UI: Internal support for custom UI list item drag & activate operators

For pose libraries, we need to be able to apply a pose whenever
activating (clicking) an item in the Pose Library asset view and blend
it by dragging (press & move). And since we want to allow Python scripts
to define what happens at least when activating an asset (so they can
define for example a custom "Apply" operator for preset assets), it
makes sense to just let them pass an operator name to the asset view
template. The template will be introduced in a following commit.

source/blender/editors/include/UI_interface.h

@@ -588,6 +588,8 @@ bool UI_block_is_empty_ex(const uiBlock *block, const bool skip_title);
 bool UI_block_is_empty(const uiBlock *block);
 bool UI_block_can_add_separator(const uiBlock *block);
 
+struct uiList *UI_list_find_mouse_over(const struct ARegion *region, const struct wmEvent *event);
+
 /* interface_region_menu_popup.c */
 /**
  * Popup Menus
@@ -2282,6 +2284,13 @@ void uiTemplateFileSelectPath(uiLayout *layout,
                               struct bContext *C,
                               struct FileSelectParams *params);
 
+struct PointerRNA *UI_list_custom_activate_operator_set(struct uiList *ui_list,
+                                                        const char *opname,
+                                                        bool create_properties);
+struct PointerRNA *UI_list_custom_drag_operator_set(struct uiList *ui_list,
+                                                    const char *opname,
+                                                    bool create_properties);
+
 /* items */
 void uiItemO(uiLayout *layout, const char *name, int icon, const char *opname);
 void uiItemEnumO_ptr(uiLayout *layout,

source/blender/editors/interface/interface_handlers.c

@@ -767,23 +767,34 @@ static uiAfterFunc *ui_afterfunc_new(void)
  * For executing operators after the button is pressed.
  * (some non operator buttons need to trigger operators), see: T37795.
  *
+ * \param context_but: A button from which to get the context from (`uiBut.context`) for the
+ *                     operator execution.
+ *
+ * \note Ownership over \a properties is moved here. The after-func owns it now.
  * \note Can only call while handling buttons.
  */
-PointerRNA *ui_handle_afterfunc_add_operator(wmOperatorType *ot, int opcontext, bool create_props)
+static void ui_handle_afterfunc_add_operator_ex(wmOperatorType *ot,
+                                                PointerRNA **properties,
+                                                int opcontext,
+                                                const uiBut *context_but)
 {
-  PointerRNA *ptr = NULL;
   uiAfterFunc *after = ui_afterfunc_new();
 
   after->optype = ot;
   after->opcontext = opcontext;
-
-  if (create_props) {
-    ptr = MEM_callocN(sizeof(PointerRNA), __func__);
-    WM_operator_properties_create_ptr(ptr, ot);
-    after->opptr = ptr;
+  if (properties) {
+    after->opptr = *properties;
+    *properties = NULL;
   }
 
-  return ptr;
+  if (context_but && context_but->context) {
+    after->context = CTX_store_copy(context_but->context);
+  }
+}
+
+void ui_handle_afterfunc_add_operator(wmOperatorType *ot, int opcontext)
+{
+  ui_handle_afterfunc_add_operator_ex(ot, NULL, opcontext, NULL);
 }
 
 static void popup_check(bContext *C, wmOperator *op)
@@ -1145,6 +1156,42 @@ static void ui_apply_but_ROW(bContext *C, uiBlock *block, uiBut *but, uiHandleBu
   data->applied = true;
 }
 
