IMGUI - Legacy GUI Scripting

IMGUI - Legacy GUI Scripting

Immediate Mode GUI (IMGUI) is a code-driven UI system used inside the OnGUI() callback. It is primarily used for custom Editor tools, debug displays, and inspector customization.

The Immediate Mode Loop

Unlike canvas-based UI systems, IMGUI elements are declared and processed on the fly during the OnGUI() rendering loop:

using UnityEngine;

public class DebugPanel : MonoBehaviour
{
    private bool _showDebug = true;

    void OnGUI()
    {
        // Renders a checkbox. Toggles the boolean state immediately.
        _showDebug = GUI.Toggle(new Rect(10, 10, 150, 20), _showDebug, "Show Debug Details");

        if (_showDebug)
        {
            // Simple text box container
            GUI.Box(new Rect(10, 40, 200, 70), "Debug Info");
            GUI.Label(new Rect(20, 60, 180, 20), "FPS: " + (1f / Time.smoothDeltaTime).ToString("F0"));
        }
    }
}

Automatic Layout (GUILayout)

Instead of manually calculating pixel positions (Rect), use GUILayout to automatically calculate sizes:

void OnGUI()
{
    GUILayout.BeginVertical("box");
    GUILayout.Label("Editor Options");
    if (GUILayout.Button("Reset Scene"))
    {
        UnityEngine.SceneManagement.SceneManager.LoadScene(0);
    }
    GUILayout.EndVertical();
}

Styles and Skins

Customize IMGUI fonts, margins, and states with GUIStyle and GUISkin structures:

public GUIStyle customStyle;

void OnGUI()
{
    GUILayout.Label("Custom Red Warning Text", customStyle);
}

TL;DR

• IMGUI UI runs in the OnGUI() callback.
• GUI requires manual Rect dimensions; GUILayout handles sizing automatically.
• Excellent for Editor scripting and debug panels; not recommended for in-game HUDs.