MooTools: Track Class Instances With the TrackInstances Mutator

(Script also on GitHub and MooTools Forge)

It’s great to track all of your classes’ instances for debugging purposes or mass altering.

Huh? Mass Altering?

  • For instances of some Audio/Video player classes: stop/pause all players
  • For instances of positioned elements: recalculate position of all elements upon window resize
  • Call hide method of all instances of a class upon an event

I needed it more than once and I used to track instances from within any class I needed it:

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var MyClass=new Class({
  initialize:function () {
      MyClass.instances.push(this);
  },
  recalcPosition:function () {
      // something that recalculates position or any other task that should happen on all instances
  }
});

MyClass.instances=[];

// from another code:
MyClass.instances.each(function (instance) {
  instance.recalcPosition();
});

However, I got too many classes with this behavior. I ended up writing a MooTools Class Mutator to automate the process:

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/*
---

script: Class.Mutators.TrackInstances.js

description: Allows a class to track its instances by having instances array as a class property

license: MIT-style license

authors:
- Elad Ossadon ( http://devign.me | http://twitter.com/elado )

requires:
- core:1.2.4

provides: [Class.Mutators.TrackInstances]

...
*/

Class.Mutators.TrackInstances=function (allow) {
  if (!allow) return;

  // save current initialize method
  var oldInit=this.prototype.initialize;
  var klass=this;

  // overwrite initialize method
  klass.prototype.initialize=function () {
      (klass.instances=klass.instances || []).push(this);
      oldInit.apply(this,arguments);
  };
};

Usage:

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var MyClass=new Class({
  initialize:function () {
  },

  TrackInstances:true,

  recalcPosition:function () {
      // something that recalculates position or any other task that should happen on all instances
  }
});

var x=new MyClass();
var y=new MyClass();

MyClass.instances; // [x, y]
MyClass.instances.length; // 2

// from another code:
window.addEvent("resize",function () {
  MyClass.instances.each(function (instance) {
      instance.recalcPosition();
  });
});

The only constraint is that the ”initialize” declaration in an object should be prior to the TrackInstances:true. That’s because mutators are called in the same loop as all other methods, and if the loop didn’t get to a certain method, then the mutator won’t know it. According to these tickets, the MooTools team won’t fix it anytime soon.