EditorHeaderItemInjector

Static utility that injects custom GUI callbacks into Unity's internal Editor header item list via reflection. Header items appear in the Inspector header area for UnityEngine.Object targets.

Definition

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

public static class EditorHeaderItemInjector

Remarks

Unity maintains an internal list of header item delegates in EditorGUIUtility.s_EditorHeaderItemsMethods. This static class uses reflection to access that list and inject custom callbacks at a specified index.

Initialization Flow

1. Static constructor reflects on EditorGUIUtility.s_EditorHeaderItemsMethods (a List<HeaderItemDelegate>)

2. Resolves the delegate type from the list's generic argument

3. If the internal list is not yet initialized (editor still loading), defers injection via EditorApplication.delayCall

4. Once initialized, flushes any queued callbacks into the list

Deferred Injection

Calls to Inject before Unity's header items list is ready are queued in methodsAwaitingInjection. Once Initialize() succeeds, all queued callbacks are injected in order.

Quick Lookup

Goal	How
Add header GUI callback	EditorHeaderItemInjector.Inject(MyCallback)
Add at specific position	EditorHeaderItemInjector.Inject(MyCallback, index: 0)
Callback signature	Func<Rect, UnityEngine.Object[], bool>

Fields

Field	Type	Access	Description
headerItemMethodDelegateType	Type	private static readonly	The delegate type used by Unity's internal header items list
initialized	bool	private static	Whether the internal list has been resolved and is ready
headerItemsMethodsField	FieldInfo	private static readonly	Reflected field for EditorGUIUtility.s_EditorHeaderItemsMethods
methodsAwaitingInjection	List<Func<Rect, Object[], bool>>	private static readonly	Queue of callbacks waiting for initialization

Methods

Inject

public static void Inject(Func<Rect, UnityEngine.Object[], bool> callback, int index = 0)

Registers a callback to be drawn in the Inspector header area.

Parameter	Type	Default	Description
callback	Func<Rect, Object[], bool>	—	GUI callback receiving the header rect and target objects. Return true if handled.
index	int	0	Position in the header items list (0 = first)

If called before the editor is fully initialized, the callback is queued and injected automatically once the internal list becomes available.

Common Pitfalls

Unity internals dependency This class reflects on EditorGUIUtility.s_EditorHeaderItemsMethods, which is a private static field. Changes to Unity's internal API between versions may break this functionality silently.

Callback delegate conversion The injected Func<Rect, Object[], bool> is converted to Unity's internal HeaderItemDelegate type via CreateDelegate. The callback method must be a static method — instance method delegates will throw.

Examples

Adding a Custom Header Item

[InitializeOnLoad]
static class MyHeaderExtension
{
    static MyHeaderExtension()
    {
        EditorHeaderItemInjector.Inject(OnHeaderGUI);
    }

    static bool OnHeaderGUI(Rect rect, UnityEngine.Object[] targets)
    {
        // Draw custom GUI in the Inspector header area
        if (GUI.Button(new Rect(rect.xMax - 50, rect.y, 50, rect.height), "Click"))
        {
            // Handle click for targets
            return true;
        }
        return false;
    }
}

See Also

• LowLevel Overview — subsystem overview

• OnGUIInjector — similar injection for editor window OnGUI