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📜  Node.js-事件发射器

📅  最后修改于: 2020-11-03 10:06:05             🧑  作者: Mango

Node中的许多对象都会发出事件，例如net.Server每次对等点连接到它时都会发出一个事件，而fs.readStream在文件打开时会发出一个事件。发出事件的所有对象都是events.EventEmitter的实例。

EventEmitter类

如上一节所述，EventEmitter类位于事件模块中。可通过以下代码访问-

// Import events module
var events = require('events');

// Create an eventEmitter object
var eventEmitter = new events.EventEmitter();

当EventEmitter实例遇到任何错误时，它将发出“错误”事件。添加新的侦听器时，将触发“ newListener”事件，而删除侦听器时，将触发“ removeListener”事件。

EventEmitter提供了on和emit之类的多个属性。 on属性用于将函数与事件绑定， emit用于触发事件。

方法

Sr.No. Method & Description
1

addListener(event, listener)

Adds a listener at the end of the listeners array for the specified event. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of event and listener will result in the listener being added multiple times. Returns emitter, so calls can be chained.
2

on(event, listener)

Adds a listener at the end of the listeners array for the specified event. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of event and listener will result in the listener being added multiple times. Returns emitter, so calls can be chained.
3

once(event, listener)

Adds a one time listener to the event. This listener is invoked only the next time the event is fired, after which it is removed. Returns emitter, so calls can be chained.
4

removeListener(event, listener)

Removes a listener from the listener array for the specified event. Caution − It changes the array indices in the listener array behind the listener. removeListener will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified event, then removeListener must be called multiple times to remove each instance. Returns emitter, so calls can be chained.
5

removeAllListeners([event])

Removes all listeners, or those of the specified event. It’s not a good idea to remove listeners that were added elsewhere in the code, especially when it’s on an emitter that you didn’t create (e.g. sockets or file streams). Returns emitter, so calls can be chained.
6

setMaxListeners(n)

By default, EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default which helps finding memory leaks. Obviously not all Emitters should be limited to 10. This function allows that to be increased. Set to zero for unlimited.
7

listeners(event)

Returns an array of listeners for the specified event.
8

emit(event, [arg1], [arg2], […])

Execute each of the listeners in order with the supplied arguments. Returns true if the event had listeners, false otherwise.

类方法

Sr.No. Method & Description
1

listenerCount(emitter, event)

Returns the number of listeners for a given event.

大事记

Sr.No. Events & Description
1

newListener

  • event − String: the event name

  • listener − Function: the event handler function

This event is emitted any time a listener is added. When this event is triggered, the listener may not yet have been added to the array of listeners for the event.
2

removeListener

  • event − String The event name

  • listener − Function The event handler function

This event is emitted any time someone removes a listener. When this event is triggered, the listener may not yet have been removed from the array of listeners for the event.

例

使用以下Node.js代码创建一个名为main.js的js文件-

现场演示
var events = require('events');
var eventEmitter = new events.EventEmitter();

// listener #1
var listner1 = function listner1() {
   console.log('listner1 executed.');
}

// listener #2
var listner2 = function listner2() {
   console.log('listner2 executed.');
}

// Bind the connection event with the listner1 function
eventEmitter.addListener('connection', listner1);

// Bind the connection event with the listner2 function
eventEmitter.on('connection', listner2);

var eventListeners = require('events').EventEmitter.listenerCount
   (eventEmitter,'connection');
console.log(eventListeners + " Listner(s) listening to connection event");

// Fire the connection event 
eventEmitter.emit('connection');

// Remove the binding of listner1 function
eventEmitter.removeListener('connection', listner1);
console.log("Listner1 will not listen now.");

// Fire the connection event 
eventEmitter.emit('connection');

eventListeners = require('events').EventEmitter.listenerCount(eventEmitter,'connection');
console.log(eventListeners + " Listner(s) listening to connection event");

console.log("Program Ended.");

现在运行main.js以查看结果-

$ node main.js

验证输出。

2 Listner(s) listening to connection event
listner1 executed.
listner2 executed.
Listner1 will not listen now.
listner2 executed.
1 Listner(s) listening to connection event
Program Ended.