How does the main thread work in Android?

The main thread (Main Thread) or UI thread in Android is responsible for rendering the user interface, handling input events, and executing most application components (Activity, Service, Broadcast Receiver).

Its operation is based on the Looper and Handler mechanisms.

1. Looper: This is a class that cyclically retrieves messages (Message) from the message queue (MessageQueue) and passes them to the appropriate Handler for processing. Each thread with a Looper is "looped". The main thread is created with a Looper automatically.
2. MessageQueue: This is a data structure that stores a list of messages to be processed. Messages can contain data (objects, primitives) and a reference to the Handler that should process them.
3. Handler: This is a class associated with the thread's Looper and MessageQueue. It allows sending messages to the MessageQueue and processing messages retrieved by the Looper. Handler can send messages either to its own thread (for scheduling tasks) or to other threads with a Looper.

Working principle:

1. The main thread's Looper starts and continuously processes the loop() cycle.
2. The system or other threads send messages (e.g., touch events, Activity lifecycle messages) to the main thread's MessageQueue via Handlers.
3. The Looper retrieves the next message from the MessageQueue.
4. The Looper passes the message to the Handler associated with it.
5. The Handler processes the message, executing the corresponding logic (e.g., updating the UI).

Important: All operations related to updating the UI should be performed only on the main thread. Performing long operations (network requests, file operations) on the main thread will block it (ANR - Application Not Responding) and cause the app to hang. For such operations, worker threads should be used (e.g., with Coroutines, RxJava, ExecutorService, AsyncTask).