Ajax Introduction

Ajax is considered to be (the abbreviation of Asynchronous JavaScript And Xml). Nowadays, the technology that allows the browser to communicate with the server without refreshing the current page is called Ajax.

Synchronization refers to the communication method in which the sender sends data and waits for the receiver to send back a response before sending the next data packet.

Asynchronous means: after the sender sends the data, it does not wait for the receiver to send back a response, and then sends the next data packet.

Usually the method of communicating with the server without refreshing the webpage:

Flash frame

Frameset: If a webpage is constructed using a set of frames, you can only update one of the frames without disturbing the entire page

XMLHttpRequest: This object is An extension to JavaScript that enables web pages to communicate with the server. The best choice for creating Ajax applications. In fact, Ajax is usually regarded as synonymous with the XMLHttpRequest (XHR) object

Basic use of ajax

Ajax is a technology that must be used in our development. Ajax is asynchronous javascript and xml, but now we usually use json to complete the data Interaction, ajax has a single responsibility, which is the interaction of data. Sending data and receiving data are its core and only functions.

　The implementation of ajax relies on XMLHttpRequest. Its basic usage is as follows:

var xhr;
window.XMLHttpRequest?xhr = new XMLHttpRequest():xhr = new ActiveXObject("Microsoft.XMLHTTP");
xhr.open("get","demo!register.action?name=zt&age=23",true);
xhr.send(null);
xhr.onreadystatechange = function(){
　　if(xhr.readyState==4&&xhr.status==200){
 　　alert(JSON.parse(xhr.responseText));
　　}
}

　The responsibility of ajax is to send data and receive data. Our basic usage process is:

　1. Get an XMLHttpRequest object

　2. Send data

　3. Receive and process the data returned by the server

　According to the above steps to implement an asynchronous request for data process, first obtain an xhr object. In modern browsers, we can obtain an xhr object directly through instantiation: var xhr = new XMLHttpRequest(); In IE5 and IE6 we must use ActiveXObject to obtain the xhr object: var xhr = new ActiveXObject("Microsoft.XMLHTTP").

At this point we have obtained the xhr object. The next step is to send data. The method of sending data is executed through the xhr.open() method. xhr.open() can receive 5 parameters. We often use the first three:

xhr.open(arg1,arg2,arg3)

arg1 indicates that the method of requesting data is generally get or post

arg2 indicates the requested server address

arg3 indicates whether this request is synchronous or asynchronous. The outstanding feature of ajax is asynchronous, so we generally use The third parameter of the asynchronous method is set to true (true means to make an asynchronous request, false means to make a synchronous request)

　The xhr.open() method only prepares a request. After calling open, it will not communicate with the server, but in Communication with the server will begin only after calling the send() function. The parameters of the send() function will be sent to the server as the request body. If we specify the request mode in the open() function as get, usually we set send() to xhr.send(null). If we want to send data through the request body, we must set the request mode of the open() function to post. At the same time, use the data we need to send as the parameter of the send() function: xhr.send(param). After calling the send() function, communication with the server begins.

All settings for xhr should be set before the send() function:

xhr.open(...);
xhr.setRequestHeader(...);
xhr.overrideMimeType(...);
xhr.onreadystatechange = function(){...};
xhr.send(...);

But since xhr.onreadystatechange is an event, it can also be executed after the send() function. For readability, we generally place xhr settings before the send() function.

　 After send(), you can monitor the status of this request through xhr.readyState and xhr.status. If xhr.readyState==4&&xhr.status==200 is met, the request is successful:

When the request is successful We can get the data returned by the server through xhr.responseText. It should be noted that xhr.responseText is a string.

ajax common API

　 The above request process is the most basic request process. The xhr object also has several commonly used methods: xhr.abort(), xhr.setRequestHeader(), xhr.overrideMimeType() .

　xhr.abort(): Terminate a request, call it directly without setting parameters

xhr.abort()

　xhr.setRequestHeader(): Set the sent request header:

xhr.setRequestHeader("Content-Type","application/json; charset=utf-8")

　The header to be set. The second parameter indicates the value of the header to be set. xhr.setRequestHeader() must be between xhr.open() and xhr.send(), otherwise an exception will be thrown. At the same time, the first parameter of xhr.setRequestHeader() is not case-sensitive as long as we write it in letters. The setting can be successful, but for readability we need to set it to the correct format.

　 xhr.overrideMimeType(): Rewrite the Content-Type of the response header:

xhr.overrideMimeType('text/plain; charset=utf-8')

　xhr.overrideMimeType() must also be set before xhr.send().

