Most of the mocking support Craft provides is inherited through Codeception (opens new window) and PHP Unit (opens new window). Their documentation provides all the basic information you need to get started.
Additionally Craft provides some minor additional support to help mock your dependencies and improve your tests.
The parameters you can pass into
# Module $module
An instance of the
yii\base\Module class to which your mocked component will be set.
# string $component
The class of the component that you want to mock
# array $methods = 
An array of methods where the
key is the method name and the
value is the returned result - can also be a callback function.
# array $constructParams = 
The parameters that must be passed into the constructor when creating the mock.
Let's say you have a module/plugin called
Mailchimp that facilitates an integration between Craft and Mailchimp. For this integration you may need to create a method called
getUsersFromMailchimp that makes a GET request to Mailchimp. Now in your test, you don't want to actually make a GET request to the Mailchimp servers. Let's say all requests to the Mailchimp servers are done via a service called
Now seeing as you don't want to make call the Mailchimp servers in a test environment you would have to mock this method. For this, you can use
mockMethods. Mocking your
Externals service would look something like this:
$this->tester->mockMethods([ Mailchimp::getInstance(), Externals::class, [ 'getUsersFromMailchimp' => [['user1'], ['user2']], ],  ]);
What the above would do is ensure that if
Mailchimp::getInstance()->externals->getUsersFromMailchimp() is called in your tests the value
[['user1'], ['user2']] will always be returned. No querying to the Mailchimp servers will be done. This ensures predictable tests that are more performant.
You can call this method multiple times in a single test if you want to mock out multiple components.
Under the hood
Codeception\Stub::construct(). You can read more about this method in the Codeception documentation.
mockCraftMethods is a pass through function that calls
mockCraftMethods can be called via
$this->tester within your tests. Argument 2, 3 and 4 of
mockMethods are applicable and available within
mockCraftMethods. The only difference is that the
Craft::$app object is passed as the first argument into the
# Full mock
Craft provides a
fullMock setting that can be enabled in your
codeception.yml file. A full explanation of this setting is given here. The
fullMock option ensures all components in
Craft::$app are set to mocks using PHP Unit. If you prefer to isolate all your dependencies during testing this option is for you.
fullMock also mocks any modules/plugins you define.