+/**
+ * \note Ownership of \a properties is moved here. The after-func owns it now.
+ *
+ * \param context_but: The button to use context from when calling or polling the operator.
+ *
+ * \returns true if the operator was executed, otherwise false.
+ */
+static bool ui_list_invoke_item_operator(bContext *C,
+                                         const uiBut *context_but,
+                                         wmOperatorType *ot,
+                                         PointerRNA **properties)
+{
+  if (!ui_but_context_poll_operator(C, ot, context_but)) {
+    return false;
+  }
+
+  /* Allow the context to be set from the hovered button, so the list item draw callback can set
+   * context for the operators. */
+  ui_handle_afterfunc_add_operator_ex(ot, properties, WM_OP_INVOKE_DEFAULT, context_but);
+  return true;
+}
+
+static void ui_apply_but_LISTROW(bContext *C, uiBlock *block, uiBut *but, uiHandleButtonData *data)
+{
+  uiBut *listbox = ui_list_find_from_row(data->region, but);
+  if (listbox) {
+    uiList *list = listbox->custom_data;
+    if (list && list->dyn_data->custom_activate_optype) {
+      ui_list_invoke_item_operator(
+          C, but, list->dyn_data->custom_activate_optype, &list->dyn_data->custom_activate_opptr);
+    }
+  }
+
+  ui_apply_but_ROW(C, block, but, data);
+}
+
 static void ui_apply_but_TEX(bContext *C, uiBut *but, uiHandleButtonData *data)
 {
   if (!data->str) {
@@ -1617,7 +1664,7 @@ static void ui_drag_toggle_set(bContext *C, uiDragToggleHandle *drag_info, const
    */
   if (drag_info->is_xy_lock_init == false) {
     /* first store the buttons original coords */
-    uiBut *but = ui_but_find_mouse_over_ex(region, xy_input[0], xy_input[1], true);
+    uiBut *but = ui_but_find_mouse_over_ex(region, xy_input[0], xy_input[1], true, NULL, NULL);
 
     if (but) {
       if (but->flag & UI_BUT_DRAG_LOCK) {
@@ -1688,7 +1735,7 @@ static int ui_handler_region_drag_toggle(bContext *C, const wmEvent *event, void
     wmWindow *win = CTX_wm_window(C);
     const ARegion *region = CTX_wm_region(C);
     uiBut *but = ui_but_find_mouse_over_ex(
-        region, drag_info->xy_init[0], drag_info->xy_init[1], true);
+        region, drag_info->xy_init[0], drag_info->xy_init[1], true, NULL, NULL);
 
     if (but) {
       ui_apply_but_undo(but);
@@ -2250,9 +2297,11 @@ static void ui_apply_but(
       ui_apply_but_TOG(C, but, data);
       break;
     case UI_BTYPE_ROW:
-    case UI_BTYPE_LISTROW:
       ui_apply_but_ROW(C, block, but, data);
       break;
+    case UI_BTYPE_LISTROW:
+      ui_apply_but_LISTROW(C, block, but, data);
+      break;
     case UI_BTYPE_DATASETROW:
       ui_apply_but_ROW(C, block, but, data);
       break;
@@ -4284,7 +4333,7 @@ static uiBut *ui_but_list_row_text_activate(bContext *C,
                                             uiButtonActivateType activate_type)
 {
   ARegion *region = CTX_wm_region(C);
-  uiBut *labelbut = ui_but_find_mouse_over_ex(region, event->x, event->y, true);
+  uiBut *labelbut = ui_but_find_mouse_over_ex(region, event->x, event->y, true, NULL, NULL);
 
   if (labelbut && labelbut->type == UI_BTYPE_TEXT && !(labelbut->flag & UI_BUT_DISABLED)) {
     /* exit listrow */
@@ -4778,6 +4827,15 @@ static int ui_do_but_EXIT(bContext *C, uiBut *but, uiHandleButtonData *data, con
       if (but->dragpoin && but->imb && ui_but_contains_point_px_icon(but, data->region, event)) {
         ret = WM_UI_HANDLER_CONTINUE;
       }
+      /* Same special case handling for UI lists. Return CONTINUE so that a tweak or CLICK event
+       * will be sent for the list to work with. */
+      const uiBut *listbox = ui_list_find_mouse_over(data->region, event);
+      if (listbox) {
+        const uiList *ui_list = listbox->custom_data;
+        if (ui_list && ui_list->dyn_data->custom_drag_optype) {
+          ret = WM_UI_HANDLER_CONTINUE;
+        }
+      }
       button_activate_state(C, but, BUTTON_STATE_EXIT);
       return ret;
     }
@@ -9289,6 +9347,128 @@ static int ui_handle_button_event(bContext *C, const wmEvent *event, uiBut *but)
   return retval;
 }
 
