Expectation documentation

More control over mock objects

The default behaviour of the mock objects in SimpleTest is either an identical match on the argument or to allow any argument at all. For almost all tests this is sufficient. Sometimes, though, you want to weaken a test case.

One place where a test can be too tightly coupled is with text matching. Suppose we have a component that outputs a helpful error message when something goes wrong. You want to test that the correct error was sent, but the actual text may be rather long. If you test for the text exactly, then every time the exact wording of the message changes, you will have to go back and edit the test suite.

For example, suppose we have a news service that has failed to connect to its remote source.

class NewsService {
    ...
    function publish(&$writer) {
        if (! $this->isConnected()) {
            $writer->write('Cannot connect to news service "' .
                    $this->_name . '" at this time. ' .
                    'Please try again later.');
        }
        ...
    }
}

Here it is sending its content to a Writer class. We could test this behaviour with a MockWriter like so...

class TestOfNewsService extends UnitTestCase {
    ...
    function testConnectionFailure() {
        $writer = &new MockWriter($this);
        $writer->expectOnce('write', array(
                'Cannot connect to news service ' .
                '"BBC News" at this time. ' .
                'Please try again later.'));
        
        $service = &new NewsService('BBC News');
        $service->publish($writer);
        
        $writer->tally();
    }
}

This is a good example of a brittle test. If we decide to add additional instructions, such as suggesting an alternative news source, we will break our tests even though no underlying functionality has been altered.

To get around this, we would like to do a regular expression test rather than an exact match. We can actually do this with...

class TestOfNewsService extends UnitTestCase {
    ...
    function testConnectionFailure() {
        $writer = &new MockWriter($this);
        $writer->expectOnce(
                'write',
                array(new WantedPatternExpectation('/cannot connect/i')));
        
        $service = &new NewsService('BBC News');
        $service->publish($writer);
        
        $writer->tally();
    }
}

Instead of passing in the expected parameter to the MockWriter we pass an expectation class called WantedPatternExpectation. The mock object is smart enough to recognise this as special and to treat it differently. Rather than simply comparing the incoming argument to this object, it uses the expectation object itself to perform the test.

The WantedPatternExpectation takes the regular expression to match in its constructor. Whenever a comparison is made by the MockWriter against this expectation class, it will do a preg_match() with this pattern. With our test case above, as long as "cannot connect" appears in the text of the string, the mock will issue a pass to the unit tester. The rest of the text does not matter.

The possible expectation classes are...

EqualExpectation	An equality, rather than the stronger identity comparison
NotEqualExpectation	An inequality comparison
IndenticalExpectation	The default mock object check which must match exactly
NotIndenticalExpectation	Inverts the mock object logic
WantedPatternExpectation	Uses a Perl Regex to match a string
NoUnwantedPatternExpectation	Passes only if failing a Perl Regex
IsAExpectation	Checks the type or class name only
NotAExpectation	Opposite of the IsAExpectation
MethodExistsExpectation	Checks a method is available on an object

Most take the expected value in the constructor. The exceptions are the pattern matchers, which take a regular expression, and the IsAExpectation and NotAExpectation which takes a type or class name as a string.

Using expectations to control stubs

The expectation classes can be used not just for sending assertions from mock objects, but also for selecting behaviour for either the mock objects or the server stubs. Anywhere a list of arguments is given, a list of expectation objects can be inserted instead.

Suppose we want an authorisation server stub to simulate a successful login only if it receives a valid session object. We can do this as follows...

Stub::generate('Authorisation');

$authorisation = new StubAuthorisation();
$authorisation->setReturnValue(
        'isAllowed',
        true,
        array(new IsAExpectation('Session', 'Must be a session')));
$authorisation->setReturnValue('isAllowed', false);

We have set the default stub behaviour to return false when isAllowed is called. When we call the method with a single parameter that is a Session object, it will return true. We have also added a second parameter as a message. This will be displayed as part of the mock object failure message if this expectation is the cause of a failure.

This kind of sophistication is rarely useful, but is included for completeness.

Creating your own expectations

The expectation classes have a very simple structure. So simple that it is easy to create your own versions for commonly used test logic.

As an example here is the creation of a class to test for valid IP addresses. In order to work correctly with the stubs and mocks the new expectation class should extend SimpleExpectation...

class ValidIp extends SimpleExpectation {
    
    function test($ip) {
        return (ip2long($ip) != -1);
    }
    
    function testMessage($ip) {
        return "Address [$ip] should be a valid IP address";
    }
}

There are only two methods to implement. The test() method should evaluate to true if the expectation is to pass, and false otherwise. The testMessage() method should simply return some helpful text explaining the test that was carried out.

This class can now be used in place of the earlier expectation classes.

Under the bonnet of the unit tester

The SimpleTest unit testing framework also uses the expectation classes internally for the UnitTestCase class. We can also take advantage of these mechanisms to reuse our homebrew expectation classes within the test suites directly.

The most crude way of doing this is to use the SimpleTest::assertExpectation() method to test against it directly...

class TestOfNetworking extends UnitTestCase {
    ...
    function testGetValidIp() {
        $server = &new Server();
        $this->assertExpectation(
                new ValidIp(),
                $server->getIp(),
                'Server IP address->%s');
    }
}

This is a little untidy compared with our usual assert...() syntax.

For such a simple case we would normally create a separate assertion method on our test case rather than bother using the expectation class. If we pretend that our expectation is a little more complicated for a moment, so that we want to reuse it, we get...

class TestOfNetworking extends UnitTestCase {
    ...
    function assertValidIp($ip, $message = '%s') {
        $this->assertExpectation(new ValidIp(), $ip, $message);
    }
    
    function testGetValidIp() {
        $server = &new Server();
        $this->assertValidIp(
                $server->getIp(),
                'Server IP address->%s');
    }
}

It is unlikely we would ever need this degree of control over the testing machinery. It is rare to need the expectations for more than pattern matching. Also, complex expectation classes could make the tests harder to read and debug. These mechanisms are really of most use to authors of systems that will extend the test framework to create their own tool set.