Can I mock singletons?

[Luc45]

Hello,

Can I mock a class with a protected/private constructor and a public static method ::instance()? (The classic Singleton setup)

Eg:

class Foo {
  protected static $instance;

  protected function __construct(){}

  public static function instance() {
    if ( ! self::$instance ) {
	self::$instance = new self;
    }

    return self::$instance;
  }
}

I know I shouldn't be using singletons anyway, but this is what the code I'm trying to test looks like.

Thanks.

[stof]

Well, you won't be able to mock the singleton directly, as you would need a way to instantiate the double object and to inject it in the Foo::$instance property.
And that's not something we want to support in Prophecy itself.

[ciaranmcnulty]

No, there's no sensible way to do that

The important thing about a singleton is that it is enforcing the constraint that there's a single instance - the global availability is an unfortunate abused side-effect of the pattern. You can refactor your objects to take the singleton in their constructor as part of testing

[Luc45]

Well, you won't be able to mock the singleton directly, as you would need a way to instantiate the double object and to inject it in the Foo::$instance property.
And that's not something we want to support in Prophecy itself.

Like Reflection? Indeed, I'm not sure how to approach that if Prophecy don't plan to support it... 🤔

You can refactor your objects to take the singleton in their constructor as part of testing

Yes, that's exactly what I'm doing. The codebase is WordPress, for an enterprise plugin that's been around for 10 years now. Getting rid of singletons is just not an option. We have now a DI container in place, and I'm injecting the dependencies in the constructor, Singletons included.

That's where the "Can I mock a singleton" question comes in. Since I'm feeding it in the constructor, I would like to mock it to test the class that takes it as a dependency.

Thanks for the help so far.

[ciaranmcnulty]

You can certainly mock it then; you won't need to call getInstance() on the mock because you already have the instance

[Luc45]

Hmm, I don't have the instance because I can't call new (protected/private constructor).

For instance:

class Foo {
    private $singleton;
 
    public function __construct( Singleton $singleton ) {
        $this->singleton = $singleton;
    }
 
    public function do_something() {
        if ( ! $this->singleton->hasCap() ) {
            // Don't do something
            return false;
        }
 
        // Do something
        return true;
    }
}

class FooTest {
    public function should_do_something_if_has_caps() {
        $singleton = $this->prophesize(Singleton::class);
        $singleton->hasCap()->willReturn( true );
        $foo = new Foo( $singleton->reveal() );
 
        $this->assertTrue( $foo->do_something() );
    }
 
    public function should_do_something_if_does_not_has_caps() {
        $singleton = $this->prophesize(Singleton::class);
        $singleton->hasCap()->willReturn( false );
        $foo = new Foo( $singleton->reveal() );
 
        $this->assertFalse( $foo->do_something() );
    }
}

I'm pretty sure (I've tried it yesterday but can't quite remember), that I can't test Foo as in the examples, because trying to mock Singleton fails because of the private/protected constructor:

[ciaranmcnulty]

Ah "can't mock private constructor" sounds like a more specific issue