+/**
+ * Activate the underlying list-row button, so the row is highlighted.
+ * Early exits if \a activate_dragging is true, but the custom drag operator fails to execute.
+ * Gives the wanted behavior where the item is activated on a tweak event when the custom drag
+ * operator is executed.
+ */
+static int ui_list_activate_hovered_row(bContext *C,
+                                        ARegion *region,
+                                        const uiList *ui_list,
+                                        const wmEvent *event,
+                                        bool activate_dragging)
+{
+  const bool do_drag = activate_dragging && ui_list->dyn_data->custom_drag_optype;
+
+  if (do_drag) {
+    const uiBut *hovered_but = ui_but_find_mouse_over(region, event);
+    if (!ui_list_invoke_item_operator(C,
+                                      hovered_but,
+                                      ui_list->dyn_data->custom_drag_optype,
+                                      &ui_list->dyn_data->custom_drag_opptr)) {
+      return WM_UI_HANDLER_CONTINUE;
+    }
+  }
+
+  const int *mouse_xy = ISTWEAK(event->type) ? &event->prevclickx : &event->x;
+  uiBut *listrow = ui_list_row_find_mouse_over(region, mouse_xy[0], mouse_xy[1]);
+  if (listrow) {
+    wmOperatorType *custom_activate_optype = ui_list->dyn_data->custom_activate_optype;
+
+    /* Hacky: Ensure the custom activate operator is not called when the custom drag operator
+     * was. Only one should run! */
+    if (activate_dragging && do_drag) {
+      ((uiList *)ui_list)->dyn_data->custom_activate_optype = NULL;
+    }
+
+    /* Simulate click on listrow button itself (which may be overlapped by another button). Also
+     * calls the custom activate operator (ui_list->custom_activate_opname). */
+    UI_but_execute(C, region, listrow);
+
+    ((uiList *)ui_list)->dyn_data->custom_activate_optype = custom_activate_optype;
+  }
+
+  return WM_UI_HANDLER_BREAK;
+}
+
+static bool ui_list_is_hovering_draggable_but(bContext *C,
+                                              const uiList *list,
+                                              const ARegion *region,
+                                              const wmEvent *event)
+{
+  /* On a tweak event, uses the coordinates from where tweaking was started. */
+  const int *mouse_xy = ISTWEAK(event->type) ? &event->prevclickx : &event->x;
+  const uiBut *hovered_but = ui_but_find_mouse_over_ex(
+      region, mouse_xy[0], mouse_xy[1], false, NULL, NULL);
+
+  if (list->dyn_data->custom_drag_optype) {
+    if (ui_but_context_poll_operator(C, list->dyn_data->custom_drag_optype, hovered_but)) {
+      return true;
+    }
+  }
+
+  return (hovered_but && hovered_but->dragpoin);
+}
+
+static int ui_list_handle_click_drag(bContext *C,
+                                     const uiList *ui_list,
+                                     ARegion *region,
+                                     const wmEvent *event)
+{
+  if (!ELEM(event->type, LEFTMOUSE, EVT_TWEAK_L)) {
+    return WM_HANDLER_CONTINUE;
+  }
+
+  int retval = WM_HANDLER_CONTINUE;
+
+  const bool is_draggable = ui_list_is_hovering_draggable_but(C, ui_list, region, event);
+  bool activate = false;
+  bool activate_dragging = false;
+
+  if (event->type == EVT_TWEAK_L) {
+    if (is_draggable) {
+      activate_dragging = true;
+      activate = true;
+    }
+  }
+  /* KM_CLICK is only sent after an uncaught release event, so the forground button gets all
+   * regular events (including mouse presses to start dragging) and this part only kicks in if it
+   * hasn't handled the release event. Note that if there's no overlaid button, the row selects
+   * on the press event already via regular UI_BTYPE_LISTROW handling. */
+  else if ((event->type == LEFTMOUSE) && (event->val == KM_CLICK)) {
+    activate = true;
+  }
+
+  if (activate) {
+    retval = ui_list_activate_hovered_row(C, region, ui_list, event, activate_dragging);
+  }
+
+  return retval;
+}
+
+static void ui_list_activate_row_from_index(
+    bContext *C, ARegion *region, uiBut *listbox, uiList *ui_list, int index)
+{
+  uiBut *new_active_row = ui_list_row_find_from_index(region, index, listbox);
+  if (new_active_row) {
+    /* Preferred way to update the active item, also calls the custom activate operator
+     * (#uiList.custom_activate_opname). */
+    UI_but_execute(C, region, new_active_row);
+  }
+  else {
+    /* A bit ugly, set the active index in RNA directly. That's because a button that's
+     * scrolled away in the list box isn't created at all.
+     * The custom activate operator (#uiList.custom_activate_opname) is not called in this case
+     * (which may need the row button context).*/
+    RNA_property_int_set(&listbox->rnapoin, listbox->rnaprop, index);
+    RNA_property_update(C, &listbox->rnapoin, listbox->rnaprop);
+    ui_apply_but_undo(listbox);
+  }
+
+  ui_list->flag |= UILST_SCROLL_TO_ACTIVE_ITEM;
+}
+
 static int ui_handle_list_event(bContext *C, const wmEvent *event, ARegion *region, uiBut *listbox)
 {
   int retval = WM_UI_HANDLER_CONTINUE;
@@ -9322,7 +9502,10 @@ static int ui_handle_list_event(bContext *C, const wmEvent *event, ARegion *regi
     }
   }
 
