JavaScript/Notes/Factory

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Contents

[edit] Factory Aspect, Added to a Decorator

This 2-part class will be a soup-to-nuts explanation of widget design, a description the Factory pattern, a recapitulation of Event Bubbling strategy, and generic concepts for creating and mixing design patterns.

[edit] Prerequisite

You'll need to have a good understanding of Functions and context, and Scope. Suggested review includes Singleton and the event bubbling section of ClassName Swap lesson.

[edit] Object Oriented Design

The basic principle for all design patterns is: Encapsulate the parts that vary.

Animation Transition

Pseudocode:

color:      rgb(12, 44, 89)  -->  rgb(233, 197, 17);
marginLeft: 0                -->  -9px;
padding:    0                -->  12px;
zIndex:     1                -->  100

Each animation's time duration is passed to it when it is called. The style values are supplied by the caller.

The part that varies is the Style Value Type, and that depends on the Style Property.

StyleTransition.js

[edit] Decorator

Factory is useful when you want to create at most one of an object for any associated element, off a bubbled event, using an object pool to "get or create" a wrapper, or Decorator.

[edit] Step 1: getById

function ElementWrapper(id, x) {
  this.id = id;
  this.x = x;
}
 
ElementWrapper.instances = {};
 
ElementWrapper.getById = function(id, x) {
  if(ElementWrapper.instances.hasOwnProperty(id)) return ElementWrapper.instances[id];
  return ElementWrapper.instances[id] = new ElementWrapper(id, x);
};

[edit] Problems

Problem: Not DRY. Cannot be arbitrarily reused other constructor functions. Problem: ArgumentList (§11.2) is set to two.

Problem: Encapsulation. The factory and the constructor are exposed publicly.

[edit] Decorator Factory Aspect

A Decorator Factory Aspect is a Factory method, added as an Aspect to a constructor of a Decorator.

Before we add a Factory to a constructor function for an element decorator, Let's define Decorator (also called a wrapper), Factory and Aspect.

Decorator Pattern
Add functionality to an object by wrapping it. (Wikipedia link)
Factory Pattern
The Factory pattern is a creational design pattern that encapsulates the processes of creating objects (Wikipedia link). That way, we can create objects lazily and pool them in an object pool.
Aspect
introduces separation of concern(s), as modularization of functionality (Wikipedia link)

[edit] Decorator Examples

Decorator is very common in JavaScript. For example: YAHOO.util.Element decorates an element, jQuery decorates a collection of elements.

[edit] Factory Example

The Factory gets or creates a decorated element. The id of the wrapper is the same as the id of the element. This is the part I want to make reusable:

/**
 * @constructor
 * @param {String} id - the id of the element and widget. 
 */
function ElementWrapper(id, x) {
  this.id = id;
  this.x = x;
}
 
// Factory. 
// TODO: How can I make this generic/reusable?
ElementWrapper.instances = {};
ElementWrapper.getById = function(id, x) {
  if(this.instances.hasOwnProperty(id)) return this.instances[id];
  return this.instances[id] = new this(id, x);
};
 
ElementWrapper.prototype = { 
  show : function() { 
    document.getElementById(this.id).style.visibility = "visible";
  }
};

[edit] Benefits

Solves the problem of creating only one decorator per element id.

By calling getElementById, the decorator can avoid some of the problems with changing node references with innerHTML (though state changes must still be managed manually).

[edit] Problem: DRY

Don't Repeat Yourself

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

It is cumbersome and error-prone to write out a Factory each time. Since this is an idiom I use a lot, it makes sense to make it reusable.

I want to have a generic getById method that can be reused and will return an instance of the constructor that it is called on. I want to be able to pass extra arguments to that constructor (varargs).

[edit] Problem: Publicly Exposed Constructor

A factory should not expose the constructor, but should hide it so that the factory method must be used. I will explain the solution to this problem in part II.

[edit] Encapsulate the Parts That Vary

What varies?

The id parameter variable of getById does not change; it will always be present in any generic Factory. The parts of the Factory that vary are: The additional zero or more arguments (varargs, this case, x), and the context, or thisArg.

Resolving the context arg is easy.

If I can solve passing varargs to a constructor in a generic context, it will be possible to create a generic Factory Aspect.

[edit] Function newApply

A way to call new with variable arguments would solve this problem. A new + apply() would provide the varargs functionality of apply, but passed to [[Construct]], not [[Call]]. (see Function.prototype.apply, [[Call]]).

The source code for newApply:

/** 
 * @param {Function} fun constructor to be invoked.
 * @param {Array} args arguments to pass to the constructor.
 * Instantiates a constructor and uses apply().
 */
function F(){};
function newApply(fun, args) {
    var i;
    F.prototype = fun.prototype; // Add prototype.
    F.prototype.constructor = fun;
 
    i = new F;
    fun.apply(i, args);  // Apply the original constructor.
    return i;
}

Usage

Early versions of this method appeared in APE JavaScript library, which has been copied.

[edit] What's newApply Good For?

Generic getById function I wanted, to use aspect with any constructor function.

[edit] Library Aspect
function getById(id) {
    if(!this.hasOwnProperty("instances")) this.instances = {};
    return this.instances[id] || (this.instances[id] = newApply(this, arguments));       
}
[edit] Implementation
function ElementWrapper(id, x, y) {
  this.id = id;
  this.x = x;
  this.y = y;
}
ElementWrapper.getById = getById;
[edit] Usage
ElementWrapper.getById("a", 2, 3).y; // 3

Example in jsbin: live example.

[edit] Using the Generic getById

This getById method used with ElementWrapper (above) or any other constructor that acts as a Decorator to an element and accepts the element's id as its first argument. All other arguments will be passed to the constructor using newApply.

