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CHAPTER 8 THE MECHANICS OF EVENT FIRING
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Listing 8-12. VB .NET Example of Locking a Resource and Then Firing an Event ' DON'T DO THIS! Public Class BadEventSource Public Delegate Sub SomethingHappenedHandler() Public OnSomethingHappened As SomethingHappenedHandler Public Sub M1() SyncLock (Me) If Not OnSomethingHappened Is Nothing Then OnSomethingHappened() End If End SyncLock End Sub End Class The code lock could cause a deadlock if a SomethingHappened subscriber caused BadEventSource.M1 to be called directly or indirectly while processing the event notification. Another type of deadlock can occur even in the absence of reentrant calls, when the event source locks a resource that is shared with other objects or components. For example, say S is a file, A is an event source, and B is an event subscriber. Assume A locks S for exclusive access and then fires an event to B using a synchronous call. When B gets the event notification, say it needs access to S before it can complete its job and return control to A. Obviously, the system will freeze, with B waiting forever for A to release the lock on S. Again, the solution is to design A so it doesn t need to lock S while it fires events.
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Dealing with Exceptions from Targets
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When an event is fired using a procedure call, there is the potential for event targets to throw exceptions while handling the event, disrupting the normal sequence of notifications. Event sources may need to be coded defensively and be prepared to handle exceptions thrown by event targets. You must consider two kinds of problems: 1. What to do if an exception occurs 2. How to proceed For the former problem, you might need to fire a different event or call a fallback method. For the latter problem, options include doing nothing, letting the exception propagate up the stack, throwing an exception of a new type, notifying the operator, and logging the exception.
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Using a Shared Resource
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Instead of using messages or procedure calls to send notifications, you can also use a shared resource. As stated earlier, messages are actually a special case of shared resource delivery, because the sender and receiver share the communication channel through which messages are delivered. Let s look at how events are fired using other kinds of shared resources.
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Shared Memory
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Shared memory has long been used in distributed systems to allow different processes to exchange information. What is noteworthy is the fact that, from the perspective of an event-based programmer, information that passes through shared memory looks just like an event notification. Why For the following reasons:
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CHAPTER 8 THE MECHANICS OF EVENT FIRING
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The sender is like an event source, because it sends out information without knowing what potential receivers will do with it. Receivers look like subscribers, because they monitor the shared memory, waiting for information to appear. Sender and receiver must agree on a designated rendezvous area in the shared memory. When the notification signal is written to the rendezvous area, the subscriber must somehow clear it after handling it; otherwise, the subscriber will go into an infinite loop and keep trying to handle the notification over and over again. If the rendezvous area is a single byte, the clearing operation might entail setting the byte to 0. If the area is a data structure, you could designate a separate byte to act as the signaling flag. You should protect the flag by a semaphore or other process synchronization mechanism to prevent concurrent writing by the sender and subscriber. Figure 8-14 shows a possible solution supporting unicast events.
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Rendezvous -IsNotificationPending : Boolean -NotificationPayload : String FireEvent()
Sender
Rendezvous
Receiver
Check IsNotificationPending() Check IsNotificationPending() Write Payload()
Wait Until IsNotificationPending Is Clear
Set Notification Flag() Get Payload() Set IsNotificationPending() Blocks Until IsNotificationPending Is Set
Figure 8-14. Using a shared Rendezvous object to send notifications A class called Rendezvous is defined to coordinate the sender and receiver. An instance of Rendezvous is stored at a given location in the shared memory, known to both sender and receiver. A flag called IsNotificationPending coordinates access to the Rendezvous payload. When the sender wishes to send a notification, it must wait for the flag to be clear, indicating that the receiver has read the last notification sent. Once the flag is clear, the sender can write the notification payload to the Rendezvous object and then set the notification flag. The receiver monitors the notification flag. When the flag is found to be set, the receiver gets the notification payload and then clears the flag. Listing 8-13 shows a C# implementation. Listing 8-13. Using a Simple Shared C# Rendezvous Object to Deliver Notifications public class Rendezvous { bool isNotificationPending; public bool IsNotificationPending { get {return isNotificationPending;} set { lock(this) { isNotificationPending = value; } string notificationPayload; public string NotificationPayload {
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