OnGUIInjector

Static utility that injects custom OnGUI callbacks into Unity editor windows by accessing their internal IMGUIContainer via reflection. Enables drawing custom GUI content in editor windows without subclassing them.

Definition

Namespace: Paragon.Editor.LowLevel Assembly: Paragon.Editor.dll

public static class OnGUIInjector

Remarks

Unity editor windows internally use a GUIView → IWindowBackend → VisualElement (visual tree) hierarchy. The root element of the visual tree is an IMGUIContainer with a private m_OnGUIHandler delegate field. This class injects a callback by appending to that delegate.

Injection Chain

1. Retrieves the windowBackend from the target ScriptableObject (which is a GUIView subclass)

2. Gets the visualTree from the IWindowBackend

3. Accesses the first child element — an IMGUIContainer

4. Reads the m_OnGUIHandler delegate and appends the callback (with deduplication)

Deduplication

The injector removes the callback before adding it (-= then +=), preventing duplicate registrations if Inject is called multiple times with the same callback.

Quick Lookup

Goal	How
Inject OnGUI callback	OnGUIInjector.Inject(editorWindow, MyOnGUI)
Callback signature	Action (no parameters)
Target parameter type	ScriptableObject (must be a GUIView-based editor window)

Fields

Field	Type	Access	Description
windowBackend	PropertyInfo	private static readonly	Reflected GUIView.windowBackend property
viewVisualTree	PropertyInfo	private static readonly	Reflected IWindowBackend.visualTree property
imguiContainerOnGUI	FieldInfo	private static readonly	Reflected IMGUIContainer.m_OnGUIHandler field

Methods

Inject

public static void Inject(ScriptableObject targetEditor, Action onGUICallback)

Injects a custom OnGUI callback into an editor window's draw loop.

Parameter	Type	Description
targetEditor	ScriptableObject	The editor window to inject into (must be a GUIView-derived type such as AppStatusBar)
onGUICallback	Action	The callback to invoke during the window's OnGUI

Common Pitfalls

Target must be a GUIView-derived ScriptableObject The targetEditor parameter accepts ScriptableObject but internally casts it as a GUIView. Passing a non-GUIView ScriptableObject will throw a NullReferenceException when accessing windowBackend.

Unity internals dependency Relies on three internal Unity types/members: GUIView.windowBackend, IWindowBackend.visualTree, and IMGUIContainer.m_OnGUIHandler. Any of these may change between Unity versions.

Visual tree structure assumption Assumes the first child (visualTree[0]) is an IMGUIContainer. If Unity changes its visual tree layout, the injection will fail with an InvalidCastException.

Examples

Injecting a Status Bar Callback

[InitializeOnLoad]
static class MyStatusBarExtension
{
    static ScriptableObject statusBar;

    static MyStatusBarExtension()
    {
        EditorApplication.delayCall += Initialize;
    }

    static void Initialize()
    {
        // Find the AppStatusBar instance
        Type appStatusBarType = typeof(UnityEditor.Editor).Assembly
            .GetType("UnityEditor.AppStatusBar");
        statusBar = (ScriptableObject)Resources
            .FindObjectsOfTypeAll(appStatusBarType).FirstOrDefault();

        if (statusBar != null)
        {
            OnGUIInjector.Inject(statusBar, OnGUI);
        }
    }

    static void OnGUI()
    {
        // Draw custom content in the status bar
        GUILayout.Label("My Custom Label");
    }
}

See Also

• LowLevel Overview — subsystem overview

• EditorHeaderItemInjector — similar injection for header items

• ParagonEntertainmentBanner — uses OnGUIInjector for status bar drawing