YUI事件体系之Y.Do

绝地灬酷狼 2023-10-17 19:41 20阅读 0赞

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 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?][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 fish
    
        var 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][Link 1]。

// 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][Link 2]。

#### 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][Link 3]。

### 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
    });

### 参考 ###

*  [YUILibrary-Do][]
 *  [YUILibrary-EventTarget][]
 *  [Wikipedia-AOP][AOP]

**发现文章有错误、对内容有疑问，都可以关注美团点评技术团队微信公众号（meituantech），在后台给我们留言。我们每周会挑选出一位热心小伙伴，送上一份精美的小礼品。快来扫码关注我们吧！**

![公众号二维码][1521521024099-image.png]

[YUI Theater _ Luke Smith_ _Events Evolved]: http://www.yuiblog.com/blog/2009/10/30/smith-yuiconf2009-events/
[10 Things I Learned While Interning at YUI]: http://net.tutsplus.com/articles/general/10-things-i-learned-while-interning-at-yui/
[How can I decided whether to choose YUI 3_s MVC or Backbone for a new project]: http://www.quora.com/How-can-I-decided-whether-to-choose-YUI-3s-MVC-or-Backbone-for-a-new-project#ld_xJGMd1_3012
[Walter Rumsby]: https://twitter.com/wrumsby/status/113568040834174976
[Kevin Isom]: https://twitter.com/kev_nz/statuses/180472697644515328
[Cross YUI Communication and Custom Events]: http://andrewwooldridge.com/blog/2011/03/08/cross-yui-communication-and-custom-events/
[AOP]: http://en.wikipedia.org/wiki/Aspect-oriented_programming
[Link 1]: http://yuilibrary.com/yui/docs/api/files/event-custom_js_event-do.js.html#l16
[Link 2]: http://yuilibrary.com/yui/docs/plugin/#methods
[Link 3]: http://yuilibrary.com/yui/docs/assets/base/myextension.js.txt
[YUILibrary-Do]: http://yuilibrary.com/yui/docs/api/classes/Do.html
[YUILibrary-EventTarget]: http://yuilibrary.com/yui/docs/event-custom/index.html
[1521521024099-image.png]: https://tech.meituan.com//img/1521521024099-image.png