Draft toggle grid as a toggle button to indicate grid visibility (#14452)

Makes the button for the Draft_ToggleGrid command act as a real toggle button to provide visual feedback of the grid visibility state.

---------

Co-authored-by: Syres916 <46537884+Syres916@users.noreply.github.com>

src/Mod/Draft/draftutils/grid_observer.py

@@ -0,0 +1,117 @@
+# ***************************************************************************
+# *                                                                         *
+# *   Copyright (c) 2024 Syres                                              *
+# *   Copyright (c) 2024 Furgo                                              *
+# *                                                                         *
+# *   This program is free software; you can redistribute it and/or modify  *
+# *   it under the terms of the GNU Lesser General Public License (LGPL)    *
+# *   as published by the Free Software Foundation; either version 2 of     *
+# *   the License, or (at your option) any later version.                   *
+# *   for detail see the LICENCE text file.                                 *
+# *                                                                         *
+# *   This program is distributed in the hope that it will be useful,       *
+# *   but WITHOUT ANY WARRANTY; without even the implied warranty of        *
+# *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         *
+# *   GNU Library General Public License for more details.                  *
+# *                                                                         *
+# *   You should have received a copy of the GNU Library General Public     *
+# *   License along with this program; if not, write to the Free Software   *
+# *   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  *
+# *   USA                                                                   *
+# *                                                                         *
+# ***************************************************************************
+"""Provide the grid observer for the Draft and BIM workbenches.
+"""
+
+import FreeCAD
+
+
+# View observer code to update the Draft_ToggleGrid command button to reflect
+# the grid's visibility status.
+# Based on view observer code to update the Draft Tray.
+if FreeCAD.GuiUp:
+    import FreeCADGui
+    from PySide import QtWidgets
+    from draftutils import gui_utils
+    from draftutils.todo import ToDo
+
+    def _update_grid_gui():
+        """Update function executed by the callback to refresh the Toggle Grid
+        button state on all toolbars and menus.
+        """
+
+        try:
+            # Get the active view.
+            view = gui_utils.get_3d_view()
+
+            # If the active view is not a 3D view, uncheck the button.
+            if view is None:
+                _set_grid_button_state(False)
+                return
+
+            # Otherwise, if there is a view, update the button's checked state.
+
+            # Retrieve the associated grid for each MDI document.
+            # [1] is the index where the grid is stored in the
+            # trackers list. See setTrackers().
+            view_idx = FreeCADGui.Snapper.trackers[0].index(view)
+            grid = FreeCADGui.Snapper.trackers[1][view_idx]
+            if grid.Visible:
+                _set_grid_button_state(True)
+            else:
+                _set_grid_button_state(False)
+
+        except Exception:
+            pass
+
+    def _view_observer_callback():
+        """Callback function to update the Toggle Grid button.
+
+        The update will only happen if either the Draft or BIM
+        workbenches are active.
+        """
+
+        ToDo.delay(_update_grid_gui, None)
+
+    _view_observer_active = False
+
+    def _view_observer_setup():
+        """Start or stop the grid observer.
+
+        Start: it is intended to happen when either the Draft or BIM workbench
+        are activated. The _view_observer_callback is connected to Qt's
+        subWindowActivated signal. The MDI area emits the subWindowActivated()
+        signal when the active window changes.
+
+        Stop: This happens when either the Draft or BIM workbenches are
+        deactivated and is the reverse of the start operation.
+        """
+
+        mw = FreeCADGui.getMainWindow()
+        mdi = mw.findChild(QtWidgets.QMdiArea)
+        global _view_observer_active
+        if not _view_observer_active:
+            # Connect callback to subWindowActivated signal
+            mdi.subWindowActivated.connect(_view_observer_callback)
+            _view_observer_active = True
+            # Trigger initial grid button update
+            _view_observer_callback()
+        else:
+            mdi.subWindowActivated.disconnect(_view_observer_callback)
+            _view_observer_active = False
+
+    def _set_grid_button_state(button_check):
+        """Sets the checked state of the Draft_ToggleGrid command.
+        This is then reflected on the associated "Toggle Grid" buttons of every
+        toolbar and menu.
+
+        Args:
+            button_check (bool): if True, check the grid button.
+
+        Returns:
+            None
+        """
+
+        action = FreeCADGui.Command.get("Draft_ToggleGrid").getAction()[0]
+        action.setCheckable(True)
+        action.setChecked(button_check)