PHP OOP classes and functions

Question

I am not sure how to name this, but here it goes. Lets suppose i have the following

class A {
    public function aa() {
       $this->bb();
    }
    public function bb() {

    }
}

class B extends a {

}

class C {
   __construct(B $service) {
       $this->service = $service;
   }
   public function aa() {

       $this->service->aa();
   }
}

My call in code will be

$C = new C(new B());
$C->aa();

So this will basically execute A:aa() which is what i want. As you can see, in A::aa() AA::bb() is called.

What I need. When AA::bb() is called i want to execute some code defined in class C, but I am not allowed to change the A class. I can only change the B class or the C class.

My idea was to add a listener in the B class and overwrite the bb() function like this

class B extends a {
    public $listener;
    bb() {
       parent::bb();
       $this->listener();
    }
}

class C {
   __construct(B $service) {
       $this->service = $service;
   }
   public function aa() {

       $this->service->listener = function() { }
       $this->service->aa();
   }
}

But I don't like this idea a lot, doesn't look like a good one. What are my options here?

Again, I CANNOT change the A class and i can only call the C class.

PHP version is 5.3

Since you cannot keep listener logic directly in class B, there are not too much options. I would move $this->service->listener = function() { } to the constructor tho. Why you don't like it? — Alex Blex
– Alex Blex, Commented Sep 20, 2017 at 13:41

tereško · Accepted Answer · 2017-09-20 13:31:38Z

1

You have two options. Extend or decorate.

First one would be kinda what you have already written, though, I would not use public visibility for the listener:

class Foo extends A {
    private $listener;
    public function setListener(callable $func) {
        $this->listener = $func;
    }
    public function bb() {
        call_user_func($this->listener);
        return parent:bb();
    }
}

In the example I passed the listener via setter injection, but you can also use constructor injection and pass the $listened in the overloaded __construct() method. When you extend a class, the "interface restriction" does not aply to the constructor's signature.

The other approach is to use a decorator:

class Foo {
    private $target;
    public function __construct(A $target) {
        $this->target = $target;
    }

    public function bb($callback) {
        $callback();
        return $this->target->bb();
    }

    public function __call($method, $arguments) {
        return call_user_func_array( 
                array( $this->target, $method ),
                $arguments
            );
    }
}

The second approach would let you alter the interface.

Which option you pick depend on the exact functionality you actually need to implement. The decorator is a solution for, when you need drastic change in the objects behavior - for example, it is really good for adding access control.

answered Sep 20, 2017 at 13:31

tereško

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2 Comments

keepwalking Over a year ago

Thanks for your answer. What i omitted from the question is that the code that has to be executed needs to be inside the C class. In your example, Foo is the B class or the C class?

tereško Over a year ago

it's the B class .. you vague class/method don't really help

bishop · Accepted Answer · 2017-09-20 13:52:14Z

I understand that you want to execute code in C after code in A completes. You cannot change A.

As written, C::aa calls A::aa, which calls A::bb and the stack unwinds. Why not just do the work in C::aa after the service call finishes?

class C {
   public function aa() {
       $this->service->aa();
       // whatever you want to do
   }
}

If, on the other hand, you need to call code after A::aa is called but before A::bb is called then the example you posted would suffice with clarity:

class B extends a {
    public $listener;
    public function bb() {
       call_user_func($this->listener);
       parent::bb();
    }
}

Note the use of call_user_func, which is necessary for PHP 5.3 to call an anonymous function stored in a member variable.

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