node.js 中 emit() 和 on() 的一个小发现

node 中的 EventEmitter 类提供了监听事件的方法 on(event, listener) 和触发事件的方法 emit(event, [arg1], [arg2], […])

emit触发信号之后，on监听到事件后程序是如何执行的呢？

var events = require("events");
var eventEmitter = new events.EventEmitter();

var connectHandler = function (){
    console.log("data_received");
    eventEmitter.emit("data_received");
}
eventEmitter.on("data_received",function(){
    console.log("data_receive ---> connection");
    eventEmitter.emit("connection");

});

eventEmitter.on("connection",function(){
    console.log(" connection ---> test");
    eventEmitter.emit("test");


});
eventEmitter.on("test",function(){

    console.log("I have listened test");

});

connectHandler();


console.log("end");

上面的代码中有三个 on() 监听事件，其中两个的回调函数内又有新的 emit()。若按照上面代码的顺序进行执行，console中的结果是
data_received data_receive ---> connection connection ---> test I have listened test end
但是如果将connectHandler中和 on中的console.log 和 emit调换顺序的话，运行会得到相反的结果：

var events = require("events");
var eventEmitter = new events.EventEmitter();

var connectHandler = function (){
    eventEmitter.emit("data_received");
    console.log("data_received");

}
eventEmitter.on("data_received",function(){
    eventEmitter.emit("connection");
    console.log("data_receive ---> connection");
});

eventEmitter.on("connection",function(){
    eventEmitter.emit("test");
    console.log(" connection ---> test");

});
eventEmitter.on("test",function(){

    console.log("I have listened test");

});

connectHandler();


console.log("end");

I have listened test connection ---> test data_receive ---> connection data_received end

这个结果说明，在emit触发事件之后，会先寻找是否存在对应的事件监听，如果有优先执行其回调函数，执行完毕后才会返回值。

node.js 中 emit() 和 on() 的一个小发现

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