UI: Support easy dropping into/onto rows in new tree-view API

Adds an easy way to add drop support for tree-view rows.

Most of the work is handled by the tree-view UI code. The tree items can
simply override a few functions (`can_drop()`, `on_drop()`,
`drop_tooltip()`) to implement their custom drop behavior.

While dragging over a tree-view item that can be dropped into/onto, the
item can show a custom and dynamic tooltip explaining what's gonna
happen on drop.

This isn't used yet, but will soon be for asset catalogs.

See documentation here:
https://wiki.blender.org/wiki/Source/Interface/Views#Further_Customizations

1 files changed, 66 insertions, 0 deletions

diff --git a/source/blender/editors/interface/interface_dropboxes.cc b/source/blender/editors/interface/interface_dropboxes.cc
new file mode 100644
index 00000000000..cb33e7f736e
--- /dev/null
+++ b/source/blender/editors/interface/interface_dropboxes.cc
@@ -0,0 +1,66 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+/** \file
+ * \ingroup edinterface
+ */
+
+#include "BKE_context.h"
+
+#include "DNA_space_types.h"
+
+#include "WM_api.h"
+
+#include "UI_interface.h"
+
+static bool ui_tree_view_drop_poll(bContext *C, wmDrag *drag, const wmEvent *event)
+{
+  const ARegion *region = CTX_wm_region(C);
+  const uiTreeViewItemHandle *hovered_tree_item = UI_block_tree_view_find_item_at(
+      region, event->x, event->y);
+  if (!hovered_tree_item) {
+    return false;
+  }
+
+  return UI_tree_view_item_can_drop(hovered_tree_item, drag);
+}
+
+static char *ui_tree_view_drop_tooltip(bContext *C,
+                                       wmDrag *drag,
+                                       const wmEvent *event,
+                                       wmDropBox *UNUSED(drop))
+{
+  const ARegion *region = CTX_wm_region(C);
+  const uiTreeViewItemHandle *hovered_tree_item = UI_block_tree_view_find_item_at(
+      region, event->x, event->y);
+  if (!hovered_tree_item) {
+    return nullptr;
+  }
+
+  return UI_tree_view_item_drop_tooltip(hovered_tree_item, C, drag, event);
+}
+
+void ED_dropboxes_ui()
+{
+  ListBase *lb = WM_dropboxmap_find("User Interface", SPACE_EMPTY, 0);
+
+  WM_dropbox_add(lb,
+                 "UI_OT_tree_view_drop",
+                 ui_tree_view_drop_poll,
+                 nullptr,
+                 nullptr,
+                 ui_tree_view_drop_tooltip);
+}