Ajax背后的想法十分简单：在浏览器中监听事件，给服务器发送后台请求，当服务器响应时更新页面的一部分。

The ideas behind Ajax are quite simple: Listen for an event in the browser, send a background request to the server, and update part of the page when server responds.

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如果操作不能很快完成，应该让它们在单独的线程中运行(“后台”或“工作”线程)。

If you have operations to perform that are not instantaneous, you should make sure to do them in separate threads (" background "or" worker "threads).

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在清单2中，您看到每个后台线程接收和打印检索特定主题tweets的结果。

Back in Listing 2, you saw each background thread receive and print the results of retrieving tweets for a particular topic.

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