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• Main Thread = UI Thread = All user input and output = Activity or fragment

All long operation must not be done on the Main Thread.

• The class AsyncTask simplify background thread creation coupled to an activity
• loader are background thread not coupled to activity
• A fragment can be created without UI. See Fragment without UI
• Android - Service (background operation without UI interaction)

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Documentation / Reference

• guide/components/processes-and-threads.html.html