YUI事件体系之Y.Do

YUI团队在种种场合不断的夸耀自己的事件体系是多么强大：

YUI 3′s Event module is one of the strengths of the library –Eric Miraglia, YUI Theater — Luke Smith: “Events Evolved”
YUI 3 is not all about DOM manipulation — it also contains a robust set of class/object management tools, not to mention our powerful custom events –Tilo Mitra, 10 Things I Learned While Interning at YUI
One of the strengths of the YUI App Framework is that it’s integrated tightly with the rest of YUI and benefits from YUI’s fantastic event system and plugin/extension infrastructure. –Ryan Grove, How can I decided whether to choose YUI 3’s MVC or Backbone for a new project?

事实的确如此吗？就使用YUI的开发者反馈来看，应该是不错的：

AFAIK YUI 3’s event system is the most sophisticated of any JavaScript framework. Am I wrong in thinking that? –Walter Rumsby
I love the event system in YUI. Pure awesomeness. –Kevin Isom
I am constantly impressed by the degree of excellence I find in working with the YUI3 framework –Andrew Wooldridge, Cross YUI Communication and Custom Events

作为一名YUI用户，我对其事件体系的强大深有体会。从本篇文章起，我将对YUI事件机制做一个全面分析。

本次我们介绍的是比较基础的两个对象Y.EventHandle和Y.Do。千里之行积于跬步，YUI整套事件机制也是从这两个对象开始构筑的。

Y.EventHandle

Y.EventHandle的作用很简单：注销事件/消息监听。

Y.EventHandle = function (evt, sub) {this.evt = evt; // 事件对象this.sub = sub; // 监听对象
};
Y.EventHandle.detach = function () {this.evt._delete(this.sub); // 执行event对象的_delete方法，注销事件/消息监听return true;
};

Y.Do

Y.Do的作用是：向对象方法前面或者后面插入其它方法(前置、后置方法)，以达到动态修改对象行为的目的。这种方式，也称作AOP。

示例

让我们先来看个简单的例子：

// 例1
YUI().use('event-custom', function (Y) {var cat = {eat: function () {console.log('eat a fish');}};cat.eat(); // output: eat a fishvar beforeHandle = Y.Do.before(function () {console.log('catch a fish');}, cat, 'eat');var afterHandle = Y.Do.after(function () {console.log('done!');}, cat, 'eat');cat.eat(); // output: catch a fish, eat, done!afterHandle.detach();cat.eat(); // output: catch a fish, eat
});

在不修改原对象方法的基础上，可以方便的添加前置、后置方法，并且注销这些方法也很容易。Y.Do非常漂亮的解决了我们动态修改对象方法的需求！很难想象，如果不用Y.Do代码会复杂成怎样。

