TilePlacer.cs

[EditorTool("Tile Placer Tool", typeof(TileData))]
class TilePlacerTool : EditorTool
{
    private GUIContent m_iconContent;

    void OnEnable()
    {
        var icon = AssetDatabase.LoadAssetAtPath<Texture2D>("Assets/Textures/Editor/TileEditorIcon.png");

        // Icon for tools toolbar
        m_iconContent = new GUIContent()
        {
            image = icon,
            text = "Tile Placer",
            tooltip = "Tile Placer"
        };
    }

    public override GUIContent toolbarIcon
    {
        get { return m_iconContent; }
    }


    public override void OnActivated()
    {
        base.OnActivated();
        SceneView.duringSceneGui += OnSceneGUI;
    }

    public override void OnWillBeDeactivated()
    {
        base.OnWillBeDeactivated();
        SceneView.duringSceneGui -= OnSceneGUI;
    }

    [Shortcut("Activate Tile Placer Tool", typeof(SceneView), KeyCode.P)]
    static void TilePlacerToolShortcut()
    {
        ToolManager.SetActiveTool<TilePlacerTool>();
    }

    private void OnSceneGUI(SceneView sceneView)
    {
        // Draw in Scene View
    }
}

[Overlay(typeof(SceneView), "Tile Placer Overlay")]
public class TilePlacerToolOverlay : IMGUIOverlay, ITransientOverlay
{
    public bool visible
    {
        get
        {
            return ToolManager.activeToolType == typeof(TilePlacerTool);
        }
    }

    public override void OnGUI()
    {
        // Draw overlay that is only visible when the TilePlacerTool is active.
    }
}