Phpunit: Add support for stubbing and mocking closures

[HeahDude]

| Q | A
| --------------------| ---------------
| PHPUnit version | 8.1
| PHP version | 7.x
| Installation Method | Composer / PHAR

Would you accept a PR to support mocking closure, to ease checking that one is called?

I.e : https://3v4l.org/Yaq5A

public function testClosureIsCalled()
{
    $callback = function () {};

    $this->assertClosureWillBeCalled($callback);

    SomeBusiness:caller($callback);
}

[sebastianbergmann]

I cannot promise acceptance of such a pull request but would, of course, consider it.

[morozov]

This is already possible by mocking __invoke() on stdClass:

public function testClosureIsCalled() : void
{
    $closure = $this->createPartialMock(stdClass::class, ['__invoke']);

    $closure->expects(
        $this->exactly(2)
    )->method('__invoke');

    $closure();
    $closure();
}

Unlike this workaround, the proposed API is not really flexible, however, it'd be nice if the workaround was formalized as a feature.

[veewee]

@morozov
The provided solution results in:

createPartialMock() called with method(s) __invoke that do not exist in stdClass. This will not be allowed in future versions of PHPUnit.

An API for this might be nice indeed!

[ChrisHSandN]

After some experimentation there still seems to be a method which works:

$mock = $this
  ->getMockBuilder(\stdclass::class)
  ->addMethods(['__invoke'])
  ->getMock();

$mock
  ->expects(self::once())
  ->method('__invoke'));

(Tested on PHPUnit 9 / PHP 7.3)