I've recently been spending more time with JavaScript. I thought I'd make something like google.maps.MVCObject (from Google Maps JavaScript API) as a way for me to learn more about JavaScript and the Closure Library. So, here's what I've got, so far.
Understanding MVCObject
First off, here's how MVCObject behaves (at least, this is my understanding).
- When a property 'foo' of objA is bound to a property 'foo' of objB, the following happens:
- Setting value x as property 'foo' on objB, results to objA.get('foo') returning the same value x.
- Calling objA.get('foo') is equivalent to calling objB.get('foo'). In fact, objA.get('foo') returns the same object returned by objB.get('foo').
- Setting value x as property 'foo' on objA is actually setting value x as property 'foo' on objB.
- Calling objB.set('foo', x) notifies objA (and all objects bound to its 'foo' property) about 'foo_changed'. This effectively makes listening to objB's 'foo_changed' event equivalent to listening to objA's 'foo_changed' event.
- When a property 'bar' of objA is bound to a property 'foo' of objB, and property 'foo' of objB is bound to a property 'foo' of objC, the following happens:
- Calling objA.set('bar') is equivalent to calling objB.set('foo'), which, in turn, is equivalent to calling objC.set('foo')
- Calling objA.get('bar') is equivalent to calling objB.get('foo'), which, in turn, is equivalent to calling objC.get('foo')
- ...You get the point.
This binding technique can save memory and provide better performance, since the object (or property value) is not duplicated. When bounded, Objects A and B actually point to the same value for their property 'foo'. So, if the object value happens to be a large object with several properties, then there's significant memory to be saved (since they point to the same object, and it is not duplicated).
MVCObject Tests
Second, here's how I started with some unit tests to help guide me in writing the MVCObject class with the Closure Library.
goog.require('goog.events');
goog.require('goog.testing.jsunit');
goog.require('example.MVCObject');
function testSetAndGet() {
var objA = new example.MVCObject();
objA.set('foo', 2);
assertEquals(2, objA.get('foo'));
assertEquals(2, objA.foo);
}
function testBindObjectAToObjectB() {
var objA = new example.MVCObject();
var objB = new example.MVCObject();
objA.bindTo('foo', objB);
objB.set('foo', 2);
assertEquals(2, objA.get('foo'));
assertEquals(objB.get('foo'), objA.get('foo'));
}
function testDispatchesChangedEventWhenPropertyIsSet() {
var objA = new example.MVCObject();
var called = false;
goog.events.listenOnce(objA, 'foo_changed', function(event) {
called = true;
});
objA.set('foo', 2);
assertTrue(called);
}
function testUnbind() {
var objA = new example.MVCObject();
var objB = new example.MVCObject();
objA.bindTo('foo', objB);
objB.set('foo', 2);
objA.unbind('foo');
objB.set('foo', 3);
assertEquals(2, objA.get('foo'));
}
MVCObject Implementation
Lastly, here's my implementation of MVCObject.
goog.provide('example.MVCObject');
goog.require('goog.events');
goog.require('goog.events.EventTarget');
/**
* Base class implementing KVO (key-value object).
*
* @constructor
* @extends {goog.events.EventTarget}
*/
example.MVCObject = function() {
// Call super constructor
goog.events.EventTarget.call(this);
};
goog.inherits(example.MVCObject, goog.events.EventTarget);
/**
* @type {string}
* @private
*/
example.MVCObject.EVENT_SUFFIX_ = '_changed';
/**
* Returns the value of a property.
*
* @param {string} key the name of the property.
* @return {*} the value of a property.
*/
example.MVCObject.prototype.get = function(key) {
var binding = this.getBindings_()[key];
if (binding) {
// Use binding to get bounded object and return its value.
return binding.target.get(binding.targetKey);
} else {
return this[key];
}
};
/**
* Sets a property.
*
* @param {string} key the name of the property.
* @param {*} value the new value of the property.
*/
example.MVCObject.prototype.set = function(key, value) {
var bindings = this.getBindings_();
if (key in bindings) {
// Use bindings[key] to get bounded object and set its value.
var binding = bindings[key];
binding.target.set(binding.targetKey, value);
} else {
this[key] = value;
this.notify(key);
}
};
/**
* Sets a collection of key-value pairs.
