jquery class inheritance

问题:

var A=function(){
};

$.extend(A.prototype, {
    init:function(){
        alert('A init');
    }
});
var B=function(){

};

$.extend(B.prototype,A.prototype,{
    init:function(){
        alert('B init');
    }
});
var p=new A();
p.init();
var x=new B();
x.init();

is the above the best way to create class and inheritance in jQuery? In B's init how do I invoke parent's init (similar to super.init() in OO languages)?

回答1:

For OO, it's best to look outside jQuery. jQuery is based on collections returned by selectors.

If you want classes, some choices are Base2, Joose, and JS.Class.

回答2:

John Resig created a snippet for simple inheritance here. http://ejohn.org/blog/simple-javascript-inheritance/

he stores the super class to a _super variable so you can call it like such

this._super();

you can reference his code snippet to get a better idea of what he another helpful post is: http://alexsexton.com/?p=51

回答3:

How to invoke parent's methods:

var B=function(){
    A.call(this);
};

$.extend(B.prototype,A.prototype,{
        init:function(){
                A.prototype.init.call(this);
                alert('B init');
        }
});

回答4:

If you don't want to depend on any other libraries, you can do this:

function A() {}
A.prototype.foo = function() {};

function B() {
    A.call(this);
    //Or, if there are arguments that need to be passed to A(),
    //this might be preferable:
    //A.apply(this, arguments);
}

B.prototype = new A();

//Or, if the browser supports ECMAScript 5 and/or you have a shim for Object.create,
//it would be better to do this:
B.prototype = Object.create(A.prototype);

$.extend(B.prototype, {
   //set the constructor property back to B, otherwise it would be set to A
   constructor: B,
   bar: function() {}
});

Make sure to define any properties in the constructor rather than on the prototype, e.g.:

function A() {
    this.baz = null;
}

This avoids having unintentionally shared prototype properties.

There are some libraries that make prototypal inheritance easier:

• https://github.com/mbrowne/simpleoo.js (my library; its documentation expounds on some of the concepts I've mentioned here)
• https://github.com/Gozala/selfish
• https://github.com/Raynos/pd

Notes:

• Any time a prototype is replaced, including by extension, it's a best practice to set its constructor property back to the correct constructor. That's why we set B.prototype.constructor to B. If you were replacing A.prototype you should do it like this:

...

A.prototype = {
    constructor: A,
    foo: function() {}
    //other methods...
}

• B.prototype = Object.create(A.prototype) is preferred over B.prototype = new A() because it helps you detect it early if you forgot to call A() from B()'s constructor; it also allows A() to have required parameters. You'll need a shim for older browsers; the simplest shim (although it doesn't support the full Object.create spec) is at the bottom of this page: http://javascript.crockford.com/prototypal.html.

回答5:

I use the same pattern and I like its conciseness.

About the lack of the "super" keyword, that's not really a problem. Thanks to Function.prototype.call() operator, you can call any function within the context of any object. So the sequence to call A.prototype.init() from B.prototype.init() is:

A.prototype.init.call(this, some parameters ...);

Also, don't forget that you may call A constructor from B constructor like this:

B = function(key, name) {
    A.call(this, key);
    this.name = name;
};

An experimented JS coder will know what happens.

So to conclude: not perfect but close enough.

回答6:

I was looking for something similar. None of the answers given really appealed to me, so I finally had a crack at it myself...

http://jsfiddle.net/tn9upue0/1/

Example Classes

• $.Animal() creates a generic animal, with a default of 4 legs, that can be passed a name in it's options, and can describe itself. $.Dog() is a subclass of Animal that goes "woof", and may know some tricks. $.Cat() is a subclass of Animal that goes "meow". $.Bird() is a subclass of Animal that has 2 legs and goes "tweet".

Class implementation

• Each animal subclass creates an instance of $.Animal called parent, which can be used later to call methods of the parent. When calling a parent method, context can be important. When it is, the method should be called via $.proxy() passing this as the context.

Example output

My name is unknown. I am an animal with 4 legs.

My name is Rover. I am an animal with 4 legs. I say "woof". I can sit, stay, and roll over.

My name is Mittens. I am an animal with 4 legs. I say "meow".

My name is unknown. I am an animal with 2 legs. I say "tweet".

Sample code

$.Animal = function (options) {
    return {
        options: options || {},

        _getName: function () {
            return this.options.name || 'unknown';
        },

        _getLegs: function () {
            return 4;
        },

        describe: function () {
            return 'My name is ' + this._getName() + '. I am an animal with ' + this._getLegs() + ' legs.';
        }
    }
};

$.Dog = function (options) {
    var parent = $.Animal(options);
    return $.extend({}, parent, {
        describe: function () {
            var s = $.proxy(parent.describe, this)() + ' I say  "woof".';
            if (this.options.tricks) {
                s += ' I can ' + this.options.tricks + '.';
            }
            return s;
        }
    });
};

$.Cat = function (options) {
    var parent = $.Animal(options);
    return $.extend({}, parent, {
        describe: function () {
            return $.proxy(parent.describe, this)() + ' I say  "meow".';
        }
    });
};

$.Bird = function (options) {
    var parent = $.Animal(options);
    return $.extend({}, parent, {
        _getLegs: function () {
            return 2;
        },

        describe: function () {
            return $.proxy(parent.describe, this)() + ' I say "tweet".';
        }
    });
};

var animal = $.Animal(),
    rover = $.Dog({name: 'Rover', tricks: 'sit, stay, and roll over'}),
    mittens = $.Cat({name: 'Mittens'}),
    bird = $.Bird();
$('#out').html(
    animal.describe() + '<br>' +
        rover.describe() + '<br>' +
        mittens.describe() + '<br>' +
        bird.describe()
);