add_action reference a class

Question

Is it possible to reference a class instead of a function in 'add_action'? Can't seem to figure it out. Here is just a basic example of the function in question.

add_action( 'admin_init', 'MyClass' );
class MyClass {
     function __construct() {
          .. This is where stuff gets done ..
     }
}

So yeah, that doesn't work. I've also tried:

$var = new MyClass();
add_action( 'admin_init', array( &$var ) );

And:

$var = new MyClass();
add_action( 'admin_init', array( &$var, '__construct' ) );

And also:

add_action( 'admin_init', 'MyClass::__construct' );

Is there anyway I can do this without having to create a separate function that loads the class? I'd like to be able to just run the classes constructor through the add_action. That's all that needs to be loaded to get the ball rolling.

Answer 1

No, you cannot 'initialise' or instantiate the class through a hook, not directly. Some additional code is always required ( and it is not a desirable thing to be able to do that, as you're opening a can of worms for yourself.

Here is a better way of doing it:

class MyClass {
     function __construct() {
          add_action('admin_init',array(&$this,'getStuffDone'));
     }
     function getStuffDone() {
          // .. This is where stuff gets done ..
     }
}
$var = new MyClass();

Of course one could create an interface class to simplify it for the general case even further:

class IGetStuffDone {
    function IGetStuffDone(){
        add_action('admin_init',array(&$this,'getStuffDone'));
    }
    public abstract function getStuffDone();
}

Which would let you say:

class CDoingThings extends IGetStuffDone {
    function getStuffDone(){
        // doing things
    }
}
$var = new CDoingThings();

Which would then automatically add all the hooks, you just need to define what exactly is being done in a subclass and then create it!

On Constructors

I wouldn't add a constructor as a hook function, it's bad practice, and can lead ot a lot of unusual events. Also in most languages a constructor returns the object that is being instantiated, so if your hook needs to return something like in a filter, it will not return the filtered variable as you want, but instead it will return the class object.

Calling a constructor or a destructor is very, very, very bad programming practice, no matter which language you're in, and should never be done.

Constructors should also construct objects, to initialise them ready for use, not for actual work. Work to be done by the object should be in a separate function.

Static class methods, and not needing to instantiate/initialise at all

If your class method is a static class method, you can pass the name of the class in quotes rather than &$this as shown below:

class MyClass {
     public static function getStuffDone() {
          // .. This is where stuff gets done ..
     }
}
add_action('admin_init',array('MyClass','getStuffDone'));

Closures & PHP 5.3

Sadly you cannot avoid the line creating the new class. The only other solution to skipping it would involve boiler plate code that still has that line, and would require PHP 5.3+ e.g.:

add_action('admin_init',function(){
    $var = new MyClass();
    $var->getStuffDone();
});

At which point you may as well skip the class, and just use a function:

add_action('admin_init',function(){
    // do stuff
});

Answer 2

Example class

Notes:

• Init the class only once
  • Call on priority 0, so you can use the same hook with the default priority later
  • Wrap it up in a ! class_exists to avoid calling it twice and place the init caller inside
• Make the init function and the class var static
• Call the constructor from inside your init, when you call the class new self.

Here's an example

if ( ! class_exists( 'WPSESampleClass' ) )
{
    // Init the class on priority 0 to avoid adding priority inside the class as default = 10
    add_action( 'init', array ( 'WPSESampleClass', 'init' ), 0 );

class WPSESampleClass
{
    /**
     * The Class Object
     */
    static private $class = null;

    public static function init()
    {
        if ( null === self::$class ) 
            self :: $class = new self;

        return self :: $class;
    }

    public function __construct()
    {
        // do stuff like add action calls:
        add_action( 'init', array( $this, 'cb_fn_name' ) );
    }

    public function cb_fn_name()
    {
        // do stuff 
    }
} // END Class WPSESampleClass

} // endif;

Php 5+

Please, leave the & out. We're already beyond php4. :)

Answer 3

Generally speaking, you wouldn't add an entire class to a hook. The add_action()/add_filter() hooks expect callback functions, which can be referenced from within a class.

Let's say that you have an init() function inside your class, that you want to hook into the WordPress init hook.

Put your add_action() call inside your class, and then identify the callback like so:

add_action( 'init', array( &$this, 'init' ) );

(Note: I'm assuming your class is properly namespaced; otherwise, be sure to namespace your callback functions.)

Answer 4

You should be able to do it by passing the class name instead of the instantiated object:

add_action( 'init', array( 'MyClass', '__construct' ) );

(In theory, your other solution should work too

$var = new MyClass();
add_action( 'admin_init', array( &$var, '__construct' ) );

Not sure off the head why it doesn't. Maybe if you don't call by reference?)