Explain the mechanism of passing a click event from the source to the handler function

In iOS, the mechanism for handling click events (for example, button presses) occurs through an event handling chain based on the UIResponder system.

1. The user taps on a UI element (for example, UIButton).
2. The system determines which object is under the touch point.
3. The event is passed to this object (usually a UIView or its subclass).
4. The object calls the associated event handler method (action), which was assigned via Target-Action or Interface Builder.

Example of assigning a handler:

button.addTarget(self, action: #selector(buttonClicked), for: .touchUpInside)

@objc func buttonClicked() {
    print("Button pressed")
}

Thus, the click event passes from the system to the specific UI element, which calls the associated handler method.