// noprotect

​

var watch = (function() {

​

  function findIndex(array, fn)

  {

    var l = array.length;

    for (var i = 0; i < l; i++)

    {

      if (typeof(fn) === "function" && fn(array[i]) || array[i] === fn)

        return i;

    }

  }

  // keep a list of all listeners

  var listeners = [];

​

  return function watch(obj, prop, callback){

    if (!(prop in obj))

    {

      throw new Error("Dangerous use of watch, " + obj + " does not have [" + prop + "]");

    }

​

    // find an existing listener for the property on the given object

    var index = findIndex(listeners, function(l) {

       return (l.obj === obj && l.prop === prop);

    });

​

    var listener = listeners[index];

    // start watching all changes

    if (!listener)

    {

      // remember the current descriptor

      var descriptor = Object.getOwnPropertyDescriptor(obj, prop);

​

      listener = { obj: obj, prop: prop, descriptor: descriptor, callbacks: [] };

      listeners.push(listener);

​

      Object.defineProperty(obj, prop, {

        set: function(value) {

          if (descriptor.set)

            descriptor.set.call(obj, value);

          else

            obj.value = value;

​

          // call all callbacks

          listener.callbacks.forEach(function(cb) {

            cb(value);

          });

        },

        get: function() {

          return descriptor.get ? descriptor.get.call(obj) : obj.value;

        }

      });

    }

​

    listener.callbacks.push(callback);

​

    var canceled = false;

    return function cancelWatch() {

      if (canceled)

        throw new Error("Already canceled!");

      canceled = true;

​

      var callbackIndex = findIndex(listener.callbacks, function(cb) { return cb === callback; });

      listener.callbacks.splice(callbackIndex, 1);

      // remove the listener as soon as every callback has been removed

      if (listener.callbacks.length === 0)

      {

        Object.defineProperty(obj, prop, listener.descriptor);

​

        var listenerIndex = findIndex(listeners, function(l) { return l === listener; });

        listeners.splice(listenerIndex, 1);

      }

    };

  };

​

})();

​

​

​

// basic object

var o = {};

o.a = 1;

​

// add watchers

var c0 = watch(o, "a", function(v) {

  console.log("watch c0: " + v);

});

var c1 = watch(o, "a", function(v) {

  console.log("watch c1: " + v);

});

​

// change value

o.a = 2;

// expect 2 console statements (c0, c1)

​

// cancel watch c0

c0();

​

// change value

o.a = 3;

// expect 1 console statement (c1)

​

// add new watcher

var c2 = watch(o, "a", function(v) {

  console.log("watch c2: " + v);

});

​

// change value

o.a = 4;

// expect 2 console statement (c1)

​

// cancel watch c1, c2

c1();

c2();

​

// change value

o.a = 5;

// expect NO console statement (c1)

​

// object with custom get/set

var square = {

  l: 2,

  get area() { return this.l * this.l; },

  set area(a) { this.l = Math.sqrt(a); }

};

​

// add new watcher

var cSquare = watch(square, "area", function(v) {

  console.log("area: " + v);

});

​

// change value

square.area = 16;

// expect 1 console statement (cSquare) with "area: 16"

console.log(square.area + " == 16");

​

// cancel watch cSquare

cSquare();

​

// change value

square.area = 25;

// expect NO console statement

console.log(square.area + " == 25");