Processing circle in webpages

In Most of the web si开发者_如何转开发tes we see the common one 'Processing cricle' for indicating page loading /image loading etc.. and making the main(parent ,back) page faded.. I would like to know what are those concepts and how could I implement those

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The circle probably indicates that there is a pending AJAX call.

Have a look at the AJAX Tutorial at w3schools for an introduction.

The "loading animations" can be generated at http://ajaxload.info/

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@aioobe has a good answer, but as you didn't provide any example pages, I'll offer other reasons for progress indicators:

• Ajax calls may be in progress
• Very long page loads can use an animated gif during the load before the load event has fired
• Flash content may be loading (similar to AJAX as it's an asynchronous HTTP request)
• multi-threaded JavaScript may be executing.

Each situation has its own implementation, and you may want to either ask a new question in regard to each method, or turn this question into a community wiki.