Last modified February 10, 2015.
This page is obsolete; current versions are on my github pages at github.bililite.com/understanding-widgets.html. This page is being kept here for historical purposes.
This was written largely to help me make sense of using UI to create my own widgets,
but I hope it may help others. "Widget" to me means a user-interface element, like
a button or something more complicated like a popup date picker, but in jQuery UI terms
it means a class, members of which are associated with HTML elements; things like
Draggable and
Sortable.
In fact, not everything that I would have called a widget uses $.widget
; the UI datepicker does not.
Dan Wellman has another tutorial that you may find helpful.
Modifying Elements: Plugins
That being as it may, let's use $.widget
.
Let's take a paragraph of class target:
<p class="target">This is a paragraph</p>
And lets make it green. We know how; $('.target').css({background: 'green'})
.
Now, make it more general-purpose: a plugin:
$.fn.green = function() {return this.css({background: 'green'})};
But this allows us to perform some behavior on the selected elements; it does not leave us with any way to
keep our plugin associated with that element, so we can do something with it later, like
$('.target').off()
to remove the green background, but only if we used green to
put it there in the beginning. We also have no way of associating
state with the element, to do $('.target').darker(), which would require knowing how green the element is now.
Keeping State in Plugins
We could create an object and associate it with an element using javascript expandos:element.myobject = new Myobject({'target': element})
. Sample code would be:
$.fn.green2 = function() {
return this.each(function(){
if (!this.green) this.green = new Green($(this)); // associate our state-keeping object with the element
this.green.setLevel(15);
});
};
$.fn.off = function() {
return this.each(function(){
if (this.green) this.green.setLevel(16);
delete this.green; // recover the memory
});
};
$.fn.darker = function() {
return this.each(function(){
if (this.green) this.green.setLevel(this.green.getLevel()-1);
});
};
$.fn.lighter = function() {
return this.each(function(){
if (this.green) this.green.setLevel(this.green.getLevel()+1);
});
};
function Green(target){
greenlevels = ['#000','#010','#020','#030','#040','#050','#060','#070','#080','#090','#0a0','#0b0','#0c0','#0d0','#0e0','#0f0','#fff'];
this.target = target; // associate the element with the object
this.level = 0;
this.getLevel = function() { return this.level; }
this.setLevel = function(x) {
this.level = Math.floor(Math.min(greenlevels.length-1, Math.max(0,x)));
this.target.css({background: greenlevels[this.level]});
}
};
But this pollutes the $.fn
namespace terribly, with off
, darker
and lighter
.
There are ways to create real namespaces within $.fn
, but the usual design pattern is to use a string to
specify which function to call. Thus, element.green2()
to instantiate the plugin,
element.green2('darker')
or element.green2('lighter')
to manipulate it:
$.fn.green2 = function(which){
return this.each(function(){
if (which === undefined){ // initial call
if (!this.green) this.green = new Green($(this)); // associate our state-keeping object with the element
this.green.setLevel(15);
}else if (which == 'off'){
if (this.green) this.green.setLevel(16);
delete this.green
}else if (which == 'darker'){
if (this.green) this.green.setLevel(this.green.getLevel()-1);
}else if (which == 'lighter'){
if (this.green) this.green.setLevel(this.green.getLevel()+1);
}
});
};
function Green(target){
greenlevels = ['#000','#010','#020','#030','#040','#050','#060','#070','#080','#090','#0a0','#0b0','#0c0','#0d0','#0e0','#0f0', '#fff'];
this.target = target; // associate the element with the object
this.level = 0;
this.getLevel = function() { return this.level; }
this.setLevel = function(x) {
this.level = Math.floor(Math.min(greenlevels.length-1, Math.max(0,x)));
this.target.css({background: greenlevels[this.level]});
}
};
<p class="target">This is a test paragraph</p>
The Problems with Associating an Object with a Plugin
But you get into trouble with circular
references (note that "this.green = new Green($(this))
" gives a DOM element a reference to a javascript object
and
"this.target = target
" gives a javascript object a reference to a DOM element) and
memory leaks:
browsers (notably Internet Explorer) uses different garbage collectors for DOM elements and javascript objects.
Circular references mean that each garbage collector thinks the other object is in use and won't delete them.
We also need to remember to reclaim the memory (with delete
) if we no longer need the plugin.
jQuery solves the circular reference problem with the $.fn.data
plugin:
$(element).data('myobject', new Myobject({'target': element}))
. But now we've got a lot of "paperwork" to
keep track of, and it hides the underlying program logic. As we know,
design patterns reflect language weakness.
If we are
constantly re-implementing a pattern, we need to abstract it and make it automatic.
Solving the Problem: $.widget
That's where$.widget
comes
in. It creates a plugin and an associated javascript class and ties an instance of that class with each
element so we can interact with the object and act on the element, without getting into trouble with
memory leaks.
