public void OnGUI()
        {
            // Reserve layout space for the progress bar, equal to the space for a
            // textfield:
            Rect progress_rect = GUILayoutUtility.GetRect(18, 18, "TextField");

            EditorGUI.ProgressBar(progress_rect, progress_complete_, progress_message_);
            EditorGUILayout.Space();

            if (bake_stage_ != BakeStage.kWaitForDoneButton)
            {
                if (GUILayout.Button("Cancel"))
                {
                    if (runner_status_ != null)
                    {
                        runner_status_.CancelTask();
                    }
                }
            }

            if (bake_stage_ == BakeStage.kWaitForDoneButton)
            {
                if (GUILayout.Button("Done"))
                {
                    bake_stage_ = BakeStage.kComplete;
                }
            }
        }