-  if (val == KM_PRESS) {
+  if (ELEM(event->type, LEFTMOUSE, EVT_TWEAK_L)) {
+    retval = ui_list_handle_click_drag(C, ui_list, region, event);
+  }
+  else if (val == KM_PRESS) {
     if ((ELEM(type, EVT_UPARROWKEY, EVT_DOWNARROWKEY) &&
          !IS_EVENT_MOD(event, shift, ctrl, alt, oskey)) ||
         ((ELEM(type, WHEELUPMOUSE, WHEELDOWNMOUSE) && event->ctrl &&
@@ -9384,12 +9567,7 @@ static int ui_handle_list_event(bContext *C, const wmEvent *event, ARegion *regi
       CLAMP(value, min, max);
 
       if (value != value_orig) {
-        RNA_property_int_set(&listbox->rnapoin, listbox->rnaprop, value);
-        RNA_property_update(C, &listbox->rnapoin, listbox->rnaprop);
-
-        ui_apply_but_undo(listbox);
-
-        ui_list->flag |= UILST_SCROLL_TO_ACTIVE_ITEM;
+        ui_list_activate_row_from_index(C, region, listbox, ui_list, value);
         redraw = true;
       }
       retval = WM_UI_HANDLER_BREAK;

source/blender/editors/interface/interface_intern.h

@@ -931,9 +931,7 @@ const char *ui_textedit_undo(struct uiUndoStack_Text *undo_stack,
                              int *r_cursor_index);
 
 /* interface_handlers.c */
-PointerRNA *ui_handle_afterfunc_add_operator(struct wmOperatorType *ot,
-                                             int opcontext,
-                                             bool create_props);
+extern void ui_handle_afterfunc_add_operator(struct wmOperatorType *ot, int opcontext);
 extern void ui_pan_to_scroll(const struct wmEvent *event, int *type, int *val);
 extern void ui_but_activate_event(struct bContext *C, struct ARegion *region, uiBut *but);
 extern void ui_but_activate_over(struct bContext *C, struct ARegion *region, uiBut *but);
@@ -1128,19 +1126,32 @@ bool ui_but_contains_point_px_icon(const uiBut *but,
 bool ui_but_contains_point_px(const uiBut *but, const struct ARegion *region, int x, int y)
     ATTR_WARN_UNUSED_RESULT;
 
-uiBut *ui_list_find_mouse_over(struct ARegion *region,
+uiBut *ui_list_find_mouse_over(const struct ARegion *region,
                                const struct wmEvent *event) ATTR_WARN_UNUSED_RESULT;
+uiBut *ui_list_find_from_row(const struct ARegion *region,
+                             const uiBut *row_but) ATTR_WARN_UNUSED_RESULT;
+uiBut *ui_list_row_find_mouse_over(const struct ARegion *region,
+                                   int x,
+                                   int y) ATTR_WARN_UNUSED_RESULT;
+uiBut *ui_list_row_find_from_index(const struct ARegion *region,
+                                   const int index,
+                                   uiBut *listbox) ATTR_WARN_UNUSED_RESULT;
 
