458 lines
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458 lines
17 KiB
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<title>SimpleTest for PHP partial mocks documentation</title>
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<span class="chosen">Partial mocks</span>
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<h1>Partial mock objects documentation</h1>
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This page...
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<ul>
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<li>
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<a href="#inject">The mock injection problem</a>.
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</li>
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<li>
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Moving creation to a <a href="#creation">protected factory</a> method.
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</li>
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<li>
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<a href="#partial">Partial mocks</a> generate subclasses.
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</li>
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<li>
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Partial mocks <a href="#less">test less than a class</a>.
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</li>
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</ul>
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<div class="content">
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<p>
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A partial mock is simply a pattern to alleviate a specific problem
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in testing with mock objects,
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that of getting mock objects into tight corners.
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It's quite a limited tool and possibly not even a good idea.
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It is included with SimpleTest because I have found it useful
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on more than one occasion and has saved a lot of work at that point.
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</p>
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<h2>
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<a class="target" name="inject"></a>The mock injection problem</h2>
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<p>
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When one object uses another it is very simple to just pass a mock
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version in already set up with its expectations.
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Things are rather tricker if one object creates another and the
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creator is the one you want to test.
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This means that the created object should be mocked, but we can
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hardly tell our class under test to create a mock instead.
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The tested class doesn't even know it is running inside a test
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after all.
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</p>
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<p>
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For example, suppose we are building a telnet client and it
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needs to create a network socket to pass its messages.
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The connection method might look something like...
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<pre>
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<strong><?php
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require_once('socket.php');
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class Telnet {
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...
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function connect($ip, $port, $username, $password) {
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$socket = new Socket($ip, $port);
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$socket->read( ... );
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...
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}
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}
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?></strong>
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</pre>
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We would really like to have a mock object version of the socket
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here, what can we do?
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</p>
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<p>
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The first solution is to pass the socket in as a parameter,
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forcing the creation up a level.
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Having the client handle this is actually a very good approach
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if you can manage it and should lead to factoring the creation from
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the doing.
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In fact, this is one way in which testing with mock objects actually
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forces you to code more tightly focused solutions.
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They improve your programming.
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</p>
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<p>
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Here this would be...
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<pre>
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<?php
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require_once('socket.php');
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class Telnet {
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...
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<strong>function connect($socket, $username, $password) {
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$socket->read( ... );
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...
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}</strong>
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}
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?>
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</pre>
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This means that the test code is typical for a test involving
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mock objects.
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<pre>
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class TelnetTest extends UnitTestCase {
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...
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function testConnection() {<strong>
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$socket = new MockSocket();
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...
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$telnet = new Telnet();
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$telnet->connect($socket, 'Me', 'Secret');
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...</strong>
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}
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}
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</pre>
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It is pretty obvious though that one level is all you can go.
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You would hardly want your top level application creating
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every low level file, socket and database connection ever
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needed.
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It wouldn't know the constructor parameters anyway.
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</p>
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<p>
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The next simplest compromise is to have the created object passed
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in as an optional parameter...
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<pre>
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<?php
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require_once('socket.php');
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class Telnet {
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...<strong>
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function connect($ip, $port, $username, $password, $socket = false) {
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if (! $socket) {
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$socket = new Socket($ip, $port);
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}
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$socket->read( ... );</strong>
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...
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return $socket;
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}
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}
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?>
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</pre>
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For a quick solution this is usually good enough.
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The test now looks almost the same as if the parameter
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was formally passed...
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<pre>
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class TelnetTest extends UnitTestCase {
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...
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function testConnection() {<strong>
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$socket = new MockSocket();
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...
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$telnet = new Telnet();
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$telnet->connect('127.0.0.1', 21, 'Me', 'Secret', $socket);
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...</strong>
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}
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}
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</pre>
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The problem with this approach is its untidiness.
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There is test code in the main class and parameters passed
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in the test case that are never used.
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This is a quick and dirty approach, but nevertheless effective
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in most situations.
