How is the user interface displayed on the screen in the application?

In iOS, the user interface rendering on the screen is performed using the UIKit rendering system (or SwiftUI in newer apps). The main element is UIView, which represents a rectangular area on the screen.

The rendering process involves several stages:

1. Creating and configuring the view hierarchy (UIView and its subclasses).
2. The system calls layoutSubviews to position elements.
3. Then, draw(_:) is called to render the view's content.
4. The rendering output is displayed on the screen via a graphics context.

For example, if you create a button:

let button = UIButton(frame: CGRect(x: 50, y: 100, width: 200, height: 50))
button.setTitle("Нажми меня", for: .normal)
view.addSubview(button)

The system will automatically render the button on the screen by calling the necessary rendering methods.

Thus, UI rendering is a process of creating a hierarchy of visual components and drawing them using iOS's built-in mechanisms.