You still need to create the constructor of your class, but instead of a real constructor function, you need
a prototype object with all the relevant methods. There are a few conventions: the function _create
is called on construction, _init
is called both on construction and for re-initializing
the function and destroy
is called on removal. All of these are predefined as empty functions but you can override them (and most likely
will need to override _init
). element
is the associated jQuery object (what we called target
above).
Widget methods that start with "_
" are pseudo-private; they cannot be called with the $(element).plugin('string') notation
var Green3 = {
_init: function() { this.setLevel(15); },
greenlevels: ['#000','#010','#020','#030','#040','#050','#060','#070','#080','#090','#0a0','#0b0','#0c0','#0d0','#0e0','#0f0', '#fff'],
level: 0,
getLevel: function() { return this.level; },
setLevel: function(x) {
this.level = Math.floor(Math.min(this.greenlevels.length-1, Math.max(0,x)));
this.element.css({background: this.greenlevels[this.level]});
},
darker: function() { this.setLevel(this.getLevel()-1); },
lighter: function() { this.setLevel(this.getLevel()+1); },
off: function() {
this.element.css({background: 'none'});
this.destroy(); // use the predefined function
}
};
Notice it's all program logic, no DOM or memory-related bookkeeping. Now we need to create a name, which must be preceded by a
namespace, like "ns.green
" . Unfortunately the namespacing is fake; the plugin is just called $().green()
. The constructor function
is $.ns.green
, but you never use that, so you might as well use the "official" namespace of "ui
". But defining the widget couldn't be easier:
$.widget("ui.green3", Green3); // create the widget
Manipulating Widgets
What about our manipulating functions? All the functions defined in the prototype that don't start with an underscore are exposed automatically:
$('.target').green3()
creates the widgets; $('.target').green3('darker')
manipulates them.
There are two kinds of plugin functions: getters and setters. Setters manipulate elements and can be chained in jQuery code; they just return the jQuery object they started with. For example, $('.demo').css({color: 'red'}).text('stuff')
. Getters return information and break the chain, like text = $('.demo').text()
. widget
distinguishes between the two by the return value: if your function returns any value other than undefined
, it assumes that it is a getter and returns that value (for the first element in the jQuery object, just like all jQuery getter functions). If it does not return a value, then it is assumed to be a setter and is called on each element in the jQuery object, and the jQuery object itself is returned for chaining.
<p class="target">This is a test paragraph.</p>
Pass arguments to the manipulating functions after the name: $('.target').green3('setLevel', 5)
.
Data for Each Widget
The astute reader will have noticed that level
is a class variable; the same variable is used
for every green3 object. This is clearly not what we want; each instance should have its own copy.
$.widget defines an object
this.options
for per-widget data.
Thus:
var Green4 = {
getLevel: function () { return this.options.level; },
setLevel: function (x) {
var greenlevels = this.options.greenlevels;
var level = Math.floor(Math.min(greenlevels.length-1, Math.max(0,x)));
this.options.level = level;
this.element.css({background: greenlevels[level]});
},
_init: function() { this.setLevel(this.getLevel()); }, // grab the default value and use it
darker: function() { this.setLevel(this.getLevel()-1); },
lighter: function() { this.setLevel(this.getLevel()+1); },
off: function() {
this.element.css({background: 'none'});
this.destroy(); // use the predefined function
},
options: { // initial values are stored in the widget's prototype
level: 15,
greenlevels: ['#000','#010','#020','#030','#040','#050','#060','#070','#080','#090','#0a0','#0b0','#0c0','#0d0','#0e0','#0f0', '#fff']
}
};
$.widget("ui.green4", Green4);
And on creating an instance of a widget, pass an options object (the way most plugins do)
and override the defaults:
$('.target').green4({level: 8})
.
Note that I also put the list of colors into the defaults object, so it too can be overridden. This widget probably shouldn't be called "green" anymore!
<p class="target">This is a test paragraph.</p>
<p class="target">
This is a test paragraph called with .green4({
level:3,
greenlevels: ['#000','#00f','#088', '#0f0', '#880', '#f00', '#808', '#fff']
}).
</p>
Callbacks, or, Keeping the Lines of Communication Open
The programmer who is embedding our widget in his page may want to do other things when the widget changes state. There are two ways to alert the calling program that something has happened:
- Tightly Coupled
- The caller can provide a function to call at the critical point. jQuery jargon calls this a "callback;" it's used in
animations and
Ajax. We can create callback functions
that the widget calls at critical points, and pass them to the widget-constructing plugin like any other option:
var Green5 = { setLevel: function(x){ //... this.element.css({background: greenlevels[level]}); var callback = this.options.change; if ($.isFunction(callback)) callback(level); }, // ... rest of widget definition }; $.widget("ui.green5", Green5); $('.target').green5({change: function(x) { alert ("The color changed to "+x); } });
- Loosely Coupled
- Also called the Observer Design Pattern, the widget sends a signal to
the programming framework and the calling program informs the framework that it wants to know about the signal. Events like
clicks and keystrokes work like this, and jQuery allows the widget to create custom events and for the calling program to
bind an event handler to that custom event:
var Green5 = { setLevel: function(x){ //... this.element.css({background: greenlevels[level]}); this.element.trigger ('green5change', level); }, // ... rest of widget definition }; $.widget("ui.green5", Green5); $('.target').green5(); $('.target').bind("green5change", function(evt,x) { alert ("The color changed to "+x); });
$.widget
allows both forms with the _trigger
method. In a widget object,
this._trigger(type, event, data)
takes a type {String}
with the
name of the event you want (use some short verb, like 'change'
) and optionally a
$.Event object (if you want to pass things like timestamps and mouse locations.
