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Practical PHP Testing Patterns: Test Spy

03.09.2011
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The concept of behavior verification consists in verifying not only the output of the System Under Test, but the calls to other components. These method calls are an output normally not visible to a caller like the test; unless he injects, instead of the real collaborator, a Test Double which can be accessed also by him.

Today we will explore the first pattern for behavior verification: the Test Spy. It is a Test Double which records calls so that assertions can be made on them later in the test (after the exercise part).

Spies gives us the ability to observe side-effects of the SUT, expressed as interactions with other objects. Usually behavior verification is taught with Mocks, which also verify the calls that are made on them but with specifications provided before the exercise phase.

Use cases

Why using a Spy instead a Mock? I bet you have heard already about Mocks at this point in the series. Here are some possible use cases on when to define Test Spies.

  • When we cannot predict what the collaborator will be called with (if we can, we may use a Mock instead).
  • When an assertion on calls is complex to define beforehand, or needs all (or more than one of) the calls to be completed before taking place.
  • The matchers used for Mocks are not sufficient for verification.
  • When a Mock that immediately threw an exception would only result in the SUT swallowing it. So a Spy is better in this case as it leaves verification for later. PHPUnit matchers in general do not always throw an exception or always wait for the end of test (it depends on the particular constraint.)
  • The behavior involves more than one collaborator (it happens sometimes). In this case, you can make an assertion using the data recorded by more than one Spy.

Implementation

1. Like with all Test Doubles, provide an alternate implementation or subclass. Probably the calls to the Spy won't return anything (or will return something canned to avoid the SUT's failure, like in a Stub). The methods implementations will just record all calls, or some property of the calls like the first parameter or their total number.

2. Inject the Test Double and exercise the SUT.

3. Make assertions on the Test Double data gathered by the Spy.

Variations

The variations of the pattern are mainly on how to retrieve the data from the Test Spy.

  • Retrieval Interface: the Test Spy is a class with additional methods, or public properties (eek) that expose the recorded data. This variation cannot be coded with PHPUnit generation of Test Doubles, only by hand-rolling them.
  • Self Shunt: the Test Spy and Test Case Object are a single object. This means we inject $this as the Test Double and we have the maximum freedom of defining new methods and access the recording. The caveat is that it can only be done with interfaces in PHPUnit, because Test Case Classes must extend PHPUnit_Framework_TestCase. That's a good reason to extract an interface, though.
  • Inner Test Double: we inject a private class (not existent in PHP) or a closure which records everything. A closure for example can access $this public properties or methods, or some other local ArrayObject passed by handler (or variable passed by reference) where the data can be kept.
  • Indirect Output Registry: same as Inner Test Double, but the target for recordings is a full-fledged object. Almost always an overkill.

Examples

The sample code shows you how to implement a Spy, in its different variations and in use cases where it actually make sense. Most of the times, Mock are used instead and the pattern is equivalent. Test Spies are a little more difficult to write, but they are invaluable in the case where your verification logic does not fit the framework of Mocks (predefined, single-object expectations).

<?php
class TestSpyTest extends PHPUnit_Framework_TestCase implements Mailer, Db
{
/**
* Other expectations would be simple to check with generated Mocks,
* but order of calls on different object is not in PHPUnit.
* The same Self-Shunting setup can be used in other tests too.
*/
public function testSendsAMailAfterUserCreationViaSelfShunting()
{
// remember, Db and Mailer would be two different objects in production
$sut = new UserDao($this, $this);
$sut->createUser(array('mail' => 'someone@example.com', 'nickname' => 'johndoe'));
$this->assertEquals(array('executeQuery', 'mail'), $this->order);
}

private $order = array();

private $queries = array();

public function executeQuery($query, array $params)
{
$this->order[] = 'executeQuery';
$this->queries[] = $query;
}

private $mails = array();

public function mail($to, $subject, $object)
{
$this->order[] = 'mail';
$this->mails[] = array('to' => $to,
'subject' => $subject,
'object' => $object);
}

public function testInnerTestDoubleArrayObject()
{
$parts = new ArrayObject();
$receiver = $this->getMock('Receiver');
$receiver->expects($this->any())
->method('definePart')
->will($this->returnCallback(function($amount) use ($parts) {
$parts[] = $amount;
}));
$sut = new RandomDivider($receiver);
$sut->divide(10);
$this->assertEquals(10, array_sum($parts->getArrayCopy()));
}

public function testInnerTestDoubleArrayPassedByReference()
{
$parts = array(); // an array would not be passed by handler by default
$receiver = $this->getMock('Receiver');
$receiver->expects($this->any())
->method('definePart')
->will($this->returnCallback(function($amount) use (&$parts) { // but we can pass it by reference
$parts[] = $amount;
}));
$sut = new RandomDivider($receiver);
$sut->divide(10);
$this->assertEquals(10, array_sum($parts));
}
}

interface Mailer
{
public function mail($to, $subject, $object);
}

interface Db
{
public function executeQuery($query, array $params);
}

class UserDao
{
private $db;
private $mailer;

public function __construct(DB $db, Mailer $mailer)
{
$this->db = $db;
$this->mailer = $mailer;
}

public function createUser(array $userDetails)
{
// internally it would use PDO
$this->db->executeQuery("INSERT INTO users ...", $userDetails);
$this->mailer->mail($userDetails['mail'], 'You have been registered on example.com', '...');
}
}

interface Receiver
{
public function definePart($amount);
}

class RandomDivider
{
private $receiver;

public function __construct(Receiver $receiver)
{
$this->receiver = $receiver;
}

public function divide($total)
{
$part = ceil(rand() * $total);
$this->receiver->definePart($part);
$this->receiver->definePart($total - $part);
}
}
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