Change notifications to use "source_" prefix.

android/src/toga_android/widgets/detailedlist.py

@@ -211,16 +211,68 @@ def change_source(self, source):
     def after_on_refresh(self, widget, result):
         self._refresh_layout.setRefreshing(False)
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def insert(self, index, item):
+        import warnings
+
+        warnings.warn(
+            "The insert() method is deprecated. Use source_insert() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_insert(index=index, item=item)
+
+    def source_insert(self, *, index, item):
         self._load_data()
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def change(self, item):
+        import warnings
+
+        warnings.warn(
+            "The change() method is deprecated. Use source_change() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_change(item=item)
+
+    def source_change(self, *, item):
         self._load_data()
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def remove(self, index, item):
+        import warnings
+
+        warnings.warn(
+            "The remove() method is deprecated. Use source_remove() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_remove(index=index, item=item)
+
+    def source_remove(self, *, index, item):
         self._load_data()
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def clear(self):
+        import warnings
+
+        warnings.warn(
+            "The clear() method is deprecated. Use source_clear() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_clear()
+
+    def source_clear(self):
         self._load_data()
 
     def _clear_selection(self):

android/src/toga_android/widgets/selection.py

@@ -42,7 +42,20 @@ def on_change(self, index):
             self.interface.on_change()
             self.last_selection = index
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def insert(self, index, item):
+        import warnings
+
+        warnings.warn(
+            "The insert() method is deprecated. Use source_insert() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_insert(index=index, item=item)
+
+    def source_insert(self, *, index, item):
         self.adapter.insert(self.interface._title_for_item(item), index)
         if self.last_selection is None:
             self.select_item(0)
@@ -51,14 +64,40 @@ def insert(self, index, item):
             self.last_selection += 1
             self.select_item(self.last_selection)
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def change(self, item):
+        import warnings
+
+        warnings.warn(
+            "The change() method is deprecated. Use source_change() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_change(item=item)
+
+    def source_change(self, *, item):
         # Instead of calling self.insert and self.remove, use direct native calls to
         # avoid disturbing the selection.
         index = self.interface._items.index(item)
         self.adapter.insert(self.interface._title_for_item(item), index)
         self.adapter.remove(self.adapter.getItem(index + 1))
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def remove(self, index, item=None):
+        import warnings
+
+        warnings.warn(
+            "The remove() method is deprecated. Use source_remove() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_remove(index=index, item=item)
+
+    def source_remove(self, *, index, item=None):
         self.adapter.remove(self.adapter.getItem(index))
 
         # Adjust the selection index, but only generate an event if the selected item
@@ -82,7 +121,20 @@ def get_selected_index(self):
         selected = self.native.getSelectedItemPosition()
         return None if selected == Spinner.INVALID_POSITION else selected
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def clear(self):
+        import warnings
+
+        warnings.warn(
+            "The clear() method is deprecated. Use source_clear() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_clear()
+
+    def source_clear(self):
         self.adapter.clear()
         self.on_change(None)
 

android/src/toga_android/widgets/table.py

@@ -173,16 +173,68 @@ def get_selection(self):
         else:
             return selection[0]
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def insert(self, index, item):
+        import warnings
+
+        warnings.warn(
+            "The insert() method is deprecated. Use source_insert() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_insert(index=index, item=item)
+
+    def source_insert(self, *, index, item):
         self.change_source(getattr(self.interface, "data", None))
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def clear(self):
+        import warnings
+
+        warnings.warn(
+            "The clear() method is deprecated. Use source_clear() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_clear()
+
+    def source_clear(self):
         self.change_source(getattr(self.interface, "data", None))
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def change(self, item):
+        import warnings
+
+        warnings.warn(
+            "The change() method is deprecated. Use source_change() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_change(item=item)
+
+    def source_change(self, *, item):
         self.change_source(getattr(self.interface, "data", None))
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def remove(self, index, item):
+        import warnings
+
+        warnings.warn(
+            "The remove() method is deprecated. Use source_remove() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_remove(index=index, item=item)
+
+    def source_remove(self, *, index, item):
         self.change_source(getattr(self.interface, "data", None))
 
     def scroll_to_row(self, index):

changes/4046.bugfix.md

@@ -0,0 +1 @@
+The `ListListener` and `TreeListener` protocols no longer incompatibly override the `insert`, `remove` and `clear` methods of `ListSource` and `TreeSource`, permitting mutable sources which are also listeners.

