UI: Add asset shelf Python UI template

Adds a new "Ui Asset Shelf" template to the script editor, demonstrating
how scripts can register own asset shelves. The asset shelf is designed
as a new standard UI element that add-ons, application templates and the
like can use, so this is useful. It's also useful as a quick way to test
and customize the asset shelf.

The script is only available in the UI if the "Asset Shelf" experimental
feature is enabled in the Preferences.

scripts/startup/bl_ui/space_text.py

@@ -287,12 +287,18 @@ class TEXT_MT_text(Menu):
 class TEXT_MT_templates_py(Menu):
     bl_label = "Python"
 
-    def draw(self, _context):
+    def draw(self, context):
+        prefs = context.preferences
+        use_asset_shelf = prefs.experimental.use_asset_shelf
+
         self.path_menu(
             bpy.utils.script_paths(subdir="templates_py"),
             "text.open",
             props_default={"internal": True},
             filter_ext=lambda ext: (ext.lower() == ".py"),
+            # Filter out asset shelf template depending on experimental "Asset Shelf" option.
+            filter_path=lambda path, use_asset_shelf=use_asset_shelf:
+                (use_asset_shelf or not path.endswith("ui_asset_shelf.py")),
         )
 
 

scripts/templates_py/ui_asset_shelf.py

@@ -0,0 +1,26 @@
+import bpy
+
+
+class MyAssetShelf(bpy.types.AssetShelf):
+    bl_space_type = 'VIEW_3D'
+    bl_idname = "my_template.my_material_asset_shelf"
+
+    @classmethod
+    def poll(cls, context):
+        return bool(context.object and context.object.mode == 'OBJECT')
+
+    @classmethod
+    def asset_poll(cls, asset):
+        return asset.file_data.id_type in {'MATERIAL', 'OBJECT'}
+
+
+def register():
+    bpy.utils.register_class(MyAssetShelf)
+
+
+def unregister():
+    bpy.utils.unregister_class(MyAssetShelf)
+
+
+if __name__ == "__main__":
+    register()