Medical Imaging Interaction Toolkit
2016.11.0
Medical Imaging Interaction Toolkit
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In this example, we complicate things further by defining a new service that uses an arbitrary number of dictionary services to perform its function. More precisely, we define a spell checker service which will aggregate all dictionary services and provide another service that allows us to spell check passages using our underlying dictionary services to verify the spelling of words. Our module will only provide the spell checker service if there are at least two dictionary services available. First, we will start by defining the spell checker service interface in a file called spellcheckservice/ISpellCheckService.h
:
The service interface is quite simple, with only one method that needs to be implemented. Because we provide an empty out-of-line destructor (defined in the file ISpellCheckService.cpp
) we must export the service interface by using the module specific SPELLCHECKSERVICE_EXPORT
macro.
In the following source code, the module needs to create a complete list of all dictionary services; this is somewhat tricky and must be done carefully if done manually via service event listners. Our module makes use of the ServiceTracker
and ServiceTrackerCustomizer
classes to robustly react to service events related to dictionary services. The module activator of our module now additionally implements the ServiceTrackerCustomizer
class to be automatically notified of arriving, departing, or modified dictionary services. In case of a newly added dictionary service, our ServiceTrackerCustomizer::AddingService()
implementation checks if a spell checker service was already registered and if not registers a new ISpellCheckService
instance if at lead two dictionary services are available. If the number of dictionary services drops below two, our ServiceTrackerCustomizer
implementation un-registers the previously registered spell checker service instance. These actions must be performed in a synchronized manner to avoid interference from service events originating from different threads. The implementation of our module activator is done in a file called spellcheckservice/Activator.cpp
:
Note that we do not need to unregister the service in stop() method, because the C++ Micro Services library will automatically do so for us. The spell checker service that we have implemented is very simple; it simply parses a given passage into words and then loops through all available dictionary services for each word until it determines that the word is correct. Any incorrect words are added to an error list that will be returned to the caller. This solution is not optimal and is only intended for educational purposes.
For an introduction how to compile our source code, see Example 1 - Service Event Listener.
After running the usCoreExamplesDriver
program we should make sure that the module from Example 1 is active. We can use the s
shell command to get a list of all modules, their state, and their module identifier number. If the Example 1 module is not active, we should load the module using the load command and the module's identifier number or name that is displayed by the s
command. Now we can load the spell checker service module by entering the l spellcheckservice
command which will also trigger the loading of the dictionaryservice module containing the english dictionary:
CppMicroServices-build> bin/usCoreExamplesDriver > l eventlistener Starting to listen for service events. > l spellcheckservice Ex1: Service of type IDictionaryService/1.0 registered. > s Id | Name | Status ----------------------------------- - | dictionaryclient | - - | dictionaryclient2 | - - | dictionaryclient3 | - - | frenchdictionary | - - | spellcheckclient | - 1 | CppMicroServices | LOADED 2 | Event Listener | LOADED 3 | Dictionary Service | LOADED 4 | Spell Check Service | LOADED >
To trigger the registration of the spell checker service from our module, we load the frenchdictionary using the l frenchdictionary
command. If the module from Example 1 is still active, then we should see it print out the details of the service event it receives when our new module registers its spell checker service:
CppMicroServices-build> bin/usCoreExamplesDriver > l frenchdictionary Ex1: Service of type IDictionaryService/1.0 registered. Ex1: Service of type ISpellCheckService/1.0 registered. >
We can experiment with our spell checker service's dynamic availability by stopping the french dictionary service; when the service is stopped, the eventlistener module will print that our module is no longer offering its spell checker service. Likewise, when the french dictionary service comes back, so will our spell checker service. We create a client for our spell checker service in Example 7. To exit the usCoreExamplesDriver
program, we use the q
command.
Next: Example 7 - Spell Checker Client Module
Previous: Example 5 - Service Tracker Dictionary Client Module