An in-depth analysis of the events module in nodejs

This article will introduce you to the events module in node in detail. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.

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The events module is the core module of node, and almost all commonly used node modules inherit it Events modules, such as http, fs, etc. This article will introduce the event mechanism in nodeJS in detail

EventEmitter

Most Node.js core APIs adopt an idiomatic asynchronous event-driven architecture, in which certain types of objects (called triggers) The named event will be triggered periodically to call the function object (listener). For example, a net.Server object will trigger an event every time there is a new connection; an fs.ReadStream will trigger an event when a file is opened; a stream will trigger an event when the data is readable.

【EventEmitter】

The EventEmitter class is defined and opened by the events module. All objects that can trigger events are instances of the EventEmitter class

var EventEmitter = require('events');
/*
{ [Function: EventEmitter]
  EventEmitter: [Circular],
  usingDomains: false,
  defaultMaxListeners: [Getter/Setter],
  init: [Function],
  listenerCount: [Function] }
 */
console.log(EventEmitter);

The EventEmitter attribute of the events module points to The module itself

var events = require('events');
console.log(events.EventEmitter === events);//true

EventEmitter is a constructor that can be used to generate an instance of the event generator emitter

var EventEmitter = require('events');
var emitter = new EventEmitter();
/*
EventEmitter {
  domain: null,
  _events: {},
  _eventsCount: 0,
  _maxListeners: undefined }
 */
console.log(emitter);

Method

[emitter.emit(eventName[, .. .args])】

eventName <any>
...args <any>

This method synchronously calls each listener registered to the event named eventName in the order in which the listeners are registered, and passes in the provided parameters. If the event has a listener, it returns true, otherwise it returns false

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.on('test1',function(){});
console.log(emitter.emit('test1'));//true
console.log(emitter.emit('test2'));//false

[emitter.on(eventName, listener)]

This method is used to add the listener function to the event named eventName. The end of the listener array

eventName <any> 事件名
listener <Function> 回调函数

　[Note] Will not check whether the listener has been added. Calling multiple times and passing in the same eventName and listener will cause the listener to be added and called multiple times

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.on('test',function(){
    console.log(1);
});
emitter.on('test',function(){
    console.log(2);
});
emitter.emit('test');//1 2

This method returns an EventEmitter reference, which can be called in a chain

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.on('test',function(){
    console.log(1);
}).on('test',function(){
    console.log(2);
});
emitter.emit('test');//1 2

[emitter.addListener( eventName, listener)】

Alias ​​of emitter.on(eventName, listener)

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.addListener('test',function(){
    console.log(1);
});
emitter.emit('test');//1

【emitter.prependListener()】

Different from the on() method, prependListener( ) method can be used to add an event listener to the beginning of the listener array

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.on('test',function(){
    console.log(1);
}).prependListener('test',function(){
    console.log(2);
});
emitter.emit('test');//2 1

[emitter.once(eventName, listener)]

This method adds a one-time listener function to the eventName event. The next time the eventName event is triggered, the listener will be removed, and then

eventName <any> 事件名
listener <Function> 回调函数

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.on('test',function(){
    console.log(1);
}).once('test',function(){
    console.log(2);
});
emitter.emit('test');//1 2
emitter.emit('test');//1

[emitter.prependOnceListener()]

is called. This method is used to add the event listener to the beginning of the listener array. . The next time the eventName event is triggered, the listener will be removed, and then call

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.on('test',function(){
    console.log(1);
}).prependOnceListener('test',function(){
    console.log(2);
});
emitter.emit('test');//2 1
emitter.emit('test');//1

[emitter.removeAllListeners([eventName])]

eventName <any>

to remove all or the listeners of the specified eventName. Returns an EventEmitter reference, which can be called in a chain

[Note] It is a bad practice to remove listeners added elsewhere in the code, especially when the EventEmitter instance is another component or module (such as socket or file stream)

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.on('test',function(){
    console.log(1);
}).removeAllListeners('test');
emitter.emit('test');//''

【emitter.removeListener(eventName, listener)】

eventName <any>
listener 

Removes the specified listener from the listener array of the event named eventName

var EventEmitter = require('events');
var emitter = new EventEmitter();
function show(){
    console.log(1);
}
emitter.on('test',show).removeListener('test',show);
emitter.emit('test');//''

[Note] removeListener will only remove at most one listener instance from the listener array. If any single listener is added multiple times to the listener array for a specified eventName, removeListener must be called multiple times to remove each instance

var EventEmitter = require('events');
var emitter = new EventEmitter();
function show(){
    console.log(1);
}
emitter.on('test',show).on('test',show).removeListener('test',show);
emitter.emit('test');//'1'

