proxy: Add API to tag proxy objects

When an application and a toolkit share the same Wayland connection,
it will receive events with each others objects. For example if the
toolkit manages a set of surfaces, and the application another set, if
both the toolkit and application listen to pointer focus events,
they'll receive focus events for each others surfaces.

In order for the toolkit and application layers to identify whether a
surface is managed by itself or not, it cannot only rely on retrieving
the proxy user data, without going through all it's own proxy objects
finding whether it's one of them.

By adding the ability to "tag" a proxy object, the toolkit and
application can use the tag to identify what the user data pointer
points to something known.

To create a tag, the recommended way is to define a statically allocated
constant char array containing some descriptive string. The tag will be
the pointer to the non-const pointer to the beginning of the array.

For example, to identify whether a focus event is for a surface managed
by the code in question:

	static const char *my_tag = "my tag";

	static void
	pointer_enter(void *data,
		      struct wl_pointer *wl_pointer,
		      uint32_t serial,
		      struct wl_surface *surface,
		      wl_fixed_t surface_x,
		      wl_fixed_t surface_y)
	{
		struct window *window;
		const char * const *tag;

		tag = wl_proxy_get_tag((struct wl_proxy *) surface);

		if (tag != &my_tag)
			return;

		window = wl_surface_get_user_data(surface);

		...
	}

	...

	static void
	init_window_surface(struct window *window)
	{
		struct wl_surface *surface;

		surface = wl_compositor_create_surface(compositor);
		wl_surface_set_user_data(surface, window);
		wl_proxy_set_tag((struct wl_proxy *) surface,
				 &my_tag);
	}

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>

src/wayland-client.c

@@ -69,6 +69,7 @@ struct wl_proxy {
 	void *user_data;
 	wl_dispatcher_func_t dispatcher;
 	uint32_t version;
+	const char * const *tag;
 };
 
 struct wl_event_queue {
@@ -2059,6 +2060,70 @@ wl_proxy_get_id(struct wl_proxy *proxy)
 	return proxy->object.id;
 }
 
+/** Set the tag of a proxy object
+ *
+ * A toolkit or application can set a unique tag on a proxy in order to
+ * identify whether an object is managed by itself or some external part.
+ *
+ * To create a tag, the recommended way is to define a statically allocated
+ * constant char array containing some descriptive string. The tag will be the
+ * pointer to the non-const pointer to the beginning of the array.
+ *
+ * For example, to define and set a tag on a surface managed by a certain
+ * subsystem:
+ *
+ * 	static const char *my_tag = "my tag";
+ *
+ * 	wl_proxy_set_tag((struct wl_proxy *) surface, &my_tag);
+ *
+ * Then, in a callback with wl_surface as an argument, in order to check
+ * whether it's a surface managed by the same subsystem.
+ *
+ * 	const char * const *tag;
+ *
+ * 	tag = wl_proxy_get_tag((struct wl_proxy *) surface);
+ * 	if (tag != &my_tag)
+ *		return;
+ *
+ *	...
+ *
+ * For debugging purposes, a tag should be suitable to be included in a debug
+ * log entry, e.g.
+ *
+ * 	const char * const *tag;
+ *
+ * 	tag = wl_proxy_get_tag((struct wl_proxy *) surface);
+ * 	printf("Got a surface with the tag %p (%s)\n",
+ * 	       tag, (tag && *tag) ? *tag : "");
+ *
+ * \param proxy The proxy object
+ * \param tag The tag
+ *
+ * \memberof wl_proxy
+ * \since 1.17.90
+ */
+WL_EXPORT void
+wl_proxy_set_tag(struct wl_proxy *proxy,
+		 const char * const *tag)
+{
+	proxy->tag = tag;
+}
+
+/** Get the tag of a proxy object
+ *
+ * See wl_proxy_set_tag for details.
+ *
+ * \param proxy The proxy object
+ *
+ * \memberof wl_proxy
+ * \since 1.17.90
+ */
+WL_EXPORT const char * const *
+wl_proxy_get_tag(struct wl_proxy *proxy)
+{
+	return proxy->tag;
+}
+
 /** Get the interface name (class) of a proxy object
  *
  * \param proxy The proxy object