Wordpress - Testing custom API endpoint with class dependency - php

Question

Sorry I feel like really stuck here.
I have a plugin introducing a new Rest API controller (WP_REST_Controller) with basically a single endpoint which uses a separate class as a client to fetch some data. Let's say:
#my_plugin.php
function register_items_routes() {
if ( ! class_exists( 'WP_REST_My_Controller' ) ) {
require_once __DIR__ . '/class-wp-my-controller.php';
}
$controller = new WP_REST_My_Controller();
$controller->register_routes();
}
add_action( 'rest_api_init', 'register_items_routes' );
_
#class-wp-my-controller.php
class WP_REST_My_Controller extends WP_REST_Controller {
/**
* Registers the routes.
*/
public function register_routes() {
$namespace = 'my/namespace';
$path = 'get-items';
register_rest_route( $namespace, '/' . $path, [
array(
'methods' => 'GET',
'callback' => array( $this, 'get_items' ),
'permission_callback' => array( $this, 'get_items_permissions_check' )
),
] );
}
public function get_items_permissions_check( $request ) {
return true;
}
/**
* Get items from My_Class and return them.
*
* #param WP_REST_Request $request The incoming HTTP request.
*
* #return WP_REST_Response|WP_Error The response containing the items in JSON, WP_Error in case of error.
*/
public function get_items( $request ) {
$client = new My_Class();
try {
$items = $client->fetch_some_items();
} catch ( Exception $e ) {
return new WP_Error(
'some-client-error',
$e->getMessage()
);
// Code to be tested. - Do some stuff with items and return.
return new WP_REST_Response( $items );
}
How am I supposed to stub the My_Class dependency from PhpUnit in order to return a predefined set of items which I could test with?
public function test_get_items() {
$request = new WP_REST_Request( 'GET', '/my/namespace/get-items' );
$data = rest_get_server()->dispatch( $request );
$expected_items = [
'some_key1' => 'some_value1',
'some_key2' => 'some_value2',
];
$this->assertTrue( count($data['items']) == count($expected_items) );
}