As a personal challenge, I find it good practice to learn new programming languages. Each programming language has it's perk and benefits, but what boggles my mind is how javascript replicates almost all of the great features of other languages, even if its not there by default.
The other day I was studying Objective-C, which is quite a good language. It has the ability of observing property changes that extends NSObject. This feature really is wonderful, and I thought, "man I wish javascript had this feature". As I thought that, I said to myself, "I could probably implement this fairly easily with getters and setter in javascript". Now all that is history, and I did it. I implemented it, I tested it, and I am now going to use it as a foundation for all of my javascript applications.
There are two global objects in by code below, window.child, and a window.parent. In the html it list some data about the parent and the child. I have set up the property observing by using the "observe" method of the objects. The callbacks on these observers update the DOM. So open up your javascript console and have some fun with this html page. Change the last name of the parent and see what happens to the child data, and by consequence the DOM. Its exciting.
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342 | <html>
<head>
<style>
body {
margin: 0;
padding: 0;
font-family: arial;
}
.person {
border-radius: 10px;
}
.child {
background-color:#123456;
color:#fff;
padding:10px;
}
.parent {
background-color:#666;
color:#fff;
padding:10px;
}
</style>
</head>
<body>
<div class="parent">
<h1>Parent</h1>
<p>First Name: <span id="parentFirstName">John</span></p>
<p>Last Name: <span id="parentLastName">Smith</span></p>
<p>Age: <span id="parentAge">28</span></p>
</div>
<div class="child">
<h1>Child</h1>
<p>First Name: <span id="childFirstName">Jane</span></p>
<p>Last Name: <span id="childLastName">Smith</span></p>
<p>Age: <span id="childAge">5</span></p>
</div>
<script>
(function(){
var Event = function (type) {
if (!(this instanceof Event)) {
return new Event(type);
}
var event = this;
event.__defineGetter__("type", function(){return type;});
};
window.makeClassObservable = function (OldClass) {
if (typeof OldClass !== "function"){
throw new Error("Expected a constructor as the first argument.");
}
//Create a new constructor and return it.
var NewClass = function () {
var instance = this;
//Old Class
if (Object !== OldClass) {
OldClass.apply(this, arguments);
}
//Listener object.
var propertyListeners = {};
var changeListeners = [];
instance.Event = Event;
//Emits a property change event, and invokes callbacks.
instance.notifyObservers = function (event) {
if (event instanceof Event && instance.notifyObservers.enabled) {
for (var y = 0 ; y < changeListeners.length ; y++){
changeListeners[y].apply(instance, arguments);
}
var listenerArray = propertyListeners[event.type];
if (listenerArray) {
for (var x = 0; x < listenerArray.length; x++) {
listenerArray[x].apply(instance, arguments);
}
}
}
};
instance.notifyObservers.enabled = true;
//Adds listeners
instance.observe = function (type, callback) {
if (arguments.length === 2){
if (!propertyListeners[type]) {
propertyListeners[type] = [];
}
propertyListeners[type].push(callback);
} else if (typeof type === "function"){
changeListeners.push(type);
}
};
//Emits the property callback once and then removes it for you.
instance.observeOnce = function (type, callback) {
callback = callback || type;
if (arguments.length === 2){
var once = function (e) {
callback.apply(this, arguments);
instance.unobserve(type, once);
};x
instance.observe(type, once);
} else if (arguments.length === 1) {
var once = function(e){
callback.apply(this, arguments);
instance.unobserve(once);
};
instance.observe(once);
}
};
//You can remove all listeners of a specified type or refer to a specific listener to remove.
instance.unobserve = function (type, callback) {
if (callback && typeof type === "string" && propertyListeners[type]) {
var listenerArray = propertyListeners[type];
var index = listenerArray.indexOf(callback);
if (index>-1){
listenerArray.splice(index,1);
}
} else if (typeof type === "function"){
for (var a in propertyListeners){
var index = propertyListeners[a].indexOf(type);
if (index > -1){
propertyListeners[a].splice(index,1);
}
}
index = changeListeners.indexOf(type);
if (index > -1){
changeListeners.splice(index,1);
}
} else if (typeof type === "string" && typeof callback === "undefined") {
propertyListeners[type] = [];
}
};
//This is the object where we store the variable values.