Don't worry about event.type
; _trigger
changes it to the constructed event name), and any data to be passed
to the handler. _trigger
creates a custom event name of widgetName+type
, like green6change
(why it doesn't do naming events this way has been the subject of some discussion), sets type+'.'+widgetName
the way jQuery expects
is beyond meevent.type = custom event name
(creating a new $.Event
if it was not provided)
and calls this.element.trigger(event, data)
and then looks for a callback with
callback = this._getData(type)
and calls it with
callback.call(this.element[0], event, data)
.
Notice that this means the function signature is slightly different for the event handler and the callback if data
is an array. element.trigger()
uses apply
to turn each item in the array into a separate argument.
So this._trigger('change', 0, ['one', 'two'])
requires an event handler of the form function(event, a, b)
and a callback of the form
function(event, data)
.
In practice, it's not as complicated as it sounds. For example, using both methods:
var Green5 = {
getLevel: function () { return this.options.level; },
setLevel: function (x) {
var greenlevels = this.options.greenlevels;
var level = Math.floor(Math.min(greenlevels.length-1, Math.max(0,x)));
this.options.level = level;
this.element.css({background: greenlevels[level]});
this._trigger('change', 0, level);
},
_init: function() { this.setLevel(this.getLevel()); }, // grab the default value and use it
darker: function() { this.setLevel(this.getLevel()-1); },
lighter: function() { this.setLevel(this.getLevel()+1); },
off: function() {
this.element.css({background: 'none'});
this._trigger('done');
this.destroy(); // use the predefined function
},
options: {
level: 15,
greenlevels: ['#000','#010','#020','#030','#040','#050','#060','#070','#080','#090','#0a0','#0b0','#0c0','#0d0','#0e0','#0f0', '#fff']
}
};
$.widget("ui.green5", Green5);
<p class="target">This is a test paragraph with green level <span class="level">undefined</span>.</p>
// The on button above does the following:
$('.target').green5({
change: function(event, level) { $('.level', this).text(level); } // callback to handle change event
});
$('.target').bind('green5done', function() { $('.level', this).text('undefined');alert('bye!') }); // event handler for done event
Involving the Mouse
Now, a lot of what we want to do with widgets involves mouse tracking, so ui.core.js
provides a mixin object that
includes lots of useful methods for the mouse. All we need to do is add the $.ui.mouse
widget to our
widget prototype:
var Green6 = {mouse-overriding function and widget-specific functions};
$.widget ('ui.green6', $.ui.mouse, Green6);
And override $.ui.mouse
's functions (_mouseStart
,
_mouseDrag
,
_mouseStop
) to do something useful,
and call this._mouseInit
in your this._init
and this._mouseDestroy
in your this.destroy
. The mouse defaults are automagically including in your options object; see the mouse code for details.
Let's add some mouse control to our greenerizer:
Green6 = $.extend({}, $.ui.green5.prototype, { // leave the old Green5 alone; create a new object
_init: function(){
$.ui.green5.prototype._init.call(this); // call the original function
this._mouseInit(); // start up the mouse handling
},
destroy: function(){
this._mouseDestroy();
$.ui.green5.prototype.destroy.call(this); // call the original function
},
// need to override the mouse functions
_mouseStart: function(e){
// keep track of where the mouse started
this.xStart = e.pageX; // not in the options object; this is not something that can be initialized by the user
this.levelStart = this.options.level;
},
_mouseDrag: function(e){
this.setLevel (this.levelStart +(e.pageX-this.xStart)/this.options.distance);
},
options: {
level: 15,
greenlevels: ['#000','#010','#020','#030','#040','#050','#060','#070','#080','#090','#0a0','#0b0','#0c0','#0d0','#0e0','#0f0', '#fff'],
distance: 10
}
});
$.widget("ui.green6", $.ui.mouse, Green6);
<p class="target">This is a test paragraph with green level <span class="level">undefined</span>.</p>
The ever-alert reader will note what we've just done: subclassed green5 to make green6, including calls
to "super" methods. This ought to be abstracted out
into its own method, something like
$.ui.green5.subclass("green6", $.ui.mouse, {mouseStart: function(){}, mouseDrag: function(){}})
but that's a topic for another day.
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