Immediate Mode GUI (IMGUI)

The “Immediate Mode” GUI system (also known as IMGUI) is an entirely separate feature to Unity’s main GameObject-based UI(User Interface) Allows a user to interact with your application. Unity currently supports three UI systems. More info
See in Glossary System. IMGUI is a code-driven GUI system, and is mainly intended as a tool for programmers. It is driven by calls to the OnGUI function on any script which implements it. For example, this code:

    void OnGUI() {
        if (GUILayout.Button("Press Me")){ 
            Debug.Log("Hello!");
        }
    }

Would result in a button displayed like so:

The Immediate Mode GUI system is commonly used for:

• Creating in-game debugging displays and tools.
• Creating custom inspectorsA Unity window that displays information about the currently selected GameObject, asset or project settings, allowing you to inspect and edit the values. More info
  See in Glossary for script components.
• Creating new editor windows and tools to extend Unity itself.

The IMGUI system is not generally intended to be used for normal in-game user interfaces that players might use and interact with. For that you should use Unity’s main GameObject-based UI system, which offers a GameObject-based approach for editing and positioning UI elements, and has far better tools to work with the visual design and layout of the UI.

“Immediate Mode” refers to the way the IMGUI is created and drawn. To create IMGUI elements, you must write code that goes into a special function named OnGUI. The code to display the interface is executed every frame, and drawn to the screen. There are no persistent gameobjectsThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary other than the object to which your OnGUI code is attached, or other types of objects in the hierarchy related to the visual elementsA node of a visual tree that instantiates or derives from the C# VisualElement class. You can style the look, define the behaviour, and display it on screen as part of the UI. More info
See in Glossary that are drawn.

IMGUI allows you to create a wide variety of functional GUIs using code. Rather than creating GameObjects, manually positioning them, and then writing a script that handles its functionality, you can do everything at once with just a few lines of code. The code produces GUI controls that are drawn and handled with a single function call.

This section explains how to use IMGUI both in your game and in extensions to the Unity editor.