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</p>
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<p>
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The next method is to pass in a factory object to do the creation...
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<pre>
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<?php
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require_once('socket.php');
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class Telnet {<strong>
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function Telnet($network) {
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$this->_network = $network;
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}</strong>
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...
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function connect($ip, $port, $username, $password) {<strong>
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$socket = $this->_network->createSocket($ip, $port);
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$socket->read( ... );</strong>
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...
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return $socket;
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}
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}
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?>
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</pre>
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This is probably the most highly factored answer as creation
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is now moved into a small specialist class.
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The networking factory can now be tested separately, but mocked
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easily when we are testing the telnet class...
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<pre>
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class TelnetTest extends UnitTestCase {
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...
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function testConnection() {<strong>
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$socket = new MockSocket();
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...
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$network = new MockNetwork();
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$network->returnsByReference('createSocket', $socket);
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$telnet = new Telnet($network);
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$telnet->connect('127.0.0.1', 21, 'Me', 'Secret');</strong>
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}
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}
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</pre>
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The downside is that we are adding a lot more classes to the
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library.
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Also we are passing a lot of factories around which will
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make the code a little less intuitive.
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The most flexible solution, but the most complex.
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</p>
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<p>
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Well techniques like "Dependency Injection" tackle the problem of
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instantiating a lot of constructor parameters.
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Unfortunately knowledge of this pattern is not widespread, and if you
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are trying to get older code to work, rearchitecting the whole
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application is not really an option.
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</p>
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<p>
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Is there a middle ground?
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</p>
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<h2>
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<a class="target" name="creation"></a>Protected factory method</h2>
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<p>
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There is a way we can circumvent the problem without creating
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any new application classes, but it involves creating a subclass
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when we do the actual testing.
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Firstly we move the socket creation into its own method...
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<pre>
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<?php
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require_once('socket.php');
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class Telnet {
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...
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function connect($ip, $port, $username, $password) {
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<strong>$socket = $this->createSocket($ip, $port);</strong>
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$socket->read( ... );
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...
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}<strong>
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protected function createSocket($ip, $port) {
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return new Socket($ip, $port);
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}</strong>
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}
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?>
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</pre>
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This is a pretty safe step even for very tangled legacy code.
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This is the only change we make to the application.
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</p>
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<p>
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For the test case we have to create a subclass so that
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we can intercept the socket creation...
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<pre>
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<strong>class TelnetTestVersion extends Telnet {
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var $mock;
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function TelnetTestVersion($mock) {
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$this->mock = $mock;
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$this->Telnet();
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}
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protected function createSocket() {
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return $this->mock;
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}
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}</strong>
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</pre>
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Here I have passed the mock in the constructor, but a
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setter would have done just as well.
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Note that the mock was set into the object variable
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before the constructor was chained.
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This is necessary in case the constructor calls
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<span class="new_code">connect()</span>.
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Otherwise it could get a null value from
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<span class="new_code">createSocket()</span>.
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</p>
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<p>
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After the completion of all of this extra work the
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actual test case is fairly easy.
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We just test our new class instead...
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<pre>
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class TelnetTest extends UnitTestCase {
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...
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function testConnection() {<strong>
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$socket = new MockSocket();
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...
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$telnet = new TelnetTestVersion($socket);
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$telnet->connect('127.0.0.1', 21, 'Me', 'Secret');</strong>
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}
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}
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</pre>
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The new class is very simple of course.
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It just sets up a return value, rather like a mock.
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It would be nice if it also checked the incoming parameters
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as well.
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Just like a mock.
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It seems we are likely to do this often, can
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we automate the subclass creation?
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</p>
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<h2>
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<a class="target" name="partial"></a>A partial mock</h2>
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<p>
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Of course the answer is "yes" or I would have stopped writing
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this by now!
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The previous test case was a lot of work, but we can
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generate the subclass using a similar approach to the mock objects.
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</p>
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<p>
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Here is the partial mock version of the test...
