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What’s an activity?
An activity is the equivalent of a Frame/Window in GUI toolkits. It takes up the entire drawable area of the screen (minus the status and title bars on top). An activity is what gets bound to the AndroidManifest.xml as the main entry point into an application. For long running tasks, it’s best to use a service that talks to this activity.
Activities are meant to display the UI and get input from the user. However, long running tasks are not meant to be spawned in a activity, since they can be frozen when the focus is switched away from them (by the user or incoming phone call or system event). Services on the other hand keep running for the duration of the user’s ‘session’ on the device.