How does the application determine which part of the interface should respond to a touch?

In iOS, the mechanism of hit-testing is used to determine which part of the interface should respond to a touch. When the user touches the screen, the system starts with the root view (UIView) and recursively checks which subview is under the touch point.

The process includes the following steps:

1. The system calls the hitTest(_:with:) method on the root UIView, passing the touch coordinates.
2. The UIView checks if the touch point is within its bounds (point(inside:with:)). If not — it returns nil.
3. If the point is inside, the UIView iterates over its subviews in reverse order (top to bottom), calling hitTest(_:with:) on them.
4. If any subview returns a non-nil value, it means that subview should handle the touch.
5. If no subview matches, the UIView itself is returned.

Thus, the application determines exactly which interface element is under the user's finger and ready to receive the event.