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<pre>
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<strong>Mock::generatePartial(
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'Telnet',
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'TelnetTestVersion',
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array('createSocket'));</strong>
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class TelnetTest extends UnitTestCase {
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...
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function testConnection() {<strong>
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$socket = new MockSocket();
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...
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$telnet = new TelnetTestVersion();
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$telnet->setReturnReference('createSocket', $socket);
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$telnet->Telnet();
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$telnet->connect('127.0.0.1', 21, 'Me', 'Secret');</strong>
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}
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}
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</pre>
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The partial mock is a subclass of the original with
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selected methods "knocked out" with test
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versions.
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The <span class="new_code">generatePartial()</span> call
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takes three parameters: the class to be subclassed,
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the new test class name and a list of methods to mock.
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</p>
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<p>
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Instantiating the resulting objects is slightly tricky.
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The only constructor parameter of a partial mock is
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the unit tester reference.
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As with the normal mock objects this is needed for sending
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test results in response to checked expectations.
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</p>
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<p>
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The original constructor is not run yet.
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This is necessary in case the constructor is going to
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make use of the as yet unset mocked methods.
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We set any return values at this point and then run the
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constructor with its normal parameters.
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This three step construction of "new", followed
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by setting up the methods, followed by running the constructor
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proper is what distinguishes the partial mock code.
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</p>
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<p>
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Apart from construction, all of the mocked methods have
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the same features as mock objects and all of the unmocked
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methods behave as before.
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We can set expectations very easily...
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<pre>
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class TelnetTest extends UnitTestCase {
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...
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function testConnection() {
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$socket = new MockSocket();
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...
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$telnet = new TelnetTestVersion();
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$telnet->setReturnReference('createSocket', $socket);
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<strong>$telnet->expectOnce('createSocket', array('127.0.0.1', 21));</strong>
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$telnet->Telnet();
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$telnet->connect('127.0.0.1', 21, 'Me', 'Secret');
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}
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}
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</pre>
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Partial mocks are not used often.
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I consider them transitory.
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Useful while refactoring, but once the application has
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all of it's dependencies nicely separated then the
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partial mocks can wither away.
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</p>
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<h2>
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<a class="target" name="less"></a>Testing less than a class</h2>
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<p>
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The mocked out methods don't have to be factory methods,
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they could be any sort of method.
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In this way partial mocks allow us to take control of any part of
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a class except the constructor.
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We could even go as far as to mock every method
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except one we actually want to test.
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</p>
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<p>
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This last situation is all rather hypothetical, as I've hardly
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tried it.
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I am a little worried that
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forcing object granularity may be better for the code quality.
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I personally use partial mocks as a way of overriding creation
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or for occasional testing of the TemplateMethod pattern.
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</p>
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<p>
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It's all going to come down to the coding standards of your
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project to decide if you allow test mechanisms like this.
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</p>
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</div>
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References and related information...
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<ul>
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<li>
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SimpleTest project page on <a href="http://sourceforge.net/projects/simpletest/">SourceForge</a>.
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</li>
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<li>
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<a href="http://simpletest.org/api/">Full API for SimpleTest</a>
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from the PHPDoc.
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</li>
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<li>
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The protected factory is described in
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<a href="http://www-106.ibm.com/developerworks/java/library/j-mocktest.html">this paper from IBM</a>.
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This is the only formal comment I have seen on this problem.
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</li>
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</ul>
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<a href="index.html">SimpleTest</a>
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<a href="mock_objects_documentation.html">Mock objects</a>
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<span class="chosen">Partial mocks</span>
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<a href="reporter_documentation.html">Reporting</a>
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<a href="expectation_documentation.html">Expectations</a>
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<a href="web_tester_documentation.html">Web tester</a>
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<a href="form_testing_documentation.html">Testing forms</a>
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<a href="authentication_documentation.html">Authentication</a>
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<a href="browser_documentation.html">Scriptable browser</a>
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</div></div>
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<div class="copyright">
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Copyright<br>Marcus Baker 2006
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</div>
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</body>
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