Mocking callables in an Expressive app

While working with Zend Expressive, a PSR-7 middleware microframework, I wanted to apply some unit testing with a nice coverage to my middlewares. Middlewares are called by the __invoke method if you provide them as an object and not as a closure. The signature of the __invoke method looks like this:

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;

class FooMiddleware 
{
    public function __invoke(
        ServerRequestInterface $request, 
        ResponseInterface $response, 
        callable $next = null
    ): ResponseInterface;
}

Additionally, my middleware implementation does some stuff, but the middleware itself does not return a response, which is fine. Instead, my implementation calls the $next middleware in line, which looks like this:

public function __invoke(
    ServerRequestInterface $request,
    ResponseInterface $response,
    callable $next
): ResponseInterface {
    // Doing the cool stuff

    // Everything ends sometime
    return $next($request, $response);
}

Now this has given me some questions on how to mock the $next argument while writing the unit test. Turns out, this is quite easy:

	
    // ...
    
    $callback = $this->createPartialMock(
        \stdClass::class, 
        ['__invoke']
    );
    $callback->expects($this->once())
        ->method('__invoke')
        ->with($serverRequest, $inputResponse)
        ->willReturn($outputResponse); // ResponseInterface

    $fooMiddleware = new FooMiddleware();
    $fooMiddlware(
        $serverRequest, // ServerRequestInterface
        $inputResponse, // ResponseInterface
        $callback       // callable
    );
    
    // ...