Slider = function(id, dir) { /* ... */ };
 
// Factory. 
Slider.getById = getById;

Then I can use:

Slider.getById( "weight", 1 );

Subsequent calls to:

Slider.getById( "weight" );

— will return the same Slider instance.

[edit] More Examples

I have used a modified version of this approach for many widgets, including

Autocomplete (can't run off Github pages because there is no server side processing). This pattern is useful for building widgets that can be initialized lazily, on a bubbled event.

(slider uses overflow hidden).

[edit] Reusable Concept

Another closely related technique is Decorator that accepts an element instead of an element's id. This is covered by getByNode.


In most patterns, encapsulating the parts that vary entails creating an class. However, in JavaScript, this Factory pattern was simple to implement by using just two functions (getById) and leveraging the dynamic nature of JavaScript.

[edit] Links

Orthogonality and the DRY Principle, A Conversation with Andy Hunt and Dave Thomas, Part II by Bill Venners March 10, 2003

[edit] Step 2: Factory

[edit] Problem: Publicly Exposed Constructor

[edit] Solution: Hide the constructor in a closure.

var ElementWrapper = (function(){ 
 
  function ElementWrapper(id, x) {
    this.id = id;
    this.x = x;
    this.show = _show;
  }
 
  function _show() { 
    document.getElementById(this.id).style.visibility = "visible";
  }
 
  var instances = {};
  return { 
    getById : function(id, x) {
      if(instances.hasOwnProperty(id)) return instances[id];
      return instances[id] = new ElementWrapper(id, x);
    }
  };
})();

[edit] Step 3: The Factory Factory

Hide the constructor on a function and pass it to makeFactory.

[edit] Solution I: A Decorator Factory Factory That Takes a Constructor

Decorator Factory Factory takes a constructor that takes an element id as its first argument and any number of extra arguments.

// Create a factory.
var ElementWrapperFactory = makeFactory(function() {
 
// The constructor.
    function ElementWrapper (id, x, y) {
      this.id = id;
      this.x = x;
      this.y = y;
      this.show = _show;
    }
    function _show() { 
      document.getElementById(this.id).style.visibility = "visible";
    }
    return ElementWrapper;
  }()
);
 
 
// The Factory Factory.
function makeFactory(ctor) { 
  var instances = {};
 
  return { 
    getById : function(id) {
      return instances[id] || (instances[id] = newApply(ctor, arguments));
    }
  };
}
 
// Implementation, or usage.
var ew = ElementWrapperFactory.getById("globalWrapper", 1, 2);
alert(ew.x); // 1.

[edit] Add newApply to Scope of makeFactory

Function rewriting.

function makeFactory(ctor) {
  return (makeFactory = function(ctor) { 
    var instances = {};
 
    return { 
      getById : function(id) {
        return instances[id] || (instances[id] = newApply(ctor, arguments));
      }
    };
  })(ctor);
 
  function F(){};
  function newApply(fun, args) {
      var i;
      F.prototype = fun.prototype; // Add prototype.
      F.prototype.constructor = fun;
 
      i = new F;
      fun.apply(i, args);  // Apply the original constructor.
      return i;
  }
}

[edit] Introduce Parameter Object

Parameter Objects are useful replacements for parameter lists, particularly long ones, or sets of parameters that always go together.

In this case, Parameter Objects are useful because they can have any number of properties needed by any constructor.

[edit] Be Smart, not Clever

By using a config parameter object, we can define a parameter list of two, thereby avoiding the need for the clever newApply trick.

[edit] Solution II: Replace ArgumentList with Parameter Object

// Create a factory.
var ElementWrapperFactory = makeFactory(function() {
    function ElementWrapper (id, config) {
      this.id = id;
      this.x = config).x;
      this.show = _show;
    }
    function _show() { 
      document.getElementById(this.id).style.visibility = "visible";
    }
    return ElementWrapper;
  }()
);
 
 
 
// Config-based Factory Factory.
function makeFactory(ctor) { 
  var instances = {};
  return { 
    getById : function(id, config)) {
      if(instances.hasOwnProperty(id)) return instances[id];
      return instances[id] = new ctor(id, config));
    }
  };
}
 
 
// Implementation, or usage.
var ew = ElementWrapperFactory.getById("globalWrapper", { x : 1 });
alert(ew.x); // 1.

[edit] Solution IIb.

[edit] The Factory Constructor

function Factory(getConstructor){
  var i = 0, ctor;
  this.getById = getById;
 
  function getById(id, config) {
    var instances = this.instances;
    if(!instances) { // First time.
      instances = this.instances = {};
      // Get the constructor.
      ctor = getConstructor(this);
    }
    return instances[id] || (instances[id] = new ctor(id, config));
  }
}

[edit] Implementation

var ElementWrapper = new Factory(function() {
 
  var defaultConfig = { 
    title : "Hella",
  };
 
  function ElementWrapperC(id, config) {
    config = config || defaultConfig;
    this.id = id;
    this.title = config.title;
    initEvents(this);
  }
  function initEvents(ew) {
    document.getElementById(ew.id).onclick = showLog;
  }
 
  function showLog(ev) {
    console.log(ev);
  };
 
  return ElementWrapperC;
});

Tip: Make a defensive copy of config properties. That way, later changes to the config won't affect your object.

[edit] Usage

ElementWrapper.getById("globalWrapper").title

[edit] Source Code

Added to APE core, in variation, many years ago, and well-used. There, the instances property is publicly accessible so that a finalizer or "clean up" function can be run, for example, to remove event handlers. https://github.com/GarrettS/ape-javascript-library/blob/master/src/APE.js#L130

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