gistfile1.txt

class aController {
    private $service;
    public function __construct(aService $service) {
        $this->service = $service;
    }

    public function get_items() {
        return $this->service->get_items();
    }

    public function register_routes() {
        // register my routes here
    }
}

// my test
function test_controller() {
    // Create mock
    $mock = Mockery::mock(aService::class);
    $expected_data = array('hi');
    $mock->shouldReceive('get_items')->andReturn($expected_data);

    // Override the routes with mock
    $setup_controller = function() use ($mock) {
        $aController = new aController($mock);
        $aController->register_routes();
    }
    add_action( 'rest_api_init', $setup_controller, 0 );

    // test
    $request  = new WP_REST_Request( 'GET',  'get_items');
    $response = rest_get_server()->dispatch( $request );
    $response_data     = $response->get_data();    
    $this->assertEquals($expected_data, $response_data);
}

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