var variables = {};
//For loop wrapped with an instant invocation of an anonomous function to preserve scope.
for (var x in instance) (function (x) {
if (typeof instance[x] !== 'function') {
variables[x] = instance[x];
var hasGetter = instance.__lookupGetter__(x);
var hasSetter = instance.__lookupSetter__(x);
//If it has a getter already, then let the getter handle the response.
if (hasGetter) {
instance.__defineGetter__(x, function () {
return hasGetter.apply(instance, arguments);
});
} else {
instance.__defineGetter__(x, function () {
return variables[x];
});
}
if (hasSetter) {
instance.__defineSetter__(x, function (val) {
var fireEvents = instance.notifyObservers.enabled;
var oldValue = variables[x];
var event = new Event(x);
event.__defineGetter__("newValue", function(){return val;});
event.__defineGetter__("oldValue", function(){return oldValue;});
event.__defineGetter__("property", function(){return x;});
event.target = instance;
if (fireEvents) {
instance.notifyObservers.enabled = false;
};
hasSetter.apply(instance, arguments);
if (fireEvents) {
instance.notifyObservers.enabled = true;
}
//Trigger event
if (oldValue !== val && instance[x]===val) instance.notifyObservers(event);
});
} else {
if (!hasGetter && !hasSetter) {
instance.__defineSetter__(x, function (val) {
var event = new Event(x);
var oldValue = variables[x];
var event = new Event(x);
event.__defineGetter__("newValue", function(){return val;});
event.__defineGetter__("oldValue", function(){return oldValue;});
event.__defineGetter__("property", function(){return x;});
variables[x] = val;
//Trigger event
if (oldValue !== val) instance.notifyObservers(event);
});
}
}
}
})(x);
return instance;
};
var oldClass = new OldClass();
NewClass.prototype = oldClass;
return NewClass;
};
})();
var Person = function(options){
if (!(this instanceof Person)){
return new Person();
}
options = options || {};
this.firstName = options.firstName || "Unknown";
this.lastName = options.lastName || "Unknown";
this.age = options.age || 0;
this.children = options.children || [];
var spouse = null;
this.__defineGetter__("spouse", function(){return spouse});
this.__defineSetter__("spouse", function(val){
if (val instanceof Person){
spouse = val;
}
});
};
window.ObservablePerson = makeClassObservable(Person);
window.parent = new ObservablePerson({
firstName:"John",
lastName:"Smith",
age: 28
});
window.child = new ObservablePerson({
firstName:"Jane",
lastName: "Smith",
age: 5
});
// Now the child automatically changes its last name if the parent does.
parent.observe("lastName", function(e){child.lastName = e.newValue});
// This is where it gets really fun and interesting.
// Listen for the parent to change and update the DOM.
parent.observe("firstName", function(e){
document.getElementById("parentFirstName").innerHTML = e.newValue;
});
parent.observe("lastName", function(e){
document.getElementById("parentLastName").innerHTML = e.newValue;
});
parent.observe("age", function(e){
document.getElementById("parentAge").innerHTML = e.newValue;
});
// Listen for the child to change and update the DOM.
child.observe("firstName", function(e){
document.getElementById("childFirstName").innerHTML = e.newValue;
});
child.observe("lastName", function(e){
document.getElementById("childLastName").innerHTML = e.newValue;
});
child.observe("age", function(e){
document.getElementById("childAge").innerHTML = e.newValue;
});
/*
This observes any changes made to an object. Just pass a callback function without a type.
It will listen for any change made and call the callback.
*/
child.observe(function(e){console.log(e.property + " was changed!");});
/*
To unobserve a global callback, just pass the function to
unobserve. e.g.
var fn = function(){};
//global observe
child.observe(fn);
child.unobserve(fn);
//specific observe
child.observe("firstName", fn);
//Clear like so
child.unobserve(fn);
//Or
child.unobserve(type, fn);
//Or, This one clears all callbacks for that specific property.
child.unobserve(property);
*/
</script>
</body>
</html>
|
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