scene_graph: Introduce sway_scene_descriptor

Across a wayland compositor, there are multiple shells: It can be
a toplevel, or a layer_shell, or even something more meta like a drag
icon or highlight indicators when dragging windows around.

This object lets us store values that represent these modes of operation
and keep track of what object is being represented.

include/sway/scene_descriptor.h

@@ -0,0 +1,25 @@
+/**
+ * Across a wayland compositor, there are multiple shells: It can be
+ * a toplevel, or a layer_shell, or even something more meta like a drag
+ * icon or highlight indicators when dragging windows around.
+ *
+ * This object lets us store values that represent these modes of operation
+ * and keep track of what object is being represented.
+ */
+#ifndef _SWAY_SCENE_DESCRIPTOR_H
+#define _SWAY_SCENE_DESCRIPTOR_H
+#include <wlr/types/wlr_scene.h>
+
+enum sway_scene_descriptor_type {
+};
+
+bool scene_descriptor_assign(struct wlr_scene_node *node,
+	enum sway_scene_descriptor_type type, void *data);
+
+void *scene_descriptor_try_get(struct wlr_scene_node *node,
+	enum sway_scene_descriptor_type type);
+
+void scene_descriptor_destroy(struct wlr_scene_node *node,
+	enum sway_scene_descriptor_type type);
+
+#endif

sway/meson.build

@@ -8,6 +8,7 @@ sway_sources = files(
 	'lock.c',
 	'main.c',
 	'realtime.c',
+	'scene_descriptor.c',
 	'server.c',
 	'swaynag.c',
 	'xdg_activation_v1.c',

sway/scene_descriptor.c

@@ -0,0 +1,66 @@
+#include <stdlib.h>
+#include <wlr/util/addon.h>
+#include "log.h"
+#include "sway/scene_descriptor.h"
+
+struct scene_descriptor {
+	void *data;
+	struct wlr_addon addon;
+};
+
+static const struct wlr_addon_interface addon_interface;
+
+static struct scene_descriptor *scene_node_get_descriptor(
+		struct wlr_scene_node *node, enum sway_scene_descriptor_type type) {
+	struct wlr_addon *addon = wlr_addon_find(&node->addons, (void *)type, &addon_interface);
+	if (!addon) {
+		return NULL;
+	}
+
+	struct scene_descriptor *desc = wl_container_of(addon, desc, addon);
+	return desc;
+}
+
+static void descriptor_destroy(struct scene_descriptor *desc) {
+	wlr_addon_finish(&desc->addon);
+	free(desc);
+}
+
+void *scene_descriptor_try_get(struct wlr_scene_node *node,
+		enum sway_scene_descriptor_type type) {
+	struct scene_descriptor *desc = scene_node_get_descriptor(node, type);
+	if (!desc) {
+		return NULL;
+	}
+
+	return desc->data;
+}
+
+void scene_descriptor_destroy(struct wlr_scene_node *node,
+		enum sway_scene_descriptor_type type) {
+	struct scene_descriptor *desc = scene_node_get_descriptor(node, type);
+	descriptor_destroy(desc);
+}
+
+static void addon_handle_destroy(struct wlr_addon *addon) {
+	struct scene_descriptor *desc = wl_container_of(addon, desc, addon);
+	descriptor_destroy(desc);
+}
+
+static const struct wlr_addon_interface addon_interface = {
+	.name = "sway_scene_descriptor",
+	.destroy = addon_handle_destroy,
+};
+
+bool scene_descriptor_assign(struct wlr_scene_node *node,
+		enum sway_scene_descriptor_type type, void *data) {
+	struct scene_descriptor *desc = calloc(1, sizeof(*desc));
+	if (!desc) {
+		sway_log(SWAY_ERROR, "Could not allocate a scene descriptor");
+		return false;
+	}
+
+	wlr_addon_init(&desc->addon, &node->addons, (void *)type, &addon_interface);
+	desc->data = data;
+	return true;
+}