Ajax学习二：异步请求

【1】http://www.ibm.com/developerworks/web/library/wa-ajaxintro2/index.html

1. web2.0? 什么是web1.0

具有完全不同的请求和响应模型的传统web

• web1.0 不支持异步请求
  
• web2.0 支持异步请求
  

2. XMLHttpRequest属性和方法

• open(): Sets up a new request to a server.
• send(): Sends a request to a server.
• abort(): Bails out of the current request.
• readyState: Provides the current HTML ready state.
• responseText: The text that the server sends back to respond to a request.

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static javascript： 代码没有放在一个方法或者函数体内，在用户与页面交互之前运行

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可以将创建XMLHttpRequest的代码放在函数里面，但是会有问题：如果你是在用户填入第14个文本框时执行Ajax代码，然后回复一个浏览器不支持的错误给用户，会导致一个非常差的体验，因为用户已经在这个页面花掉了10分钟的时间。所以还是建议使用static的代码。

 

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ajax sandbox:  【2】http://www.cnblogs.com/ComeOn/archive/2008/01/30/1058942.html

AJAX中的沙箱安全模型主要指的是跨域脚本调用的问题。比如我们存在在www.cnblogs.com中的脚本如果想访问www.163.com 脚本是违反沙箱安全模型的。结果肯定会出错。

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http ready state

• 0: The request is uninitialized (before you've called open()).
• 1: The request is set up, but hasn't been sent (before you've called send()).
• 2: The request was sent and is being processed (you can usually get content headers from the response at this point).
• 3: The request is being processed; often some partial data is available from the response, but the server hasn't finished with its response.
• 4: The response is complete; you can get the server's response and use it.