In such cases, you need to override the onreadystatechange function of the XMLHttpRequest instance so that you can set a callback function for the asynchronous request.
在这种情况下，需要覆盖XMLHttpRequest实例的onreadystatechange函数以便您可以为这个异步请求设置一个callback函数。
The AsyncToken class provides a place to set additional or token-level data for asynchronous RPC operations.
此asynctoken类提供了一个位置，可用来为异步rpc操作设置额外的或标记(token)级的数据。
Using events - a signal is set as soon as the asynchronous call is finished.
l利用事件——当异步调用完成时设定的信号。
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