How do the Handler and Looper components interact in the message processing system?

The Android message processing system is based on the interaction between the Handler and Looper components. Looper is responsible for an infinite loop of processing messages from the message queue (MessageQueue), while Handler serves as an interface for sending and processing these messages.

When a Handler is created, it is associated with the Looper of the current thread. The Handler places messages or Runnable objects into the Looper's message queue. The Looper repeatedly retrieves messages from the queue and passes them back to the Handler for processing in the handleMessage() method.

Thus, the Handler acts as an intermediary for sending and processing messages, and the Looper provides a continuous cycle of processing these messages in the thread.

Example:

// In the thread, create a Looper and Handler
Looper.prepare();
Handler handler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
        // Process the message
    }
};
Looper.loop();

// Sending a message
Message msg = handler.obtainMessage();
handler.sendMessage(msg);