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。