Events

Unity calls your OnGUI kind of functions multiple times a frame to handle your GUI. For complex views, understanding Unity GUI Events can help you write more performant views.

You can get the current event for the current OnGUI call with Event.current.

private void OnGUI()
{
    var editorEvent = Event.current;
    if (editorEventType == EventType.ScrollWheel)
    {
        var scrollDelta = editorEvent.delta;
        // ...
    }
}

The most important events are Layout and Repaint.

• Layout is called before any other event. GUILayout calls use this to initialize themselves. Layout events may occur multiple times in one frame.
• Repaint is called only once at the end of a frame.

User interaction

There is also GUI.changed which you can use to detect user interaction.

private void OnGUI()
{
    var table = DrawTable(titles);
    
    if (GUI.changed)
    {
        // Users pressed something in the table.
        // Get the index and do stuff
        // once in a frame.
        var index = table.SelectedIndex;
        // ...
    }
}