PHP OOP: do something when object is assigned as a property

Question

Consider this hypothetical scenario:

I have a Stomach class, which has a contents property, to which objects of class Food should be assigned. One way to do that would be to use a kind of a setter, e.g. setContents($food).

But lets suppose that we assign food directly, as in $stomach->contents = $orange.

Suppose also that whenever a Food object is assigned to contents we need to change the object's eaten property to true. Is there a way to do that without using a method of an object that it's being assigned to (in this case, the $stomach's setter)?

Basically, my question is: can we call a method whenever an object is assigned as a property to another object? Also, even if it is possible, is it bad design? Sorry if this is a stupid question, all of this is pretty new to me.

Answer 1

The best OOP solution here would be to create a method that indicates an action, like eat().

To ensure that the right Object is eaten, you could define an Interface (Say Food). This interface may define a method setEaten(). The Stomach (though I would prefer Mammal or something similar that actually can eat) can then call setEaten().

Since it created some controversy in the comments, I want to point out that an object definition should as closely as possible reflect what it actually does. To reduce object coupling it is best to avoid directly accessing object properties from an other class, however there is nothing wrong with using setters instead of actions when it makes sense (note that I used one in the definition of Food), and it may often depend on the developer view.

However, here it makes sense. Consider the case "A monkey eats a banana" (for simplicity please allow me to just materialize that banana out of thin air).

$m = new Monkey();
$m->eat(new Banana());

perfect. Now lets try a setter.

$m->setContents(new Banana());

Now we have a problem, the monkey already contains a lot of things, like bones, blood, muscles a brain, etc. So setContents makes no sense here. You could try $m->getStomach()->setContents(new Banana()) but that would just increase object coupling.

Answer 2

I agree with @dualed about the eat method. That said the way to acheive this to make all properties private/protected and then use __get/__set to proxy to the setters.

class Stomach {

   protected $contents;


   public function setContents(Food $food) {
      $this->contents = $food;
      $food->eaten = true;    
   }


   public function __set($name, $value) {
      $method = array($this, 'set' . $name);
      if(is_callable($method)) {
         return call_user_func_array($method, array($value));
      }
   }

  public function __get($name) {
     $method = array($this, 'get'.$name);
     if(is_callable($method)) {
        return call_user_func($method);
     }
  }
}

Youll notic i use is_callable as opposed to method_exists because if youre working on somehting complex with virtual methods is_callable should take those in to account whereas method_exists relies on the method being defined in the class hierarchy.

Answer 3

__set function will help you here. But make sure you dont have any property named contents defined in class. Do it like this

class Stomach{
    private $props = array();
    public __set($prop, $value){
        if($prop === 'contents' and $value instanceof Food){
            $this->prop[$prop] = $value;
        }
    }
}