OnGUI

OnGUI is a scripting function in the Unity game engine used for immediate mode graphical user interface (GUI) rendering. Unlike retained mode GUI systems where the system maintains a scene graph of UI elements, OnGUI works by drawing UI elements directly on the screen during each frame. This means that every time the frame is rendered, the OnGUI function is called to redraw the entire UI.

The OnGUI function is typically placed within a MonoBehaviour script. When it's called, you can use a variety of built-in Unity GUI functions to draw elements like labels, buttons, text fields, and sliders. For example, `GUI.Label()` draws text, and `GUI.Button()` creates a clickable button. The position and size of these elements are specified using Rect structures.

While OnGUI is straightforward for simple debugging GUIs, prototypes, or very basic in-game interfaces, it is generally not recommended for complex or performance-critical user interfaces in production games. This is because its immediate mode nature can lead to performance overhead, as the entire UI is recalculated and redrawn every frame, regardless of whether it has changed. For more advanced and performant UI development in Unity, the engine provides the UI Toolkit and the older Unity UI (UGUI) system, which are retained mode solutions.