+typedef bool (*uiButFindPollFn)(const uiBut *but, const void *customdata);
 uiBut *ui_but_find_mouse_over_ex(const struct ARegion *region,
                                  const int x,
                                  const int y,
-                                 const bool labeledit) ATTR_WARN_UNUSED_RESULT;
+                                 const bool labeledit,
+                                 const uiButFindPollFn find_poll,
+                                 const void *find_custom_data) ATTR_WARN_UNUSED_RESULT;
 uiBut *ui_but_find_mouse_over(const struct ARegion *region,
                               const struct wmEvent *event) ATTR_WARN_UNUSED_RESULT;
 uiBut *ui_but_find_rect_over(const struct ARegion *region,
                              const rcti *rect_px) ATTR_WARN_UNUSED_RESULT;
 
-uiBut *ui_list_find_mouse_over_ex(struct ARegion *region, int x, int y) ATTR_WARN_UNUSED_RESULT;
+uiBut *ui_list_find_mouse_over_ex(const struct ARegion *region,
+                                  int x,
+                                  int y) ATTR_WARN_UNUSED_RESULT;
 
 bool ui_but_contains_password(const uiBut *but) ATTR_WARN_UNUSED_RESULT;
 
@@ -1152,6 +1163,7 @@ uiBut *ui_but_next(uiBut *but) ATTR_WARN_UNUSED_RESULT;
 uiBut *ui_but_first(uiBlock *block) ATTR_WARN_UNUSED_RESULT;
 uiBut *ui_but_last(uiBlock *block) ATTR_WARN_UNUSED_RESULT;
 
+uiBut *ui_block_active_but_get(const uiBlock *block);
 bool ui_block_is_menu(const uiBlock *block) ATTR_WARN_UNUSED_RESULT;
 bool ui_block_is_popover(const uiBlock *block) ATTR_WARN_UNUSED_RESULT;
 bool ui_block_is_pie_menu(const uiBlock *block) ATTR_WARN_UNUSED_RESULT;

source/blender/editors/interface/interface_panel.c

@@ -2653,7 +2653,7 @@ static void panel_activate_state(const bContext *C, Panel *panel, const uiHandle
 
     /* Initiate edge panning during drags for scrolling beyond the initial region view. */
     wmOperatorType *ot = WM_operatortype_find("VIEW2D_OT_edge_pan", true);
-    ui_handle_afterfunc_add_operator(ot, WM_OP_INVOKE_DEFAULT, true);
+    ui_handle_afterfunc_add_operator(ot, WM_OP_INVOKE_DEFAULT);
   }
   else if (state == PANEL_STATE_ANIMATION) {
     panel_set_flag_recursive(panel, PNL_SELECT, false);

source/blender/editors/interface/interface_query.c

@@ -266,11 +266,29 @@ bool ui_but_contains_point_px_icon(const uiBut *but, ARegion *region, const wmEv
   return BLI_rcti_isect_pt(&rect, x, y);
 }
 
+static uiBut *ui_but_find(const ARegion *region,
+                          const uiButFindPollFn find_poll,
+                          const void *find_custom_data)
+{
+  LISTBASE_FOREACH (uiBlock *, block, &region->uiblocks) {
+    LISTBASE_FOREACH_BACKWARD (uiBut *, but, &block->buttons) {
+      if (find_poll && find_poll(but, find_custom_data) == false) {
+        continue;
+      }
+      return but;
+    }
+  }
+
+  return NULL;
+}
+
 /* x and y are only used in case event is NULL... */
 uiBut *ui_but_find_mouse_over_ex(const ARegion *region,
                                  const int x,
                                  const int y,
-                                 const bool labeledit)
+                                 const bool labeledit,
+                                 const uiButFindPollFn find_poll,
+                                 const void *find_custom_data)
 {
   uiBut *butover = NULL;
 
@@ -282,6 +300,9 @@ uiBut *ui_but_find_mouse_over_ex(const ARegion *region,
     ui_window_to_block_fl(region, block, &mx, &my);
 
