Asset System: Data-block asset metadata storage, reading and API

Asset metadata is what turns a regular data-block into an asset. It is a small
data-structure, but a key part of the technical design of the asset system.

The design foresees that asset data-blocks store an `ID.asset_data` pointer of
type `AssetMetaData`. This data **must not** have dependencies on other
data-blocks or data-block data, it must be an independent unit. That way we can
read asset-metadata from .blends without reading anything else from the file.
The Asset Browser will use this metadata (together with the data-block name,
preview and file path) to represent assets in the file list.

Includes:
* New `ID.asset_data` for asset metadata.
* Asset tags, description and custom properties.
* BKE code to manage asset meta-data and asset tags.
* Code to read asset data from files, without reading IDs.
* RNA for asset metadata (including tags)

Part of the first Asset Browser milestone. Check the #asset_browser_milestone_1
project milestone on developer.blender.org.

Differential Revision: https://developer.blender.org/D9716

Reviewed by: Bastien Montagne, Brecht Van Lommel

1 files changed, 62 insertions, 0 deletions

diff --git a/source/blender/makesdna/DNA_asset_types.h b/source/blender/makesdna/DNA_asset_types.h
new file mode 100644
index 00000000000..671012e54ef
--- /dev/null
+++ b/source/blender/makesdna/DNA_asset_types.h
@@ -0,0 +1,62 @@
+/*
+ * 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 DNA
+ */
+
+#ifndef __DNA_ASSET_TYPES_H__
+#define __DNA_ASSET_TYPES_H__
+
+#include "DNA_listBase.h"
+
+/**
+ * \brief User defined tag.
+ * Currently only used by assets, could be used more often at some point.
+ * Maybe add a custom icon and color to these in future?
+ */
+typedef struct AssetTag {
+  struct AssetTag *next, *prev;
+  char name[64]; /* MAX_NAME */
+} AssetTag;
+
+/**
+ * \brief The meta-data of an asset.
+ * By creating and giving this for a data-block (#ID.asset_data), the data-block becomes an asset.
+ *
+ * \note This struct must be readable without having to read anything but blocks from the ID it is
+ *       attached to! That way, asset information of a file can be read, without reading anything
+ *       more than that from the file. So pointers to other IDs or ID data are strictly forbidden.
+ */
+typedef struct AssetMetaData {
+  /** Custom asset meta-data. Cannot store pointers to IDs (#STRUCT_NO_DATABLOCK_IDPROPERTIES)! */
+  struct IDProperty *properties;
+
+  /** Optional description of this asset for display in the UI. Dynamic length. */
+  char *description;
+  /** User defined tags for this asset. The asset manager uses these for filtering, but how they
+   * function exactly (e.g. how they are registered to provide a list of searchable available tags)
+   * is up to the asset-engine. */
+  ListBase tags; /* AssetTag */
+  short active_tag;
+  /** Store the number of tags to avoid continuous counting. Could be turned into runtime data, we
+   * can always reliably reconstruct it from the list. */
+  short tot_tags;
+
+  char _pad[4];
+} AssetMetaData;
+
+#endif /* __DNA_ASSET_TYPES_H__ */