Operators: Add option to give a modal operator priority in event handling

This may be used by add-ons like screencast keys, to log events before a
modal operator like transform handles them.

Pull Request: https://projects.blender.org/blender/blender/pulls/122226

source/blender/makesrna/intern/rna_wm.cc

@@ -493,6 +493,12 @@ const EnumPropertyItem rna_enum_operator_type_flag_items[] = {
      "before beginning the operation"},
     {OPTYPE_PRESET, "PRESET", 0, "Preset", "Display a preset button with the operators settings"},
     {OPTYPE_INTERNAL, "INTERNAL", 0, "Internal", "Removes the operator from search results"},
+    {OPTYPE_MODAL_PRIORITY,
+     "MODAL_PRIORITY",
+     0,
+     "Modal Priority",
+     "Handle events before other modal operators without this option. Use with caution, do not "
+     "modify data that other modal operators assume is unchanged during their operation"},
     {0, nullptr, 0, nullptr, nullptr},
 };
 

source/blender/windowmanager/WM_types.hh

@@ -196,6 +196,9 @@ enum {
    * Even so, accessing from the menu should behave usefully.
    */
   OPTYPE_DEPENDS_ON_CURSOR = (1 << 11),
+
+  /** Handle events before modal operators without this flag. */
+  OPTYPE_MODAL_PRIORITY = (1 << 12),
 };
 
 /** For #WM_cursor_grab_enable wrap axis. */

source/blender/windowmanager/intern/wm_event_system.cc

@@ -2465,6 +2465,36 @@ static void wm_event_modalkeymap_end(wmEvent *event, const wmEvent_ModalMapStore
   }
 }
 
+/**
+ * Insert modal operator into list of modal handlers, respecting priority.
+ */
+static void wm_handler_operator_insert(wmWindow *win, wmEventHandler_Op *handler)
+{
+  if (handler->op->type->flag & OPTYPE_MODAL_PRIORITY) {
+    BLI_addhead(&win->modalhandlers, handler);
+    return;
+  }
+
+  LISTBASE_FOREACH (wmEventHandler *, handler_iter, &win->modalhandlers) {
+    if (handler_iter->type == WM_HANDLER_TYPE_UI) {
+      /* UI always has priority. */
+      continue;
+    }
+    if (handler_iter->type == WM_HANDLER_TYPE_OP) {
+      wmEventHandler_Op *handler_iter_op = (wmEventHandler_Op *)handler_iter;
+      if (handler_iter_op->op->type->flag & OPTYPE_MODAL_PRIORITY) {
+        /* Keep priority operators in front. */
+        continue;
+      }
+
+      BLI_insertlinkbefore(&win->modalhandlers, handler_iter, handler);
+      return;
+    }
+  }
+
+  BLI_addtail(&win->modalhandlers, handler);
+}
+
 /**
  * \warning this function removes a modal handler, when finished.
  */
@@ -4373,7 +4403,7 @@ void WM_event_add_fileselect(bContext *C, wmOperator *op)
   handler->context.area = root_area;
   handler->context.region = root_region;
 
-  BLI_addhead(&root_win->modalhandlers, handler);
+  wm_handler_operator_insert(root_win, handler);
 
   /* Check props once before invoking if check is available
    * ensures initial properties are valid. */
@@ -4476,7 +4506,7 @@ wmEventHandler_Op *WM_event_add_modal_handler_ex(wmWindow *win,
   handler->context.region_type = handler->context.region ? handler->context.region->regiontype :
                                                            -1;
 
-  BLI_addhead(&win->modalhandlers, handler);
+  wm_handler_operator_insert(win, handler);
 
   if (op->type->modalkeymap) {
     WM_window_status_area_tag_redraw(win);