node.js实现回调

向回调函数传递额外的参数

        在调用函数中，使用匿名函数中实现需传递的参数，再次匿名函数内调用回调函数。

var events = require("events");
 
function CarShow() {
    events.EventEmitter.call(this);
    this.seeCar = function (make) {
        this.emit('sawCar', make);
    }
}
CarShow.prototype.__proto__ = events.EventEmitter.prototype;
var show = new CarShow();
 
function logCar(make) {
    console.log("Saw a "+make);
}
 
function logColorCar(make, color) {
    console.log("Saw a %s %s ", color, make);
}
 
show.on("sawCar", logCar);
show.on("sawCar", function (make) {
    var colors = ["red", "blue", "black", "pink", "green"];
    var color = colors[Math.floor(Math.random()*3)];
    logColorCar(make, color);
});
show.seeCar("Ferrari");
show.seeCar("Porsche");
show.seeCar("Bugatti");

在回调中实现闭包

        如果某个回调函数需要访问父函数的作用域的变量，就需要使用闭包，在函数块内部封装一个异步调用，并传入所需要的变量。

function logCar(logMsg, callback) {
    process.nextTick(function () {
        callback(logMsg);
    });
}
var cars = ["猎豹", "捷达", "朗逸"];
for(var idx in cars){
    var msg = "Saw a "+cars[idx];
    logCar(msg, function () {
        console.log("Normal Callback "+ msg);
    });
}
 
for(var idx in cars){
    var msg = "Saw a "+cars[idx];
    (function (msg) {
        logCar(msg, function () {
            console.log("Closure Callback "+ msg);
        })
    })(msg);
}
//Normal Callback Saw a 朗逸
//Normal Callback Saw a 朗逸
//Normal Callback Saw a 朗逸
//Closure Callback Saw a 猎豹
//Closure Callback Saw a 捷达
//Closure Callback Saw a 朗逸

链式回调

        使用异步函数时，如果两个函数都在事件队列上，则无法保证它们的运行顺序。解决方法是让来自异步函数的回调再次调用该函数，直到没有更多的工作要做，以执行链式回调。

function logCar(car, callback) {
    console.log("Saw a %$", car);
    if(cars.length){
        process.nextTick(function () {
            callback();
        });
    }
}
 
function logCars(cars) {
    var car = cars.pop();
    logCar(car, function () {
        logCars(cars);
    });
}
 
var cars = ["猎豹", "捷达", "朗逸"];
logCars(cars);