In Example 3 <example3>, we create a simple client bundle for our dictionary service. The problem with that client was that it did not monitor the dynamic availability of the dictionary service, thus an error would occur if the dictionary service disappeared while the client was using it. In this example we create a client for the dictionary service that monitors the dynamic availability of the dictionary service. The result is a more robust client.
The functionality of the new dictionary client is essentially the same as the old client, it reads words from standard input and checks for their existence in the dictionary service. Our bundle uses its bundle context to register itself as a service event listener; monitoring service events allows the bundle to monitor the dynamic availability of the dictionary service. Our client uses the first dictionary service it finds. The source code for our bundle is as follows in a file called Activator.cpp:
dictionaryclient2/Activator.cpp
The client listens for service events indicating the arrival or departure of dictionary services. If a new dictionary service arrives, the bundle will start using that service if and only if it currently does not have a dictionary service. If an existing dictionary service disappears, the bundle will check to see if the disappearing service is the one it is using; if it is it stops using it and tries to query for another dictionary service, otherwise it ignores the event.
Like normal, we must create a manifest.json file that contains the meta-data for our bundle:
dictionaryclient2/resources/manifest.json
As in Example 3, we must link our bundle to the dictionaryservice bundle:
dictionaryclient2/CMakeLists.txt
After running the usTutorialDriver executable, and starting the event listener bundle, we can use the start dictionaryclient2 command to start our robust dictionary client bundle:
CppMicroServices-debug> bin/usTutorialDriver
> start eventlistener
Starting to listen for service events.
> start dictionaryclient2
Ex1: Service of type IDictionaryService registered.
Enter a blank line to exit.
Enter word:The above command starts the bundle and it will use the main thread to prompt us for words. Enter one word at a time to check the words and enter a blank line to stop checking words. To reload the bundle, we must first use the stop dictionaryclient2 command to stop the bundle, then the start dictionaryclient2 command to re-start it. To test the dictionary service, enter any of the words in the dictionary (e.g., "welcome", "to", "the", "micro", "services", "tutorial") or any word not in the dictionary.
Since this client monitors the dynamic availability of the dictionary service, it is robust in the face of sudden departures of the dictionary service. Further, when a dictionary service arrives, it automatically gets the service if it needs it and continues to function. These capabilities are a little difficult to demonstrate since we are using a simple single-threaded approach, but in a multi-threaded or GUI-oriented application this robustness is very useful.