Example 3 - Dictionary Client Module

This example creates a module that is a client of the dictionary service implemented in Example 2. In the following source code, our module uses its module context to query for a dictionary service. Our client module uses the first dictionary service it finds and if none are found it simply prints a message saying so and stops. Using a service is the same as using any C++ class. The source code for our module is as follows in a file called dictionaryclient2/Activator.cpp:

#include <iostream>

#include "IDictionaryService.h"

#include <usModuleActivator.h>
#include <usModuleContext.h>

US_USE_NAMESPACE

namespace {

/**
 * This class implements a module activator that uses a dictionary service to check for
 * the proper spelling of a word by check for its existence in the dictionary.
 * This modules uses the first service that it finds and does not monitor the
 * dynamic availability of the service (i.e., it does not listen for the arrival
 * or departure of dictionary services). When loading this module, the thread
 * calling the Load() method is used to read words from standard input. You can
 * stop checking words by entering an empty line, but to start checking words
 * again you must unload and then load the module again.
 */
class US_ABI_LOCAL Activator : public ModuleActivator
{

public:

  /**
   * Implements ModuleActivator::Load(). Queries for all available dictionary
   * services. If none are found it simply prints a message and returns,
   * otherwise it reads words from standard input and checks for their
   * existence from the first dictionary that it finds.
   *
   * \note It is very bad practice to use the calling thread to perform a lengthy
   *       process like this; this is only done for the purpose of the tutorial.
   *
   * @param context the module context for this module.
   */
  void Load(ModuleContext *context)
  {
    // Query for all service references matching any language.
    std::vector<ServiceReference<IDictionaryService> > refs =
        context->GetServiceReferences<IDictionaryService>("(Language=*)");

    if (!refs.empty())
    {
      std::cout << "Enter a blank line to exit." << std::endl;

      // Loop endlessly until the user enters a blank line
      while (std::cin)
      {
        // Ask the user to enter a word.
        std::cout << "Enter word: ";

        std::string word;
        std::getline(std::cin, word);

        // If the user entered a blank line, then
        // exit the loop.
        if (word.empty())
        {
          break;
        }

        // First, get a dictionary service and then check
        // if the word is correct.
        IDictionaryService* dictionary = context->GetService<IDictionaryService>(refs.front());
        if ( dictionary->CheckWord( word ) )
        {
          std::cout << "Correct." << std::endl;
        }
        else
        {
          std::cout << "Incorrect." << std::endl;
        }

        // Unget the dictionary service.
        context->UngetService(refs.front());
      }
    }
    else
    {
      std::cout << "Couldn't find any dictionary service..." << std::endl;
    }
  }

  /**
   * Implements ModuleActivator::Unload(). Does nothing since
   * the C++ Micro Services library will automatically unget any used services.
   * @param context the context for the module.
   */
  void Unload(ModuleContext* /*context*/)
  {
    // NOTE: The service is automatically released.
  }

};

}

US_EXPORT_MODULE_ACTIVATOR(Activator)

Note that we do not need to unget or release the service in the Unload() method, because the C++ Micro Services library will automatically do so for us.

Since we are using the IDictionaryService interface defined in Example 1, we must link our module to the dictionaryservice module:

set(_srcs Activator.cpp)

set(dictionaryclient_DEPENDS dictionaryservice)
CreateExample(dictionaryclient ${_srcs})

After running the usCoreExamplesDriver executable, and loading the event listener module, we can use the l dictionaryclient command to load our dictionary client module:

CppMicroServices-debug> bin/usCoreExamplesDriver
> l eventlistener
Starting to listen for service events.
> l dictionaryclient
Ex1: Service of type IDictionaryService/1.0 registered.
Enter a blank line to exit.
Enter word:

The above command loads the module and its dependencies (the dictionaryservice module) in a single step. When we load the module, 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 module, we must use the s command to get the module identifier number for the module and first use the u <id> command to unload the module, then the l <id> command to re-load 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.

This example client is simple enough and, in fact, is too simple. What would happen if the dictionary service were to unregister suddenly? Our client would abort with a segmentation fault due to a null pointer access when trying to use the service object. This dynamic service availability issue is a central tenent of the service model. As a result, we must make our client more robust in dealing with such situations. In Example 4, we explore a slightly more complicated dictionary client that dynamically monitors service availability.

Next: Example 4 - Robust Dictionary Client Module

Previous: Example 2 - Dictionary Service Module