Annotate the source

3 files changed, 277 insertions, 0 deletions

diff --git a/tinywl/Makefile b/tinywl/Makefile
index 3ecd4670..4d988fa2 100644
--- a/tinywl/Makefile
+++ b/tinywl/Makefile
@@ -1,5 +1,8 @@
 WAYLAND_PROTOCOLS=/usr/share/wayland-protocols
 
+# wayland-scanner is a tool which generates C headers and rigging for Wayland
+# protocols, which are specified in XML. wlroots requires you to rig these up
+# to your build system yourself and provide them in the include path.
 xdg-shell-protocol.h:
 	wayland-scanner server-header \
 		$(WAYLAND_PROTOCOLS)/stable/xdg-shell/xdg-shell.xml $@
diff --git a/tinywl/README.md b/tinywl/README.md
index 4be9852c..caacc412 100644
--- a/tinywl/README.md
+++ b/tinywl/README.md
@@ -31,3 +31,17 @@ keybindings. TinyWL supports the following keybindings:
 
 - `Alt+Escape`: Terminate the compositor
 - `Alt+F1`: Cycle between windows
+
+## Limitations
+
+Notable omissions from TinyWL:
+
+- HiDPI support
+- Any kind of configuration, e.g. output layout
+- Any protocol other than xdg-shell (e.g. layer-shell, for
+  panels/taskbars/etc; or Xwayland, for proxied X11 windows)
+- Optional protocols, e.g. screen capture, primary selection, virtual
+  keyboard, etc. Most of these are plug-and-play with wlroots, but they're
+  omitted for brevity.
+- Damage tracking, which tracks which parts of the screen are changing and
+  minimizes redraws accordingly.
diff --git a/tinywl/tinywl.c b/tinywl/tinywl.c
index d370b04d..069f6451 100644
--- a/tinywl/tinywl.c
+++ b/tinywl/tinywl.c
@@ -24,6 +24,7 @@
 #include <wlr/util/log.h>
 #include <xkbcommon/xkbcommon.h>
 
+/* For brevity's sake, struct members are annotated where they are used. */
 enum tinywl_cursor_mode {
 	TINYWL_CURSOR_PASSTHROUGH,
 	TINYWL_CURSOR_MOVE,
@@ -97,6 +98,7 @@ struct tinywl_pointer {
 };
 
 static void focus_view(struct tinywl_view *view, struct wlr_surface *surface) {
+	/* Note: this function only deals with keyboard focus. */
 	if (view == NULL) {
 		return;
 	}
@@ -104,31 +106,60 @@ static void focus_view(struct tinywl_view *view, struct wlr_surface *surface) {
 	struct wlr_seat *seat = server->seat;
 	struct wlr_surface *prev_surface = seat->keyboard_state.focused_surface;
 	if (prev_surface == surface) {
+		/* Don't re-focus an already focused surface. */
 		return;
 	}
 	if (prev_surface) {
+		/*
+		 * Deactivate the previously focused surface. This lets the client know
+		 * it no longer has focus and the client will repaint accordingly, e.g.
+		 * stop displaying a caret.
+		 */
 		struct wlr_xdg_surface *previous = wlr_xdg_surface_from_wlr_surface(
 					seat->keyboard_state.focused_surface);
 		wlr_xdg_toplevel_set_activated(previous, false);
 	}
 	struct wlr_keyboard *keyboard = wlr_seat_get_keyboard(seat);
+	/* Move the view to the front */
 	wl_list_remove(&view->link);
 	wl_list_insert(&server->views, &view->link);
+	/* Activate the new surface */
 	wlr_xdg_toplevel_set_activated(view->xdg_surface, true);
+	/*
+	 * Tell the seat to have the keyboard enter this surface. wlroots will keep
+	 * track of this and automatically send key events to the appropriate
+	 * clients without additional work on your part.
+	 */
 	wlr_seat_keyboard_notify_enter(seat, view->xdg_surface->surface,
 		keyboard->keycodes, keyboard->num_keycodes, &keyboard->modifiers);
 }
 
 static void keyboard_handle_modifiers(
 		struct wl_listener *listener, void *data) {
+	/* This event is raised when a modifier key, such as shift or alt, is
+	 * pressed. We simply communicate this to the client. */
 	struct tinywl_keyboard *keyboard =
 		wl_container_of(listener, keyboard, modifiers);
+	/*
+	 * A seat can only have one keyboard, but this is a limitation of the
+	 * Wayland protocol - not wlroots. We assign all connected keyboards to the
+	 * same seat. You can swap out the underlying wlr_keyboard like this and
+	 * wlr_seat handles this transparently.
+	 */
 	wlr_seat_set_keyboard(keyboard->server->seat, keyboard->device);
+	/* Send modifiers to the client. */
 	wlr_seat_keyboard_notify_modifiers(keyboard->server->seat,
 		&keyboard->device->keyboard->modifiers);
 }
 
 static bool handle_keybinding(struct tinywl_server *server, xkb_keysym_t sym) {
+	/*
+	 * Here we handle compositor keybindings. This is when the compositor is
+	 * processing keys, rather than passing them on to the client for its own
+	 * processing.
+	 *
+	 * This function assumes Alt is held down.
+	 */
 	switch (sym) {
 	case XKB_KEY_Escape:
 		wl_display_terminate(server->wl_display);
@@ -155,13 +186,16 @@ static bool handle_keybinding(struct tinywl_server *server, xkb_keysym_t sym) {
 
 static void keyboard_handle_key(
 		struct wl_listener *listener, void *data) {
+	/* This event is raised when a key is pressed or released. */
 	struct tinywl_keyboard *keyboard =
 		wl_container_of(listener, keyboard, key);
 	struct tinywl_server *server = keyboard->server;
 	struct wlr_event_keyboard_key *event = data;
 	struct wlr_seat *seat = server->seat;
 
