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When an application is launched, the system creates a process for it (or more, if it declares services in separate processes) and a main thread (UI thread). As discussed in the previous labs, any code you write in your components (unless they are Intent Services) is run on the main thread. But an app might need to perform processing in worker threads, for example, in case of networking or database operations. If we block the UI thread while performing computational intensive code or waiting for certain events, the system will display an ANR (Application Not Responding) dialog.
For working with threads and scheduling tasks we can leverage language-specific APIs for threading, Android specific components or many third-party libraries.
Calling methods from the Android UI API must be done on the UI thread. For example, if you want to show a toast from a worker-thread, use runOnUiThread.