     LISTBASE_FOREACH_BACKWARD (uiBut *, but, &block->buttons) {
+      if (find_poll && find_poll(but, find_custom_data) == false) {
+        continue;
+      }
       if (ui_but_is_interactive(but, labeledit)) {
         if (but->pie_dir != UI_RADIAL_NONE) {
           if (ui_but_isect_pie_seg(block, but)) {
@@ -310,7 +331,7 @@ uiBut *ui_but_find_mouse_over_ex(const ARegion *region,
 
 uiBut *ui_but_find_mouse_over(const ARegion *region, const wmEvent *event)
 {
-  return ui_but_find_mouse_over_ex(region, event->x, event->y, event->ctrl != 0);
+  return ui_but_find_mouse_over_ex(region, event->x, event->y, event->ctrl != 0, NULL, NULL);
 }
 
 uiBut *ui_but_find_rect_over(const struct ARegion *region, const rcti *rect_px)
@@ -351,7 +372,7 @@ uiBut *ui_but_find_rect_over(const struct ARegion *region, const rcti *rect_px)
   return butover;
 }
 
-uiBut *ui_list_find_mouse_over_ex(ARegion *region, int x, int y)
+uiBut *ui_list_find_mouse_over_ex(const ARegion *region, int x, int y)
 {
   if (!ui_region_contains_point_px(region, x, y)) {
     return NULL;
@@ -369,11 +390,77 @@ uiBut *ui_list_find_mouse_over_ex(ARegion *region, int x, int y)
   return NULL;
 }
 
-uiBut *ui_list_find_mouse_over(ARegion *region, const wmEvent *event)
+uiBut *ui_list_find_mouse_over(const ARegion *region, const wmEvent *event)
 {
+  if (event == NULL) {
+    /* If there is no info about the mouse, just act as if there is nothing underneath it. */
+    return NULL;
+  }
   return ui_list_find_mouse_over_ex(region, event->x, event->y);
 }
 
+uiList *UI_list_find_mouse_over(const ARegion *region, const wmEvent *event)
+{
+  uiBut *list_but = ui_list_find_mouse_over(region, event);
+  if (!list_but) {
+    return NULL;
+  }
+
+  return list_but->custom_data;
+}
+
+static bool ui_list_contains_row(const uiBut *listbox_but, const uiBut *listrow_but)
+{
+  BLI_assert(listbox_but->type == UI_BTYPE_LISTBOX);
+  BLI_assert(listrow_but->type == UI_BTYPE_LISTROW);
+  /* The list box and its rows have the same RNA data (active data pointer/prop). */
+  return ui_but_rna_equals(listbox_but, listrow_but);
+}
+
+static bool ui_but_is_listbox_with_row(const uiBut *but, const void *customdata)
+{
+  const uiBut *row_but = customdata;
+  return (but->type == UI_BTYPE_LISTBOX) && ui_list_contains_row(but, row_but);
+}
+
+uiBut *ui_list_find_from_row(const ARegion *region, const uiBut *row_but)
+{
+  return ui_but_find(region, ui_but_is_listbox_with_row, row_but);
+}
+
+static bool ui_but_is_listrow(const uiBut *but, const void *UNUSED(customdata))
+{
+  return but->type == UI_BTYPE_LISTROW;
+}
+
+uiBut *ui_list_row_find_mouse_over(const ARegion *region, const int x, const int y)
+{
+  return ui_but_find_mouse_over_ex(region, x, y, false, ui_but_is_listrow, NULL);
+}
+
+struct ListRowFindIndexData {
+  int index;
+  uiBut *listbox;
+};
+
+static bool ui_but_is_listrow_at_index(const uiBut *but, const void *customdata)
+{
+  const struct ListRowFindIndexData *find_data = customdata;
+
+  return ui_but_is_listrow(but, NULL) && ui_list_contains_row(find_data->listbox, but) &&
+         (but->hardmax == find_data->index);
+}
+
+uiBut *ui_list_row_find_from_index(const ARegion *region, const int index, uiBut *listbox)
+{
+  BLI_assert(listbox->type == UI_BTYPE_LISTBOX);
+  struct ListRowFindIndexData data = {
+      .index = index,
+      .listbox = listbox,
+  };
+  return ui_but_find(region, ui_but_is_listrow_at_index, &data);
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */

source/blender/editors/interface/interface_templates.c

@@ -5835,6 +5835,15 @@ static void uilist_free_dyn_data(uiList *ui_list)
     return;
   }
 