changes/4046.removal.md

@@ -0,0 +1 @@
+Sources now look for methods with names of the form `source_{notification}`, rather than just `notification` when the `notify` method is called.  If you have a custom listener class with methods like `change`, `insert`, `remove` or `clear`, you should re-name them to have a `source_` prefix: `source_change`, `source_insert` and so on.

cocoa/src/toga_cocoa/widgets/detailedlist.py

@@ -176,20 +176,72 @@ def create(self):
     def change_source(self, source):
         self.native_detailedlist.reloadData()
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def insert(self, index, item):
+        import warnings
+
+        warnings.warn(
+            "The insert() method is deprecated. Use source_insert() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_insert(index=index, item=item)
+
+    def source_insert(self, *, index, item):
         self.native_detailedlist.reloadData()
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def change(self, item):
+        import warnings
+
+        warnings.warn(
+            "The change() method is deprecated. Use source_change() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_change(item=item)
+
+    def source_change(self, *, item):
         self.native_detailedlist.reloadData()
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def remove(self, index, item):
+        import warnings
+
+        warnings.warn(
+            "The remove() method is deprecated. Use source_remove() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_remove(index=index, item=item)
+
+    def source_remove(self, *, index, item):
         self.native_detailedlist.reloadData()
 
         # After deletion, the selection changes, but Cocoa doesn't send
         # a tableViewSelectionDidChange: message.
         self.interface.on_select()
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def clear(self):
+        import warnings
+
+        warnings.warn(
+            "The clear() method is deprecated. Use source_clear() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_clear()
+
+    def source_clear(self):
         self.native_detailedlist.reloadData()
 
     def set_refresh_enabled(self, enabled):

cocoa/src/toga_cocoa/widgets/selection.py

@@ -42,7 +42,20 @@ def set_color(self, color):
     def set_background_color(self, color):
         pass
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def insert(self, index, item):
+        import warnings
+
+        warnings.warn(
+            "The insert() method is deprecated. Use source_insert() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_insert(index=index, item=item)
+
+    def source_insert(self, *, index, item):
         # Issue 2319 - if item titles are not unique, macOS will move the existing item,
         # rather than creating a duplicate item. To work around this, create an item
         # with a temporary but unique name, then change the name. `_title_for_item()`
@@ -59,22 +72,61 @@ def insert(self, index, item):
         if len(self.interface.items) == 1:
             self.interface.on_change()
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def change(self, item):
+        import warnings
+
+        warnings.warn(
+            "The change() method is deprecated. Use source_change() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_change(item=item)
+
+    def source_change(self, *, item):
         index = self.interface._items.index(item)
         native_item = self.native.itemAtIndex(index)
         native_item.title = self.interface._title_for_item(item)
         # Changing the item text can change the layout size
         self.interface.refresh()
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def remove(self, index, item):
+        import warnings
+
+        warnings.warn(
+            "The remove() method is deprecated. Use source_remove() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_remove(index=index, item=item)
+
+    def source_remove(self, *, index, item):
         selection_change = self.native.indexOfSelectedItem == index
 
         self.native.removeItemAtIndex(index)
 
         if selection_change:
             self.interface.on_change()
 
+    # Alias for backwards compatibility:
+    # March 2026: In 0.5.3 and earlier, notification methods
+    # didn't start with 'source_'
     def clear(self):
+        import warnings
+
+        warnings.warn(
+            "The clear() method is deprecated. Use source_clear() instead.",
+            DeprecationWarning,
+            stacklevel=1,
+        )
+        self.source_clear()
+
+    def source_clear(self):
         self.native.removeAllItems()
         self.interface.on_change()