register child class in another plugin

Question

I think this should be relatively easy, but somehow I cannot figure out how to do this. essentially there's a preexisting plugin that does this:

add_action('widgets_init', create_function('', 'register_widget("Foo");'));
if ( ! class_exists( 'Foo' ) ) {
class Foo extends WP_Widget {
        /*constructs etc*/
}}

now i would like to write another plugin (extension essentially), that extends the above. I tried to do something like the following (and also tried - in vain - to find some examples/documentation )

add_action('widgets_init', create_function('', 'register_widget("Bar");'));
class Bar extends Foo {     
    function __construct() {
        /*do stuff*/
    }
}

but all i ever get is

<b>Fatal error</b>:  Class 'Foo' not found in ...

so I suppose it's a question of making 'Bar' execute AFTER Foo, but I dunno how.

any hints/ideas/snippets or even just links to docs etc would be much appreciated as I am failing at the first hurdle here..:(

Answer 1

I question the wisdom of extending a class from another plugin. You have no idea what will happen next time that other plugin gets updated.

However, generally speaking you can control when in a hook "queue" a function runs by passing a third parameter-- a priority. So pass a priority high enough and your function should run after the other plugin's function.

add_action(
  'widgets_init', 
  create_function('', 'register_widget("Bar");')
  ,100
);

I don't know if that will work in your case. I've never tried it.

You should also be aware that you could have trouble because of plugin load order.

Changing Plugin Load Order
Does an activated plugin automatically mean its methods are available to other WP functions?

widget_init runs after plugins_loaded so that probably won't be an issue in you case.

On further investigation... I get errors involving missing variables required by the constructor. The following works.

if ( ! class_exists( 'Foo' ) ) {
  class Foo extends WP_Widget {
      /*constructs etc*/
      function __construct($id = 'twidg', $descr = 'Test Widget', $opts = array()) {
    $widget_opts = array();
    parent::__construct($id,$descr,$widget_opts);
      /*do stuff*/
      }
  }
}
function rw_cb() {
register_widget("Foo");
}
add_action('widgets_init', 'rw_cb');

class Bar extends Foo {     
    function __construct() {
      $widget_opts = array();
      parent::__construct('twidgextended','Test Widget 2',$widget_opts);
    }
}
function rw_cb_2() {
  register_widget("Bar");
}
add_action('widgets_init', 'rw_cb_2');

When each component is pasted into a dummy plugin file, it works. The only time I get a fatal error involving a class file not found is if I try to activate the plugin with the Bar class definition, before the plugin with the Foo definition, which makes sense.

Wrapping that block as follows solves that issue though and you can activate in any order.

if (class_exists('Foo')) {
  class Bar extends Foo {     
      function __construct() {
    $widget_opts = array();
    parent::__construct('twidgextended','Test Widget 2',$widget_opts);
      }
  }

  function rw_cb_2() {
    register_widget("Bar");
  }
  add_action('widgets_init', 'rw_cb_2');
}

Answer 2

ah, but you did point me in the right direction (specifically your second link). the following (as it stands in my scenario at least ) seems to do the trick. thank you.

  add_action( 'widgets_init', 'extend_foo' );
function extend_foo(){
    if (class_exists( 'Foo' ) ) {
        class Bar extends Foo {
            function __construct() {
                parent::__construct();          
            }   
        }
    }
}

(although I'm not sure if this is the most elegant way of doing things though...but if it works....)