　[Note] Once an event is triggered, all bindings The listeners to it will be triggered in order. This means that any call to removeListener() or removeAllListeners() after the event fires but before the last listener has finished executing will not remove them from emit() . Subsequent events will occur as expected. Because listeners are managed using internal arrays, calling this will change the position index of any listeners registered after the listener has been removed. Although this does not affect the order in which the listeners are called, it means that the copy of the listener array returned by the emitter.listeners() method needs to be recreated

var EventEmitter = require('events');
var emitter = new EventEmitter();
function show1(){
    console.log(1);
    emitter.removeListener('test',show2);
}
function show2(){
    console.log(2);
}
emitter.on('test',show1).on('test',show2);
emitter.emit('test');//1 2
emitter.emit('test');//1

Settings

[emitter.eventNames( )】

Returns an array listing the events for which the trigger has registered listeners. The value in the array is a string or symbol

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.addListener('test1',function(){console.log(1);});
emitter.addListener('test2',function(){console.log(2);});
console.log(emitter.eventNames());//[ 'test1', 'test2' ]

[emitter.listenerCount(eventName)]

eventName <any> 正在被监听的事件名

Returns the number of listeners that are listening to the event named eventName

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.addListener('test',function(){console.log(1);});
emitter.addListener('test',function(){console.log(1);});
console.log(emitter.listenerCount('test'));//2

【emitter.listeners(eventName)】

eventName <any>

Returns a copy of the listener array for the event named eventName

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.addListener('test',function(){console.log(1);});
emitter.addListener('test',function(){console.log(2);});
console.log(emitter.listeners('test'));//[ [Function], [Function] ]emitter.listeners('test')[0]();//1

【emitter.getMaxListeners()】

Returns EventEmitter The current maximum listener limit value

var EventEmitter = require('events');
var emitter = new EventEmitter();
console.log(emitter.getMaxListeners());//10

[emitter.setMaxListeners(n)]

　　默认情况下，如果为特定事件添加了超过 10 个监听器，则 EventEmitter 会打印一个警告。 此限制有助于寻找内存泄露。 但是，并不是所有的事件都要被限为 10 个。 emitter.setMaxListeners() 方法允许修改指定的 EventEmitter 实例的限制。 值设为 Infinity（或 0）表明不限制监听器的数量。返回一个 EventEmitter 引用，可以链式调用

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){});
/*
Warning: Possible EventEmitter memory leak detected. 11 a listeners added. Use emitter.setMaxListeners() to increase limit
 */

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.setMaxListeners(11);
emitter.on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){});

【EventEmitter.defaultMaxListeners】
　　每个事件默认可以注册最多10个监听器。单个EventEmitter实例的限制可以使用emitter.setMaxListeners(n)方法改变。所有EventEmitter实例的默认值可以使用EventEmitter.defaultMaxListeners属性改变

　　[注意]设置 EventEmitter.defaultMaxListeners 要谨慎，因为会影响所有EventEmitter 实例，包括之前创建的。因而，调用 emitter.setMaxListeners(n) 优先于 EventEmitter.defaultMaxListeners

var EventEmitter = require('events');var emitter = new EventEmitter();
EventEmitter.defaultMaxListeners = 11;
emitter.on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){}).on('a',function(){});

事件

【'newListener' 事件】

eventName <any> 要监听的事件的名称
listener  事件的句柄函数

　　EventEmitter 实例会在一个监听器被添加到其内部监听器数组之前触发自身的 'newListener' 事件

　　注册了 'newListener' 事件的监听器会传入事件名与被添加的监听器的引用。事实上，在添加监听器之前触发事件有一个微妙但重要的副作用： 'newListener' 回调中任何额外的被注册到相同名称的监听器会在监听器被添加之前被插入 

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.on('newListener',function(){
    console.log(2);
})
emitter.on('test',function(){
    console.log(1);
})

emitter.emit('test');//2 1

var EventEmitter = require('events');
var emitter = new EventEmitter();
emitter.on('test',function(){
    console.log(1);
})
emitter.on('newListener',function(){
    console.log(2);
})
emitter.emit('test');//1

【'removeListener' 事件】

eventName <any> 事件名
listener  事件句柄函数

　　'removeListener' 事件在 listener 被移除后触发

var EventEmitter = require('events');
var emitter = new EventEmitter();
function show(){
    console.log(1);
}
emitter.on('removeListener',function(){
    console.log(2);//2})
emitter.on('test',show).removeListener('test',show);

var EventEmitter = require('events');
var emitter = new EventEmitter();
function show(){
    console.log(1);
}
emitter.on('test',show).removeListener('test',show);
emitter.on('removeListener',function(){
    console.log(2);//''})

var EventEmitter = require('events');
var emitter = new EventEmitter();
function show(){
    console.log(1);
}
emitter.removeListener('test',show);
emitter.on('removeListener',function(){
    console.log(2);//''})

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