Explain the mechanism of message processing and distribution system within the application.

In iOS, the message processing and distribution system is based on the 'Responsibility Chain' pattern and is implemented through UIResponder objects and RunLoop.

When a user interacts with the application (for example, taps on the screen), an event is created and passed through the hierarchy of UIResponder objects (UIView, UIViewController, UIWindow, etc.). Each object can handle the event or pass it further.

Additionally, a notification mechanism (NotificationCenter) is used for broadcasting messages between application components.

Main components:

• RunLoop — the event processing loop that waits for and distributes events.
• UIEvent — the event object, such as a touch.
• UIResponder — the base class that handles events and passes them further.

Thus, the system provides asynchronous and flexible processing of user and system events within the application.