JCL Help:BaseServices.Notifications
JCL Help: Notifier/listener pattern
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Summary
The notifier/listener pattern allows for both generalizing the sending of notifications to multiple interested parties (listeners) and ease of adding new ways of processing notifications without needing to change any code.
An example of this pattern can be found in .NET, where one can find a Trace class and a number of derived TraceListener classes. Using that example, the Trace class is an IJclNotifier implementer and the TraceListener an IJclListener interface. However, the .NET Trace and TraceListener setup is a specific implementation and doesn't need an IJclNotificationMessage implementer; the Trace class simply calls specific methods of the TraceListener. The JEDI Code Library provides a more generic approach to this pattern.
NotifiersA notifier (any class implementing IJclNotifier) has only two distinct tasks:
- Managing a list of listeners (IJclListener implementing classes)
- Notify all registered listeners
The first task is generally performed before a certain process within an application is started or after it has finished.
The second task is performed by the class or methods that are part of a process. At any moment a message (IJclNotificationMessage implementing class) can be constructed and passed on to the Notification method.
ListenersA listener (any class implementing IJclListener) has a single method that will be called by a notifier whenever its notify method is called. The method will specify the message that was passed to the notifier's notify method. A listener can check incoming notifications for additional or derived interface(s) and either process the notification or ignore it based on those interface(s).
During processing the additional or derived interface(s) may provide additional information regarding the nature of the notification.
Notification messagesA notification message (any class implementing IJclNotificationMessage) is just place holder. Normally, one would either create a derived interface or add additional interfaces to the message implementation class, adding such information as context or other data that may be of interest to listeners.
Setting up a notifier/listener patternIn general setting up a specific notification sub system involves the steps as outlined below. It's not unthinkable that for certain situations you'll need additional steps or may skip steps.
1 create a notifier The JEDI Code Library simplifies this task: create an instance of TJclBaseNotifier and you're good to go in most cases. Assign the instance to a variable declared as IJclNotifier, so that you can use the notifier (the class itself declares the methods protected since it is assumed they will be used through the interface reference at all times).
2 create notification messages Notification messages describe to the listener what has happened. If all you need is a simple notification a proces has finished, you could suffice with TJclBaseNotificationMessage. In most cases, however, you will need to declare additional interfaces (use Ctrl+Shift+G to create a GUID). These interfaces can have additional properties and/or functions to provide data to the listener, but a message supporting a certain interface could be all the information you'd need. Then you'll need to create one or more classes that will implement these interfaces (in whatever combination you need).
3 create a listener In most cases you could simply derive a class from TJclBaseListener. Alternatively you could create a new class to implement the IJclListener interface, or - in case of a VCL application - have your MainForm implement the interface. The interface has only one method and it will be passed an IJclNotificationMessage instance.
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