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CHAPTER 8 THE MECHANICS OF EVENT FIRING
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Function FireEventY() As String If Not OnEventX Is Nothing Then Return OnEventX() End If End Function ' compute a default or fallback value Function DoSomethingElse() As String Return "" End Function
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VB .NET EVENTS CAN T RETURN A VALUE
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There is a small but significant difference between C# event and VB .NET Event keywords. In C#, an event can define a return value. In VB .NET, an Event is not allowed to define a return value. In VB .NET, to fire an event that defines a return value, you must use a delegate. Here s an example: Public Delegate Function MyDelegate() As String Public OnEventX As MyDelegate Function FireEventX() As String If Not OnEventX Is Nothing Then Return OnEventX() 'fire the event using a delegate End If End Function
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When no listeners are available and no alternative courses of action are possible, it may be necessary for the sender to throw an exception. A special exception might be defined for this purpose, such as ExceptionNoSubscribersAvailable. The exception handler might react by notifying the end user, making an entry in a log journal, or even shutting the system down.
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Two Notifications Might Be Better Than One
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As you ve seen, using a notification to retrieve data has drawbacks. Besides the problem of how to proceed in the absence of subscribers, another problem is that the notifications often must be sent using synchronous procedure calls. When the sender requests data, it generally must block and wait until the subscribers return control. An alternative technique is based on the use of two notifications to achieve the original goal of sending a notification and getting data back. If A is the event source and B is the event target from which data is returned, the diagram in Figure 8-12 shows the two notifications. When A sends the DataRequest notification, the sender blocks. Once B gets the data requested, it makes a reentrant DataReturned call back into A, which is still blocked on the original DataRequest call. Using synchronous calls this way can be cumbersome, and the call that B makes into A requires A to be reentrant. An alternative is to use asynchronous calls from A to B, as shown in Figure 8-13. Using an asynchronous call, A can continue with its internal tasks while B processes the DataRequest notification. The technique is even more powerful when A needs information that comes from different subscribers. It can use separate asynchronous calls to fire events to all the necessary subscribers, so all the work in the receivers is carried out in parallel instead of sequentially.
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CHAPTER 8 THE MECHANICS OF EVENT FIRING
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Data Request Data Returned
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Figure 8-12. Using two notifications to retrieve data
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Data Request
A B () Data Request
Data Returned
Data Returned
Figure 8-13. Using an asynchronous call from A to B
Avoiding Deadlocks
There is always the possibility of deadlocks in systems that have reentrant calls or concurrent code, unless you take precautions. Deadlocks are typically caused by the incorrect use of process synchronization primitives, such as monitors. When designing an object or component that fires events to the surrounding world, you have to be especially careful on where to use resource locks in relation to fired events. The first and foremost rule to avoid deadlocks in event-based code is this: Never fire events after locking a resource. The reason is obvious: You can t predict who is going to handle the notifications, and there is the possibility that the event handler may need the locked resource. In terms of lockable resources, there are two fundamental types: code and data. Listing 8-11 and Listing 8-12 show what not to do, because they lock a section of code before firing an event. The section of code protected by the lock is called a critical section, and only one thread can enter it at any given time. Listing 8-11. C# Example of Locking a Resource and Then Firing an Event // DON'T DO THIS! public class EventSource { public delegate void SomethingHappenedHandler(); public event SomethingHappenedHandler OnSomethingHappened; public void M1() { lock (this) { if (OnSomethingHappened != null) OnSomethingHappened(); } } }
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