源代码分析

接下来，让我们看看YUI的内部实现吧。这是多么有趣的事，就像小时候买把手枪，想不明白为什么可以射击，就砸开一看究竟。

为了更容易的看懂代码的核心，我做了适当的简化，感兴趣的朋友可以去看未删节的源码。

// 代码版本为YUI3.4.1，YUI3.5.0对Y.Do的实现有所改进
var DO_BEFORE = 0,DO_AFTER = 1;
Y.Do = {// 缓存处理对象objs: {},before: function (fn, obj, sFn) {return this._inject(DO_BEFORE, fn, obj, sFn);},after: function (fn, obj, sFn) {return this._inject(DO_AFTER, fn, obj, sFn);},_inject: function (when, fn, obj, sFn) {var id = Y.stamp(obj), o, sid;if (!this.objs[id]) this.objs[id] = {};o = this.objs[id];if (!o[sFn]) {// 创建保存对象、方法名的Method对象o[sFn] = new Y.Do.Method(obj, sFn);// 修改对象方法obj[sFn] = function () {return o[sFn].exec.apply(o[sFn], arguments);};}sid = id + Y.stamp(fn) + sFn;// 注册插入方法o[sFn].register(sid, fn, when);// 返回EventHandle对象，方便注销return new Y.EventHandle(o[sFn], sid);}
}Y.Do.Method = function (obj, sFn) {this.obj = obj;this.methodName = sFn;this.method = obj[sFn];this.before = {};this.after = {};
};
Y.Do.Method.prototype.register = function (sid, fn, when) {if (when) {this.after[sid] = fn;} else {this.before[sid] = fn;}
};
// 注销插入方法
Y.Do.Method.prototype._delete = function (sid) {delete this.before[sid];delete this.after[sid];
};
Y.Do.Method.prototype.exec = function () {var before = this.before,after = this.after,i, ret;// 执行插入前面的方法for (i in before) {if (before.hasOwnProperty(i)) {ret = before[i].apply(this.obj, arguments);}}// 执行原方法ret = this.method.apply(this.obj, arguments);// 执行插入后面的方法for (i in after) {if (after.hasOwnProperty(i)) {ret = after[i].apply(this.obj, arguments);}}return ret;
};

适用场景

a) 动态修改对象方法

请参照例1。

b) 动态修改原型方法

原型也是对象，所以，另外一个适用场景就是修改原型方法。

// 例2
YUI().use('event-custom', function (Y) {function Car(brand) {this.brand = brand;};Car.prototype.start = function () {console.log('start');};var myCar = new Car('bmw');Y.Do.before(function () {console.log('open the door');}, Car.prototype, 'start');Y.Do.after(function () {console.log('the car is started!');}, Car.prototype, 'start');myCar.start(); // output: open the door, start, the car is started!
});

c) 动态修改宿主方法

为宿主对象添加插件时，插件往往需要在宿主一些方法前后执行某些操作。YUI提供了一个很好的例子。

d) 动态修改被扩展对象方法

为对象添加扩展时，扩展对象往往需要在被扩展对象一些方法前后执行某些操作。YUI提供了一个很好的例子。

进阶使用

由于简化代码，省略了一些细节。Y.Do还有很多功能，例如：可以根据前置方法返回值阻止默认方法执行、替换参数等等。下面介绍一些这样的进阶使用方式：

// 例3
YUI().use('event-custom', function (Y) {function Car(brand, degree) {this.brand = brand;this.degree = degree || 0;};Car.prototype.shift = function (degree) {console.log('change to ' + degree);};var myCar = new Car('bmw');// 多个前置方法Y.Do.before(function (degree) {console.log('prepare to change');}, Car.prototype, 'shift');Y.Do.before(function (degree) {console.log('prepare to change again');}, Car.prototype, 'shift');myCar.shift(1); // output: prepare to change, prepare to change again, change to 1// 多个后置方法Y.Do.after(function (degree) {console.log('already change');}, Car.prototype, 'shift');Y.Do.after(function (degree) {console.log('already change again');}, Car.prototype, 'shift');myCar.shift(2); // output: ..., change to 2, already change, already change again // 中止执行Y.Do.before(function (degree) {if (degree < 0) {console.log('halt, too low!');return new Y.Do.Halt();}}, Car.prototype, 'shift');myCar.shift(-1); // output: ..., halt, too low! // 阻止默认方法Y.Do.before(function (degree) {if (degree > 4) {console.log('prevent changing, too high!');return new Y.Do.Prevent();}}, Car.prototype, 'shift');myCar.shift(5); // output: ..., prevent changing, too high!, already change, ... // 替换参数Y.Do.before(function (degree) {var d = Math.floor(degree);if (degree !== d) {return new Y.Do.AlterArgs('degree should be a integer', [d]);}}, Car.prototype, 'shift');myCar.shift(2.5); // output: ..., change to 2, ... // 替换返回值Y.Do.after(function (degree) {if (degree === 0) {return new Y.Do.AlterReturn('', 'wow, your car now has no power');}}, Car.prototype, 'shift');var ret = myCar.shift(0); // output: ..., change to 0, ... console.log(ret); // wow, your car now has no power
});