ENH: Display a dialog to save trans and fids before quitting the coreg app

mne/gui/_coreg.py

@@ -174,6 +174,8 @@ def _get_default(var, val):
         self._to_cf_t = None
         self._omit_hsp_distance = 0.0
         self._fiducials_file = None
+        self._trans_saved = False
+        self._fids_saved = False
         self._fid_colors = tuple(
             DEFAULTS['coreg'][f'{key}_color'] for key in
             ('lpa', 'nasion', 'rpa'))
@@ -226,10 +228,36 @@ def _get_default(var, val):
         self._renderer = _get_renderer(
             size=self._defaults["size"], bgcolor=self._defaults["bgcolor"])
         self._renderer.enable_depth_peeling()
-        self._renderer._window_close_connect(self._clean)
         self._renderer.set_interaction(interaction)
         self._renderer._status_bar_initialize()
 
+        # connect callback to close event
+        def closeEventCallback():
+            from ..viz.backends.renderer import MNE_3D_BACKEND_TESTING
+            if not self._trans_saved:
+                from PyQt5.QtWidgets import QMessageBox
+                if MNE_3D_BACKEND_TESTING:
+                    ret = QMessageBox.Cancel
+                else:
+                    ret = QMessageBox.warning(
+                        self._renderer._window,
+                        "CoregistrationUI",
+                        "The Head<>MRI transform has not been saved. "
+                        "Do you want to save it?",
+                        QMessageBox.Save | QMessageBox.Cancel,
+                        QMessageBox.Save
+                    )
+                if ret == QMessageBox.Save:
+                    self._forward_widget_command(
+                        "save_trans", "set_value", None)
+                else:
+                    assert ret == QMessageBox.Cancel
+            if not self._fids_saved:
+                self._forward_widget_command(
+                    "save_mri_fids", "set_value", None)
+            self._clean()
+        self._renderer._window_close_connect(closeEventCallback)
+
         # coregistration model setup
         self._immediate_redraw = (self._renderer._kind != 'qt')
         self._info = info
@@ -941,6 +969,8 @@ def _reset(self, keep_trans=False):
         self._update_plot()
         self._update_parameters()
         self._update_distance_estimation()
+        self._trans_saved = False
+        self._fids_saved = False
 
     def _forward_widget_command(self, names, command, value,
                                 input_value=True, output_value=False):
@@ -1232,11 +1262,13 @@ def _save_mri_fiducials(self, fname):
         )
         self._set_fiducials_file(fname)
         self._display_message(f"Saving {fname}... Done!")
+        self._fids_saved = True
 
     def _save_trans(self, fname):
         write_trans(fname, self.coreg.trans, overwrite=True)
         self._display_message(
             f"{fname} transform file is saved.")
+        self._trans_saved = True
 
     def _load_trans(self, fname):
         mri_head_t = _ensure_trans(read_trans(fname, return_all=True),

mne/viz/backends/_qt.py

@@ -506,13 +506,22 @@ def _window_initialize(self):
         self._window = self.figure.plotter.app_window
         self._window.setLocale(QLocale(QLocale.Language.English))
         self._window.signal_close.connect(self._window_clean)
+        self._window_close_callbacks = list()
+
+        # patch closeEvent
+        def closeEvent(event):
+            for callback in self._window_close_callbacks:
+                callback()
+            self._window.signal_close.emit()
+            event.accept()
+        self._window.closeEvent = closeEvent
 
     def _window_clean(self):
         self.figure.plotter = None
         self._interactor = None
 
     def _window_close_connect(self, func):
-        self._window.signal_close.connect(func)
+        self._window_close_callbacks.append(func)
 
     def _window_get_dpi(self):
         return self._window.windowHandle().screen().logicalDotsPerInch()