Preserving test cases when removing the tested classes

Scenario

There is a class \App\Example1\Formatter with a single public method format(string). This method receives a string, reverses the sequence if its words, and in the resulting set of words appends an increasing natural number to each word.

For example:

Input	Output
a b c	c1 b2 a3
foo bar baz	baz1 bar2 foo3

This method is unit-tested, see \App\Example1\Tests\Unit\FormatterTest. It tests that the method correctly transforms the text as in the table above:

# FormatterTestCase
self::assertSame('c1 b2 a3', $formatter->format('a b c'));
self::assertSame('baz1 bar2 foo3', $formatter->format('foo bar baz'));

We now want to refactor the two actions of the method (word sequence reversal and numbering) into two separate services. We do this in namespace Example2: we create WordReverser and WordNumberer services. We unit-test them.

The test cases for WordReverser:

# WordReverserTestCase
self::assertSame('c b a', (new WordReverser())->reverseWords('a b c'));
self::assertSame('Carl Bob Ann', (new WordReverser())->reverseWords('Ann Bob Carl'));

The test cases for WordNumberer:

# WordNumbererTestCase
self::assertSame('a1 b2 c3', (new WordNumberer())->numberWords('a b c'));
self::assertSame('Ann1 Bob2 Carl3', (new WordNumberer())->numberWords('Ann Bob Carl'));

The problem

We refactored the service Formatter, we unit-tested the newly created services WordNumberer and WordReverser. Is it enough testing? Definitely not. We lost the testing we had in FormatterTestCase. There, we ensured that word sequence reversal and numbering were both performed and that they were performed in the correct order: first the ordering, then the numbering. Let's call these formatting requirements. Now the responsibility to correctly call the new services lies with whoever was calling Formatter - in our case it is \App\Example2\Controller. We need to keep the FormatterTestCase valid even after refactoring.

Solution 1 - testing the argument to WordNumberer

The solution to ensure that the formatting requirements are still followed by \App\Example2\Controller, we should ensure that WordNumberer (the second to be called) is called with the reversed order of words. We do this in \App\Example2\Tests\Unit\ControllerTest::testDisplayText:

$wordNumberer = self::createMock(WordNumberer::class);
$wordNumberer
    ->expects(self::any())
    ->method('numberWords')
    ->with('there hello') # this ensures that the result of WordReverser is passed to WordNumberer
    ->willReturn('there1 hello2')
;

Solution 2 - an integration test

Integration tests instead of testing how a class performs in isolation, also test how it interacts with its dependencies (other classes). In these tests we can catch bugs such as an incorrect order of calls to services - which is exactly what we need to ensure that the Controller adheres to the formatting requirements.

We created such an integration test in Example3: \App\Example3\Tests\Integration\ControllerTest. The test case is as follows:

$controller = new Controller(new WordReverser(), new WordNumberer(), 'hello there');
self::assertSame('there1 hello2', $controller->getDisplayText());

Here, we see for certain that the functionality is intact after refactoring, without needing to test for the value of the argument to WordNumberer::numberWords as we did in Solution 1.

Final words

I have run into a lot of cases where I removed old methods and/or classes, created new methods and/or classes that were to be used instead of the old ones, and simply testing the new ones was not enough. By refactoring sometimes you lose tests that test the bigger picture/the interaction of classes. Unit testing as in Solution 1 can help, but I much more prefer to cover the code by integration tests as in Solution 2.