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While C# events can be used with delegates that define a return type, VB .NET events can t. A VB .NET Event field can only be declared with a delegate associated with a Sub. Delegates associated with a Function can t be used with Event fields.
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VB .NET defines a special RaiseEvent keyword to make calls through an event. When using RaiseEvent, you specify the name of the Event field to use. You don t have to check if the event is bound, because RaiseEvent returns with no errors if there are no methods to call, as shown in Listing 5-7. Listing 5-7. A VB .NET Event Source Using Events Public Class MyEventSource2 Public Delegate Sub MyDelegateType( ) Public Event event1 As MyDelegateType
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CHAPTER 5 A SURVEY OF COMMERCIAL SYSTEMS
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Public Sub FireEvent1( ) RaiseEvent event1( ) End Sub End Class The VB .NET Event keyword doesn t support += and -= operators, as C# does. To add and remove handlers, you must use the VB AddHandler/RemoveHandler statements, as shown in Listing 5-8. Listing 5-8. Binding an Event Field to an Event Target, Using VB .NET 'create source and target Dim object1 As New MyEventSource2 Dim object2 As New MyEventTarget2 'bind event source to target AddHandler object1.Event1, AddressOf object2.MyMethod 'unbind event source from target RemoveHandler object1.Event1, AddressOf object2.MyMethod 'fire event to target object1.FireEvent1() If the binder needs to bind an event to a method contained in the Binder class, VB .NET supports an alternative binding technique based on the WithEvents keyword. If you declare an event source object with this keyword, VB .NET lets you designate a handler by adorning the method with a Handles <object.eventName> suffix, as shown in Listing 5-9. Listing 5-9. Using the WithEvent and Handles Keywords to Bind an Event Source to a Handler Public Class MyEventSource3 Public Event Event1() Public Sub FireEvent1() RaiseEvent Event1() End Sub End Class Public Class Binder3 Private WithEvents object1 As MyEventSource2 Public Sub Binder3() object1 = New MyEventSource2 End Sub 'declare an event handler for MyEventSource3.Event1 Public Sub MyHandler() Handles object1.Event1 'do something... End Sub End Class
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The key thing to remember about the WithEvents technique is this: You can only use it to bind events to your own handler methods. In other words, the binder is also the event target.
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Typed Object Calls, Using Interfaces
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Typed object calls are method calls that go through classes or interfaces. The caller has a reference to a class or interface, and designates at compile time the method to call. Figure 5-4 shows the callercallee relationship.
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MyEventSource -myReference : MyEventTarget MyEventTarget +Method1()
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Figure 5-4. A typed object call, using a class reference With a typed object call, the caller is type-coupled to the callee s class. To eliminate the direct type coupling between caller and callee, you can have the callee implement an interface and use the interface to make the call to Method1, as shown in Figure 5-5.
MyEventSource -myReference : MyInterface interface MyInterface +Method1()
MyEventTarget +Method1()
Figure 5-5. A typed object call, using an interface The use of interfaces in typed object calls is often advantageous, compared to calling through a class, because interfaces can reduce the magnitude of intercomponent coupling. By packaging many interfaces together into a single component, event sources incur coupling only to the interface component, as shown in Figure 5-6. In a large system, all the commonly used interfaces might be packaged together, and that package would tend to be at the center of the system s coupling diagram.
CHAPTER 5 A SURVEY OF COMMERCIAL SYSTEMS
Component Containing Common Interfaces
MySource1 -myReference : MyInterface1
interface MyInterface1 +MyMethod()
interface MyInterface2 +MyMethod()
MySource2 -myReference : MyInterface2
MyTarget1 +MyMethod()
MyTarget2 +MyMethod()
Figure 5-6. Packaging interfaces separately, to reduce intercomponent coupling in a system
JavaBeans
JavaBeans are essentially Java objects that support properties, methods, and events. JavaBeans also carry metadata, allowing tools to inspect their internal structure at run time to aid a tool-assisted composition of JavaBeans. The JavaBeans event model is largely based on the Observer design pattern.2 The object that publishes events is called the event source. An object receiving event notifications from an event source is called a listener. Listeners are required to implement the EventListener interface directly or indirectly. Event notifications are sent using a typed object call from the source to the listener, as shown in Figure 5-7.
EventObject -source : Object MyEventSource -listener : MyCustomListener MyCustomEvent +addMyCustomListener() +removeMyCustomListener() myCustomEvent
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