Detailed introduction to the observer pattern of JavaScript design patterns

Javascript is active in an event-driven environment, such as mouse response, event callback, network request, etc. Observer mode is also called publisher-subscriber (publisher-subscriber) Pattern deals with the relationship between objects, their behaviors and states, and manages the relationship between people and tasks.

1. The most common observer pattern

1.1 Event listener

document.body.addEventListener('click', function () {
    console.log('you clicked me, poor guy!')
});

This is the simplest and most common observer pattern, except click In addition to , there are also load, blur, drag, focus, mouseover, etc. An event listener (listener) is different from an event handler (handler). In an event listener, an event can be associated with multiple listeners, and each listener independently processes the monitored message; the event handler is responsible for executing and processing the event. After the associated function, an event can have a processing function:

var dom = $('.dom');
var listener1 = function(e){
    //do one thing
}
var listener2 = function(e){
    //do another thing
}
addEvent(dom,'click',listener1);
addEvent(dom,'click',listener2);

In this example of the event listener, listener1 and listener2 are both dom elements The listeners will execute their respective functions when the dom is clicked;

var dom = document.getElementById('dom');
var handler1 = function(e){
    //do one thing
}
var handler2 = function(e){
    //do another thing
}
dom.onclick = handler1;
dom.onclick = handler2;

In the example of this event handler, handler1 will not be executed, only handler2 is an assignment operation.

1.2 Animation

The observer mode is widely used in animation. The start, completion, pause, etc. of animation require an observer to determine the behavior and status of the object.

//定义动画
var Animation = function(){
    this.onStart = new Publisher;  //关于Publisher的设计将在1.3节介绍
    this.onComplete = new Publisher;
    this.onTween = new Publisher;
}
//定义一个原型方法
Animation.prototype.look = function(){
    this.onStart.deliver('animation started!');
    this.onTween.deliver('animation is going on!');
    this.onComplete.deliver('animation completed!');  
};

//实例一个box对象
var box = new Animation();

//定义三个函数作为subscribers
var openBox = function(msg){
    console.log(msg)
}
var checkBox = function(msg){
    console.log(msg)
}
var closeBox = function(msg){
    console.log(msg)
}

//订阅事件
openBox.subscribe(box.onStart);
checkBox.subscribe(box.onTween);
closeBox.subscribe(box.onComplete);

//调用方法
box.look()

//animation started!
//animation is going on!
//animation completed!

1.3 Construction of Observer

First, a publisher is needed. First define a constructor and an array for it to save subscriber information:

function Publisher(){
    this.subscribes = [];
}

The publisher has the function of publishing messages, and defines a prototype function of deliver:

Publisher.prototype.deliver = function(data){
    this.subscribes.forEach(function(fn){
        fn(data);
    });
    return this;
}

Next Construct a subscription method:

Function.prototype.subscribe = function(publisher){
    var that = this;
    var alreadyExists = publisher.subscribes.some(function(el){
        return el === that;
    });
    if(!alreadyExists){
        publisher.subscribes.push(this);
    }
    return this;
}

Add the subscribe method directly to the Function prototype so that all functions can call this method. In this way, the construction is completed. For usage methods, please refer to the use case of 1.2 Animation.
A more intuitive explanation (take onStart as an example): When the box object executes the look method, execute onStart.deliver(), publishes the onStart event and broadcasts the notification 'animation started!'. At this time, it has been monitoring onStartopenBox Listen to the information released by the event and print it out.

1.4 Another way to build an observer

This method imitates the event processing mechanism of nodejs, and the code is relatively concise:

    var scope = (function() {
    //消息列表
    var events = {};
    return {
        //订阅消息
        on:function(name,hander){
            var index = 0;  //记录消息时间的索引
            if(events[name]){  
                //消息名已存在，将处理函数放到该消息的事件队列中
                index = events[name].push(hander) - 1; 
            }else{
                events[name] = [hander];
            }
            //返回当前消息处理事件的移除函数
            return function(){
                events[name].splice(index,1);
            }
        },
        //关闭消息
        off:function(name){
            if(!events[name]) return;
            //消息存在，删除消息
            delete events[name];
        },
        //消息发布
        emit:function(name,msg){
            //消息不存在，不处理
            if(!events[name]) return;
            //消息存在，将该事件处理队列中每一个函数都执行一次
            events[name].forEach(function(v,i){
                v(msg);
            });
        }
    }
})();

var sayHello = scope.on('greeting',function(msg){
    console.log('订阅消息：' + msg);
});

var greeting = function(msg){
    console.log('发布消息：' + msg);
    scope.emit('greeting', msg);
}

greeting('hello Panfen!')

1.5 Observer mode in nodejs Implementation plan

There is an events module in nodejs to implement the observer mode. You can refer to Nodejs API-Events to talk about the observer mode. Most modules integrate the events module, so you can directly use emit to launch events and on Listen for events, or define it first like below;

var EventEmitter = require('events').EventEmitter;
var life = new EventEmitter();
life.setMaxListeners(11);       //设置最大监听数，默认10

//发布和订阅sendName
life.on('sendName',function(name){
    console.log('say hello to '+name);
});
life.emit('sendName','jeff');

//发布和订阅sendName2
function sayBeautiful(name){
    console.log(name + ' is beautiful');
}
life.on('sendName2',sayBeautiful);
life.emit('sendName2','jeff');

Common methods:

• hasConfortListener: Used to determine whether the emitted event has a listener

• removeListener: Remove the listener

• listenerCount: The total number of all listeners for this event

• removeAllListeners: Remove All (or one) event listeners

pushandlisten The logic is suitable for situations where you want to separate human behavior from application behavior. For example: when the user clicks a tab on the navigation bar, a submenu containing more options will open. Generally, the user will choose to listen to the click event directly if he knows which element. The disadvantage of this is that it is not the same as the click event. The events are directly tied together. A better approach is to create an observable onTabChange object and associate several observer implementations.

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