+  if (dyn_data->custom_activate_opptr) {
+    WM_operator_properties_free(dyn_data->custom_activate_opptr);
+    MEM_freeN(dyn_data->custom_activate_opptr);
+  }
+  if (dyn_data->custom_drag_opptr) {
+    WM_operator_properties_free(dyn_data->custom_drag_opptr);
+    MEM_freeN(dyn_data->custom_drag_opptr);
+  }
+
   MEM_SAFE_FREE(dyn_data->items_filter_flags);
   MEM_SAFE_FREE(dyn_data->items_filter_neworder);
   MEM_SAFE_FREE(dyn_data->customdata);
@@ -6848,6 +6857,46 @@ void uiTemplateList(uiLayout *layout,
                     NULL);
 }
 
+/**
+ * \return: A RNA pointer for the operator properties.
+ */
+PointerRNA *UI_list_custom_activate_operator_set(uiList *ui_list,
+                                                 const char *opname,
+                                                 bool create_properties)
+{
+  uiListDyn *dyn_data = ui_list->dyn_data;
+  dyn_data->custom_activate_optype = WM_operatortype_find(opname, false);
+  if (!dyn_data->custom_activate_optype) {
+    return NULL;
+  }
+
+  if (create_properties) {
+    WM_operator_properties_alloc(&dyn_data->custom_activate_opptr, NULL, opname);
+  }
+
+  return dyn_data->custom_activate_opptr;
+}
+
+/**
+ * \return: A RNA pointer for the operator properties.
+ */
+PointerRNA *UI_list_custom_drag_operator_set(uiList *ui_list,
+                                             const char *opname,
+                                             bool create_properties)
+{
+  uiListDyn *dyn_data = ui_list->dyn_data;
+  dyn_data->custom_drag_optype = WM_operatortype_find(opname, false);
+  if (!dyn_data->custom_drag_optype) {
+    return NULL;
+  }
+
+  if (create_properties) {
+    WM_operator_properties_alloc(&dyn_data->custom_drag_opptr, NULL, opname);
+  }
+
+  return dyn_data->custom_drag_opptr;
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */

source/blender/makesdna/DNA_screen_types.h

@@ -287,6 +287,11 @@ typedef struct uiListDyn {
   int *items_filter_flags;
   /** Org_idx -> new_idx, items_len length. */
   int *items_filter_neworder;
+
+  struct wmOperatorType *custom_drag_optype;
+  struct PointerRNA *custom_drag_opptr;
+  struct wmOperatorType *custom_activate_optype;
+  struct PointerRNA *custom_activate_opptr;
 } uiListDyn;
 
 typedef struct uiList { /* some list UI data need to be saved in file */
@@ -313,6 +318,12 @@ typedef struct uiList { /* some list UI data need to be saved in file */
   int filter_flag;
   int filter_sort_flag;
 
+  /** Operator executed when activating an item. */
+  const char *custom_activate_opname;
+  /** Operator executed when dragging an item (item gets activated too, without running
+   * custom_activate_opname above). */
+  const char *custom_drag_opname;
+
   /* Custom sub-classes properties. */
   IDProperty *properties;
 

source/blender/windowmanager/intern/wm_operators.c

@@ -604,6 +604,12 @@ void WM_operator_properties_create(PointerRNA *ptr, const char *opstring)
  * used for keymaps and macros */
 void WM_operator_properties_alloc(PointerRNA **ptr, IDProperty **properties, const char *opstring)
 {
+  IDProperty *tmp_properties = NULL;
+  /* Allow passing NULL for properties, just create the properties here then. */
+  if (properties == NULL) {
+    properties = &tmp_properties;
+  }
+
   if (*properties == NULL) {
     IDPropertyTemplate val = {0};
     *properties = IDP_New(IDP_GROUP, &val, "wmOpItemProp");