Fix: Nodes: bad ui for adding panel toggles

Adding panel toggles in nodegroups have somewhat of a UX antipattern. When
running the operator, it checks for conditions that indicate it should not run,
and if those are hit, it cancels execution and mentions the invalid condition in
the footer bar.

This is not ideal, the user should not have to call the operator to find out
whether it can be called.

Why it got implemented like this is likely a consequence of all interface items
being the same "New Item" operator. Poll functions cannot use operator
properties, so variants of the same operator cannot check for different
conditions for execution.

This is a problem for panel toggles, as they have more restrictions to when they
can be added that don't apply to other interface items.

This patch creates a separate operator for adding panel toggles. This allows the
condition checks to be implemented in the poll function, which enables greying
out the operator buttons and showing on tooltips what condition is invalid.

Pull Request: https://projects.blender.org/blender/blender/pulls/146379

scripts/startup/bl_operators/node.py

@@ -916,32 +916,15 @@ class NODE_OT_interface_item_new(NodeInterfaceOperator, Operator):
     bl_label = "New Item"
     bl_options = {'REGISTER', 'UNDO'}
 
-    def get_items(_self, context):
-        items = [
-            ('INPUT', "Input", ""),
-            ('OUTPUT', "Output", ""),
-            ('PANEL', "Panel", ""),
-        ]
-
-        if context is None:
-            return items
-
-        snode = context.space_data
-        tree = snode.edit_tree
-        interface = tree.interface
-
-        active_item = interface.active
-        # Panels have the extra option to add a toggle.
-        if active_item and active_item.item_type == 'PANEL':
-            items.append(('PANEL_TOGGLE', "Panel Toggle", ""))
-
-        return items
-
     item_type: EnumProperty(
         name="Item Type",
         description="Type of the item to create",
-        items=get_items,
-        default=0,
+        items=(
+            ('INPUT', "Input", ""),
+            ('OUTPUT', "Output", ""),
+            ('PANEL', "Panel", ""),
+        ),
+        default='INPUT',
     )
 
     # Returns a valid socket type for the given tree or None.
@@ -977,18 +960,6 @@ class NODE_OT_interface_item_new(NodeInterfaceOperator, Operator):
             item = interface.new_socket("Socket", socket_type=self.find_valid_socket_type(tree), in_out='OUTPUT')
         elif self.item_type == 'PANEL':
             item = interface.new_panel("Panel")
-        elif self.item_type == 'PANEL_TOGGLE':
-            active_panel = active_item
-            if len(active_panel.interface_items) > 0:
-                first_item = active_panel.interface_items[0]
-                if type(first_item) is bpy.types.NodeTreeInterfaceSocketBool and first_item.is_panel_toggle:
-                    self.report({'INFO'}, "Panel already has a toggle")
-                    return {'CANCELLED'}
-            item = interface.new_socket(active_panel.name, socket_type='NodeSocketBool', in_out='INPUT')
-            item.is_panel_toggle = True
-            interface.move_to_parent(item, active_panel, 0)
-            # Return in this case because we don't want to move the item.
-            return {'FINISHED'}
         else:
             return {'CANCELLED'}
 
@@ -1003,6 +974,55 @@ class NODE_OT_interface_item_new(NodeInterfaceOperator, Operator):
         return {'FINISHED'}
 
 
+class NODE_OT_interface_item_new_panel_toggle(Operator):
+    '''Add a checkbox to the currently selected panel'''
+    bl_idname = "node.interface_item_new_panel_toggle"
+    bl_label = "New Panel Toggle"
+    bl_options = {'REGISTER', 'UNDO'}
+
+    @staticmethod
+    def get_panel_toggle(panel):
+        if len(panel.interface_items) > 0:
+            first_item = panel.interface_items[0]
+            if type(first_item) is bpy.types.NodeTreeInterfaceSocketBool and first_item.is_panel_toggle:
+                return first_item
+
+        return None
+
+    @classmethod
+    def poll(cls, context):
+        try:
+            snode = context.space_data
+            tree = snode.edit_tree
+            interface = tree.interface
+
+            active_item = interface.active
+
+            if active_item.item_type != 'PANEL':
+                cls.poll_message_set("Active item is not a panel")
+                return False
+
+            if cls.get_panel_toggle(active_item) is not None:
+                cls.poll_message_set("Panel already has a toggle")
+                return False
+
+            return True
+        except AttributeError:
+            return False
+
+    def execute(self, context):
+        snode = context.space_data
+        tree = snode.edit_tree
+
+        interface = tree.interface
+        active_panel = interface.active
+
+        item = interface.new_socket(active_panel.name, socket_type='NodeSocketBool', in_out='INPUT')
+        item.is_panel_toggle = True
+        interface.move_to_parent(item, active_panel, 0)
+        return {'FINISHED'}
+
+
 class NODE_OT_interface_item_duplicate(NodeInterfaceOperator, Operator):
     """Add a copy of the active item to the interface"""
     bl_idname = "node.interface_item_duplicate"
@@ -1309,6 +1329,7 @@ classes = (
     NODE_OT_add_closure_zone,
     NODE_OT_collapse_hide_unused_toggle,
     NODE_OT_interface_item_new,
+    NODE_OT_interface_item_new_panel_toggle,
     NODE_OT_interface_item_duplicate,
     NODE_OT_interface_item_remove,
     NODE_OT_interface_item_make_panel_toggle,