Boost: How do I write a unit test for a signal?

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Today, while writing some unit tests, I encountered a challenge. The user story was that, when a Person’s details are updated, the display should be updated to reflect the changes.

I’d implemented this feature using a signal on the person class that will be called whenever any details are updated:

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class person
{
public:
    // Set the Person's name.
    void name(const std::string & name)
    {
        name_ = name;
        updated(*this);
    };
    // An signal that will be called when the person's details are updated.
    boost::signal<void(const person & person)> updated;
private:
    // The person's name.
    std::string name_;
};

This is a fairly standard application of an observer pattern that you might find in any MVC application.

But the question is, using the Boost unit test framework, how can I test if my signal has been called?

The mock signal handler

To test the signal handler, we’ll use a functor as a mock signal handler, that sets an internal flag when it gets called. In the functor’s destructor, we’ll do a test on the flag to make sure it got set:

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struct mock_handler
{
    mock_handler(const person & expected_person) :
        has_been_called_(false), expected_person_(expected_person) {};
    // The signal handler function.
    void operator()(const person & person)
    {
        has_been_called_ = true;
        BOOST_CHECK_EQUAL(&person == &expected_person_, true);
    };
    // This handler must be called before it goes out of scope.
    ~mock_handler()
    {
        BOOST_CHECK_EQUAL(has_been_called_, true);
    };
private:
    bool has_been_called_;
    const person & expected_person_;
};

The test case

Once we’ve written a mock, the test case is pretty simple. Note that I wrap my handler with a boost::ref, so that it doesn’t get copied.

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// Test that setting a new name triggers the person.updated signal.
BOOST_AUTO_TEST_CASE(setting_name_triggers_update_signal)
{
    person subject;
    mock_handler handler(subject);
    subject.updated.connect(boost::ref(handler));
    // Change the person's name, triggering the updated signal.
    subject.name("Richard");
}

This works great. And if we comment out the updated signal call in person::name():

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Running 1 test case... person_test.cpp(49): error in "setting_name_triggers_update_signal": check has_been_called_ == true failed [0 != 1]
*** 1 failure detected in test suite "tests"

..then the test case will fail accordingly.

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