with the knowledge of B. Also, any objects that wish to handle notifications from A need to be of type B, or derived from B. With untyped calls, A does need to know about type B. Objects wishing to handle notifications from A don’t need to be of any special type. How important this distinction is usually depends on the size of the system you’re building, and whether you have control of both the sender and receiver of notifications. Wiring Diagrams When you develop an EBA, most of the exchanges between the parts are based on notifications. Keep in mind that notifications can be exchanged between parts that don’t reference each other. If you used a class diagram to describe this system, it would tell you little about how the system works, because there would be few or no The delegate can then be used to define a field to hold a reference to a method with the given signature: public event SomeHandler myHandler; At runtime, myHandler would be initialized somehow (the details of which are not important here) to point at the method Figure 2: A simple wiring diagram showing untyped call notifications. July 2008 l www.ddj.com l Dr. Dobb’s Journal 23 http://www.leadtools.com/dobbs http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - July 2008

Dr. Dobb's Journal - July 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Developer’s Notebook Engineers Without Borders Conversations Patricia Tries Event-Based Architectures Graphs Versus Objects Lock-Free Queues Dr. Dobb’s Architecture & Design World Java and the Nokia N10 Internet Tablet Effective Concurrency The Agile Edge Swaine’s Flames

Dr. Dobb's Journal - July 2008

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