javascript-回调

什么是回调函数

//Note that the item in the click method's parameter is a function, not a variable.
//The item is a callback function
$("#btn_1").click(function() {
  alert("Btn 1 Clicked");
});

click方法，会调用我们传递给它的回调函数

var friends = ["Mike", "Stacy", "Andy", "Rick"];

friends.forEach(function (eachName, index){
console.log(index + 1 + ". " + eachName); // 1. Mike, 2. Stacy, 3. Andy, 4. Rick
});

回调函数是如何实现的

回调函数在调用的函数体内回调
回调函数是闭包的：回调函数可以访问包含函数的变量，甚至是全局变量

实现回调函数的基本原则

命名函数/匿名函数


// global variable
var allUserData = [];

// generic logStuff function that prints to console
function logStuff (userData) {
    if ( typeof userData === "string")
    {
        console.log(userData);
    }
    else if ( typeof userData === "object")
    {
        for (var item in userData) {
            console.log(item + ": " + userData[item]);
        }
    }
}

// A function that takes two parameters, the last one a callback function
function getInput (options, callback) {
    allUserData.push (options);
    callback (options);
}

// When we call the getInput function, we pass logStuff as a parameter.
// So logStuff will be the function that will called back (or executed) inside the getInput function
getInput ({name:"Rich", speciality:"JavaScript"}, logStuff);
//  name: Rich
// speciality: JavaScript

解决回调函数this的问题

//Note that we have added an extra parameter for the callback object, called "callbackObj"
function getUserInput(firstName, lastName, callback, callbackObj)  {
    // Do other stuff to validate name here

    // The use of the Apply function below will set the this object to be callbackObj
    callback.apply (callbackObj, [firstName, lastName]);
}

多重回调函数也是允许的，就像传递多个变量一样

function successCallback() {
    // Do stuff before send
}

function successCallback() {
    // Do stuff if success message received
}

function completeCallback() {
    // Do stuff upon completion
}

function errorCallback() {
    // Do stuff if error received
}

$.ajax({
    url:"http://fiddle.jshell.net/favicon.png",
    success:successCallback,
    complete:completeCallback,
    error:errorCallback
});

回调地狱

var p_client = new Db('integration_tests_20', new Server("127.0.0.1", 27017, {}), {'pk':CustomPKFactory});
p_client.open(function(err, p_client) {
    p_client.dropDatabase(function(err, done) {
        p_client.createCollection('test_custom_key', function(err, collection) {
            collection.insert({'a':1}, function(err, docs) {
                collection.find({'_id':new ObjectID("aaaaaaaaaaaa")}, function(err, cursor) {
                    cursor.toArray(function(err, items) {
                        test.assertEquals(1, items.length);

                        // Let's close the db
                        p_client.close();
                    });
                });
            });
        });
    });
});

遇到之后的解决方法

命名并定义你的函数，然后传递函数名作为回调，而不是在主函数的参数列表里定义一个匿名函数。
模块化：把你的代码划分成一个个模块，这样你可以空出一部分代码块做特殊的工作。然后你可以将这个模型引入到你的大型应用程序中。

实现自己的回调函数

// First, setup the generic poem creator function; it will be the callback function in the getUserInput function below.
function genericPoemMaker(name, gender) {
    console.log(name + " is finer than fine wine.");
    console.log("Altruistic and noble for the modern time.");
    console.log("Always admirably adorned with the latest style.");
    console.log("A " + gender + " of unfortunate tragedies who still manages a perpetual smile");
}

//The callback, which is the last item in the parameter, will be our genericPoemMaker function we defined above.
function getUserInput(firstName, lastName, gender, callback) {
    var fullName = firstName + " " + lastName;

    // Make sure the callback is a function
    if (typeof callback === "function") {
    // Execute the callback function and pass the parameters to it
    callback(fullName, gender);
    }
}

调用getUserInput函数并传递genericPoemMaker函数作为回调

getUserInput("Michael", "Fassbender", "Man", genericPoemMaker);
// Output
/* Michael Fassbender is finer than fine wine.
Altruistic and noble for the modern time.
Always admirably adorned with the latest style.
A Man of unfortunate tragedies who still manages a perpetual smile.
*/

function greetUser(customerName, sex)  {
   var salutation  = sex && sex === "Man" ? "Mr." : "Ms.";
  console.log("Hello, " + salutation + " " + customerName);
}

// Pass the greetUser function as a callback to getUserInput
getUserInput("Bill", "Gates", "Man", greetUser);

// And this is the output
Hello, Mr. Bill Gates