Asset Browser: Support dragging assets into catalogs

With this it is possible to select any number of assets in the Asset
Browser and drag them into catalogs. The assets will be moved to that
catalog then. However, this will only work in the "Current File" asset
library, since that is the only library that allows changing assets,
which is what's done here.

While dragging assets over the tree row, a tooltip is shown explaining
what's going to happen.

In preparation to this, the new UI tree-view API was already extended
with custom drop support, see 4ee2d9df428d.

----

Changes here to the `wmDrag` code were needed to support dragging multiple
assets. Some of it is considered temporary because a) a proper #AssetHandle
design should replace some ugly parts of this patch and b) the multi-item
support in `wmDrag` isn't that great yet. The entire API will have to be
written anyway (see D4071).

Maniphest Tasks: T91573

Differential Revision: https://developer.blender.org/D12713

Reviewed by: Sybren Stüvel

1 files changed, 31 insertions, 6 deletions

diff --git a/source/blender/windowmanager/WM_types.h b/source/blender/windowmanager/WM_types.h
index 588c895c742..7f52bef3203 100644
--- a/source/blender/windowmanager/WM_types.h
+++ b/source/blender/windowmanager/WM_types.h
@@ -915,12 +915,16 @@ typedef void (*wmPaintCursorDraw)(struct bContext *C, int, int, void *customdata
 
 #define WM_DRAG_ID 0
 #define WM_DRAG_ASSET 1
-#define WM_DRAG_RNA 2
-#define WM_DRAG_PATH 3
-#define WM_DRAG_NAME 4
-#define WM_DRAG_VALUE 5
-#define WM_DRAG_COLOR 6
-#define WM_DRAG_DATASTACK 7
+/** The user is dragging multiple assets. This is only supported in few specific cases, proper
+ * multi-item support for dragging isn't supported well yet. Therefore this is kept separate from
+ * #WM_DRAG_ASSET. */
+#define WM_DRAG_ASSET_LIST 2
+#define WM_DRAG_RNA 3
+#define WM_DRAG_PATH 4
+#define WM_DRAG_NAME 5
+#define WM_DRAG_VALUE 6
+#define WM_DRAG_COLOR 7
+#define WM_DRAG_DATASTACK 8
 
 typedef enum wmDragFlags {
   WM_DRAG_NOP = 0,
@@ -953,6 +957,25 @@ typedef struct wmDragAsset {
   struct bContext *evil_C;
 } wmDragAsset;
 
+/**
+ * For some specific cases we support dragging multiple assets (#WM_DRAG_ASSET_LIST). There is no
+ * proper support for dragging multiple items in the `wmDrag`/`wmDrop` API yet, so this is really
+ * just to enable specific features for assets.
+ *
+ * This struct basically contains a tagged union to either store a local ID pointer, or information
+ * about an externally stored asset.
+ */
+typedef struct wmDragAssetListItem {
+  struct wmDragAssetListItem *next, *prev;
+
+  union {
+    struct ID *local_id;
+    wmDragAsset *external_info;
+  } asset_data;
+
+  bool is_external;
+} wmDragAssetListItem;
+
 typedef char *(*WMDropboxTooltipFunc)(struct bContext *,
                                       struct wmDrag *,
                                       const struct wmEvent *event,
@@ -979,6 +1002,8 @@ typedef struct wmDrag {
 
   /** List of wmDragIDs, all are guaranteed to have the same ID type. */
   ListBase ids;
+  /** List of `wmDragAssetListItem`s. */
+  ListBase asset_items;
 } wmDrag;
 
 /**