* @param {Object} values a collection of key-value pairs.
*/
example.MVCObject.prototype.setValues = function(values) {
for (var key in values) {
this.set(key, values[key]);
}
};
/**
* Lazily initialize bindings map.
* @return {Object} the bindings map.
* @private
*/
example.MVCObject.prototype.getBindings_ = function() {
if (!this.bindings_) {
this.bindings_ = {};
}
return this.bindings_;
};
/**
* Adds a binding between the given property and a target property.
*
* @param {string} key the name of the property.
* @param {example.MVCObject} target the target object.
* @param {string} targetKey the name of the property on the target object.
* @param {boolean=} opt_noNotify optional flag to indicate that no
* *_changed call-back shall be called.
* @private
*/
example.MVCObject.prototype.addBinding_ = function(
key, target, targetKey, opt_noNotify) {
this.getBindings_()[key] = {
target: target,
targetKey: targetKey
};
if (!opt_noNotify) {
this.notify(key);
}
};
/**
* Lazily initialize listeners map.
* @return {Object} the listeners map.
* @private
*/
example.MVCObject.prototype.getListeners_ = function() {
if (!this.listeners_) {
this.listeners_ = {};
}
return this.listeners_;
};
/**
* Binds the property identified by 'key' to the specified target.
*
* @param {string} key the name of the property to be bound.
* @param {example.MVCObject} target the object to bind to.
* @param {string=} opt_targetKey the optional name of the property on the
* target, if different from the name of the property on the observer.
* @param {boolean=} opt_noNotify optional flag to indicate that *_changed
* call-back shall not be called upon binding.
*/
example.MVCObject.prototype.bindTo = function(
key, target, opt_targetKey, opt_noNotify) {
opt_targetKey = opt_targetKey || key;
this.unbind(key);
this.getListeners_()[key] =
goog.events.listen(
target, opt_targetKey + example.MVCObject.EVENT_SUFFIX_,
function() { this.notify(key); }, undefined, this);
this.addBinding_(key, target, opt_targetKey, opt_noNotify);
};
/**
* Removes a binding of a property from its current target. Un-binding
* will set the unbound property to the current value. The object will
* not be notified, as the value has not changed.
*
* @param {string} key the name of the property to be un-bound.
*/
example.MVCObject.prototype.unbind = function(key) {
var listeners = this.getListeners_();
var listenerKey = listeners[key];
if (listenerKey) {
goog.events.unlistenByKey(listenerKey);
delete listeners[key];
this[key] = this.get(key);
delete this.getBindings_()[key];
}
};
/**
* Removes all bindings.
*/
example.MVCObject.prototype.unbindAll = function() {
var listeners = this.getListeners_();
for (key in listeners) {
this.unbind(key);
}
};
/**
* Notify all observers of a change on this property. This notifies both
* objects that are bound to the object's property as well as the object
* that it is bound to.
*
* @param {string} key the name of the property that was changed.
*/
example.MVCObject.prototype.notify = function(key) {
this.changed(key);
// Dispatch *_changed event to all listeners
this.dispatchEvent(key + example.MVCObject.EVENT_SUFFIX_);
};
/**
* Generic handler for state changes. Override this in derived classes to
* handle arbitrary state changes.
*
* @param {string} key the name of the property that changed.
*/
example.MVCObject.prototype.changed = function(key) {
};
/**
* @override
*/
example.MVCObject.prototype.disposeInternal = function() {
example.MVCObject.superClass_.disposeInternal.call(this);
this.unbindAll();
};
I find Closure's way of declaring classes and methods to be very close to Java. It's event management makes user-defined custom events, and DOM/browser events, the same. It also provides a JsUnit-based testing framework. It's very rich in features.
It's making me think (or re-think) if there is value in having all these features under one roof. I mean, when I'm considering building a single-page app, I'd go look at backbone.js, which uses underscore.js. This may also mean using jQuery for DOM manipulation. I find that most (if not all) of the features of these three (3) JavaScript libraries can be found in Google's Closure Library (and possibly more). Will there be significant code savings when using one common JavaScript library?
All in all, I'm liking Closure. I'll spend more time learning it.
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