Ajax synchronization, Asynchronization, and instance demonstration

There are two forms of submission in Ajax: synchronous and asynchronous.
XMLHTTP. Open ("get", URL, true );

XMLHTTP. Open ("get", URL, false );

Synchronization: after submission, wait for the server to respond. After receiving the data returned by the server, run the following code:
Asynchronous: In contrast to the above, after submission, the following code continues to be executed, while listening continues in the background. After the server responds, the program processes the Code accordingly, the advantage of asynchronous operations is that you can continue to do other things on the client without waiting for the server.

Example:

Function dbonclick (){

VaR req
=
New
XMLHttpRequest ();

VaR url = "www.google.cn ";

Req. Open (
"
Get
"
, URL,
True
);
//
Establish a connection with the server (Request Method post or get, URL, true indicates asynchronous, false indicates synchronous)





Req. onreadystatechange
=
Callback;
//
Specify callback function


Req. Send (
Null
);
//
Send request

Alert ("asynchronous mode"); // if it is an asynchronous request, this sentence



Directly execute the preceding sentence,



The server does not need to wait for the server to return the response before execution. The server response is monitored by the callback function.

// For Synchronous requests, this sentence can be executed only when the server returns a response

}

 

//
Callback function, which processes responses to the server and monitors the response status.

Function callback (){

If
(Req. readystate
=
4
)
//
If the request status is 4, the request is successful.

{

If
(Req. Status
=
200
)
//
HTTP status 200 indicates OK


{
Dosomething;
//
All statuses are successful, and the corresponding code is executed.

Alert (req.




Responsetext
);

} Else {

Alert (
"
Server return status
"
 
+
Req. statustext );

}

} Else {

Alert ("loading data ...");

}

}

 

XMLHTTPRequest object methods and attributes

Fang Method	Description Description
Abort ()	Stop current request
GetAllResponseHeaders ()	Set HTTP All Response Headers in the request are used as keys. / Return Value Pair
GetResponseHeader ("Header ")	Returns the string value of the specified header.
Open ("method", "url ")	Create a call to the server. Method The parameter can be Get , Post Or Put . URL The parameter can be relative URL Or absolute URL . This method also includes 3 Optional parameters
Send (content)	Send a request to the server
SetRequestHeader ("Header", "value ")	Set the specified header to the provided value. You must call Open ()

 

Owner Sex	Description Description
Onreadystatechange	This event processor is triggered when each state changes, usually calling Javascript Function
Readystate	Request status. Yes 5 Optional values: 0 = Not initialized, 1 = Loading, 2 = Loaded, 3 = Interacting, 4 = Complete
Responsetext	Server Response, represented as a string
Responsexml	The response of the server, expressed XML . This object can be parsed into Dom Object
Status	Server HTTP Status Code ( 200 Corresponding OK , 404 Corresponding Not found (Not found), and so on)
Statustext	HTTP The corresponding text of the status code ( OK Or Not found (Not found) and so on)