Question

I want to use goog.addSingletonGetter() (http://closure-library.googlecode.com/svn/!svn/bc/4/trunk/closure/goog/docs/closure_goog_testing_singleton.js.source.html#line56) to add a getInstance() method to a class with a constructor that accepts arguments:

Foo = function(x, y) {...};

Is there a way for me to specify goog.addSingletonGetter(Foo, arg1, arg2)? Calls to Foo.getInstance() would then lazily return the instantiated Foo object.

Answer 1

Closure: The Definitive Guide on page 70 defines goog.addSingletonGetter() as follows:

For a class with a constructor that takes zero arguments, goog.addSingletonGetter()
adds a static method to its constructor function named getInstance() that returns the
same instance of that object whenever it is called. Generally, it should be used instead
of the constructor function if it exists.

One approach would be to create a singleton following the Instance in a Static Property design pattern presented in JavaScript Patterns on page 143 with the addition of a static getInstance() function.

/**
 * @param {string=} opt_x
 * @param {string=} opt_y
 * @constructor
 */
Foo = function(opt_x, opt_y) {

  if(Foo.instance_) {
    return Foo.instance_;
  }

  /**
   * @type {string}
   * @private
   */
  this.x_ = goog.isDefAndNotNull(opt_x) ? opt_x : 'default_X';

  /**
   * @type {string}
   * @private
   */
  this.y_ = goog.isDefAndNotNull(opt_y) ? opt_y : 'default_Y';

  Foo.instance_ = this;
};


/**
 * Get the singleton instance of Foo.
 * @param {string=} opt_x
 * @param {string=} opt_y
 */
Foo.getInstance = function(opt_x, opt_y) {
  if (Foo.instance_) {
    return Foo.instance_;
  }
  return new Foo(opt_x, opt_y);
};

The advantage to using this pattern is that it protects you from accidentally constructing multiple instances in the event that someone were to write:

var foo = new Foo();

... 
// thousands of lines later
var snafoo = new Foo(); // Returns the singleton instance.