JSON.parse() and JSON.stringify() use

　　JSON.parse()用來將一個對象轉(zhuǎn)換為字符串，JSON.stringify()用來將一個字符串轉(zhuǎn)換為對象。在利用ajax進(jìn)行數(shù)據(jù)交互的過程中返回的數(shù)據(jù)多數(shù)的時候是一個JSON格式的字符串，如果服務(wù)器給我們返回了數(shù)據(jù)此時我們就需要利用JSON.parse()來解析返回的數(shù)據(jù)（xhr.responseText即為服務(wù)器返回的數(shù)據(jù)）：

xhr.onreadystatechange = function(){
　　if(xhr.readyState==4&&xhr.status==200){
 var data = JSON.parse(xhr.responseText);
　　}
}

　　在使用post方式發(fā)送數(shù)據(jù)的過程中，如果不是文件上傳一般情況下傳輸?shù)囊彩且粋€JSON數(shù)據(jù)，要想能夠成功的發(fā)送到后臺就需要用JSON.stringify()來將JSON對象來轉(zhuǎn)換為一個字符串，同時Content-Type要設(shè)置為application/json：

var sendData = {name:"zt",age:23};
...
xhr.setRequestHeader("Content-Type","application/json; charset=utf-8");
xhr.send(JSON.stringify(sendData));

　　另外利用JSON.parse()和JSON.stringify()可以實(shí)現(xiàn)一個對象的深拷貝功能：

var sendData = {name:"zt",age:23};
var copyData = JSON.parse(JSON.stringify(sendData));

$.ajax基本使用

　　為了方便使用JQ為我們封裝好了一個ajax來方便我們的使用：

$.ajax({
 type:"post",//請求方式
 url:"url",//請求地址
 data:"...",//發(fā)送至服務(wù)端的數(shù)據(jù)
 contentType:"...",//設(shè)置發(fā)送數(shù)據(jù)的類型如果data是一個json字符串這里要設(shè)置為application/json
 success:function(data){...},//請求成功的回調(diào)函數(shù)data可看做是服務(wù)器返回的數(shù)據(jù)
 error:function(){...}//請求失敗的回調(diào)函數(shù)
 });

　　或者：

$.ajax({
 type:"post",
 url:"url",
 data:"...",
 contentType:"...",
 })
 .done(function(data){...})
 .fail(function(){...});

　　回調(diào)函數(shù)中的data即為服務(wù)器返回的數(shù)據(jù)的一個代理，直接使用即可。

　　為了簡化我們的開發(fā)JQ提供了一些全局設(shè)置函數(shù)包括$.ajaxSetup()、$.()ajaxStart()、$().ajaxStop()、$().ajaxComplete()、$().ajaxError()、$().ajaxSuccess()、$().ajaxSend()。

　　$.ajaxSetup()用來設(shè)置基本的參數(shù)例如：

$.ajaxSetup({
 type:"post",
 contentType:"application/json; charset=utf-8"
 });

　　我們在使用$.ajax時可以直接這樣設(shè)置：

$.ajax({
url:"",
success:function(){...},
error:function(){...}
})

　　最終等價于：

$.ajax({
type:"post",
contentType:"application/json; charset=utf-8",
url:"",
success:function(){...},
error:function(){...}
})

　　$().ajaxStart()、$().ajaxStop()、$().ajaxComplete()、$().ajaxError()、$().ajaxSuccess()、$().ajaxSend()都是用來設(shè)置一些全局回調(diào)函數(shù)的。例如我們在提交數(shù)據(jù)時為了防止多次提交我們需要在發(fā)送請求時產(chǎn)生一個loading遮罩在數(shù)據(jù)發(fā)送完成后取消遮罩，如果在每一次ajax請求時我們都設(shè)置一次就會很麻煩，此時我們就可以用全局回調(diào)函數(shù)來簡化我們的操作：

　　利用全局事件在請求開始時產(chǎn)生一個遮罩在請求完成時取消遮罩：

   
$(document).ajaxStart(function(){
 loadingMask.show();
 });
 $(document).ajaxComplete(function(){
 loadingMask.hide();
 });

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