Most of the mocking support Craft provides is inherited through Codeception and PHP Unit. 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.

# mockMethods

The parameters you can pass into mockMethods are:

# 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 Externals

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:

        '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 mockMethods uses Codeception\Stub::construct(). You can read more about this method in the Codeception documentation.

# mockCraftMethods

mockCraftMethods is a pass through function that calls mockMethods. mockCraftMethods can be called via $this->tester within your tests. The only difference is that the Craft::$app object is passed as the first argument into the mockMethods call. Argument 2, 3 and 4 of mockMethods are applicable and available within mockCraftMethods.

# 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.