+	/* Translate libinput keycode -> xkbcommon */
 	uint32_t keycode = event->keycode + 8;
+	/* Get a list of keysyms based on the keymap for this keyboard */
 	const xkb_keysym_t *syms;
 	int nsyms = xkb_state_key_get_syms(
 			keyboard->device->keyboard->xkb_state, keycode, &syms);
@@ -169,12 +203,15 @@ static void keyboard_handle_key(
 	bool handled = false;
 	uint32_t modifiers = wlr_keyboard_get_modifiers(keyboard->device->keyboard);
 	if ((modifiers & WLR_MODIFIER_ALT) && event->state == WLR_KEY_PRESSED) {
+		/* If alt is held down and this button was _pressed_, we attempt to
+		 * process it as a compositor keybinding. */
 		for (int i = 0; i < nsyms; i++) {
 			handled = handle_keybinding(server, syms[i]);
 		}
 	}
 
 	if (!handled) {
+		/* Otherwise, we pass it along to the client. */
 		wlr_seat_set_keyboard(seat, keyboard->device);
 		wlr_seat_keyboard_notify_key(seat, event->time_msec,
 			event->keycode, event->state);
@@ -188,6 +225,8 @@ static void server_new_keyboard(struct tinywl_server *server,
 	keyboard->server = server;
 	keyboard->device = device;
 
+	/* We need to prepare an XKB keymap and assign it to the keyboard. This
+	 * assumes the defaults (e.g. layout = "us"). */
 	struct xkb_rule_names rules = { 0 };
 	struct xkb_context *context = xkb_context_new(XKB_CONTEXT_NO_FLAGS);
 	struct xkb_keymap *keymap = xkb_map_new_from_names(context, &rules,
@@ -198,6 +237,7 @@ static void server_new_keyboard(struct tinywl_server *server,
 	xkb_context_unref(context);
 	wlr_keyboard_set_repeat_info(device->keyboard, 25, 600);
 
+	/* Here we set up listeners for keyboard events. */
 	keyboard->modifiers.notify = keyboard_handle_modifiers;
 	wl_signal_add(&device->keyboard->events.modifiers, &keyboard->modifiers);
 	keyboard->key.notify = keyboard_handle_key;
@@ -205,15 +245,22 @@ static void server_new_keyboard(struct tinywl_server *server,
 
 	wlr_seat_set_keyboard(server->seat, device);
 
+	/* And add the keyboard to our list of keyboards */
 	wl_list_insert(&server->keyboards, &keyboard->link);
 }
 
 static void server_new_pointer(struct tinywl_server *server,
 		struct wlr_input_device *device) {
+	/* We don't do anything special with pointers. All of our pointer handling
+	 * is proxied through wlr_cursor. On another compositor, you might take this
+	 * opportunity to do libinput configuration on the device to set
+	 * acceleration, etc. */
 	wlr_cursor_attach_input_device(server->cursor, device);
 }
 
 static void server_new_input(struct wl_listener *listener, void *data) {
+	/* This event is raised by the backend when a new input device becomes
+	 * available. */
 	struct tinywl_server *server =
 		wl_container_of(listener, server, new_input);
 	struct wlr_input_device *device = data;
@@ -225,6 +272,9 @@ static void server_new_input(struct wl_listener *listener, void *data) {
 		server_new_pointer(server, device);
 		break;
 	}
+	/* We need to let the wlr_seat know what our capabilities are, which is
+	 * communiciated to the client. In TinyWL we always have a cursor, even if
+	 * there are no pointer devices, so we always include that capability. */
 	uint32_t caps = WL_SEAT_CAPABILITY_POINTER;
 	if (!wl_list_empty(&server->keyboards)) {
 		caps |= WL_SEAT_CAPABILITY_KEYBOARD;
@@ -235,10 +285,17 @@ static void server_new_input(struct wl_listener *listener, void *data) {
 static void seat_request_cursor(struct wl_listener *listener, void *data) {
 	struct tinywl_server *server = wl_container_of(
 			listener, server, request_cursor);
+	/* This event is rasied by the seat when a client provides a cursor image */
 	struct wlr_seat_pointer_request_set_cursor_event *event = data;
 	struct wlr_seat_client *focused_client =
 		server->seat->pointer_state.focused_client;
+	/* This can be sent by any client, so we check to make sure this one is
+	 * actually has pointer focus first. */
 	if (focused_client == event->seat_client) {
+		/* Once we've vetted the client, we can tell the cursor to use the
+		 * provided surface as the cursor image. It will set the hardware cursor
+		 * on the output that it's currently on and continue to do so as the
+		 * cursor moves between outputs. */
 		wlr_cursor_set_surface(server->cursor, event->surface,
 				event->hotspot_x, event->hotspot_y);
 	}
@@ -247,6 +304,13 @@ static void seat_request_cursor(struct wl_listener *listener, void *data) {
 static bool view_at(struct tinywl_view *view,
 		double lx, double ly, struct wlr_surface **surface,
 		double *sx, double *sy) {
+	/*
+	 * XDG toplevels may have nested surfaces, such as popup windows for context
+	 * menus or tooltips. This function tests if any of those are underneath the
+	 * coordinates lx and ly (in output Layout Coordinates). If so, it sets the
+	 * surface pointer to that wlr_surface and the sx and sy coordinates to the
+	 * coordinates relative to that surface's top-left corner.
+	 */
 	double view_sx = lx - view->x;
 	double view_sy = ly - view->y;
 
@@ -270,6 +334,8 @@ static bool view_at(struct tinywl_view *view,
 static struct tinywl_view *desktop_view_at(
 		struct tinywl_server *server, double lx, double ly,
 		struct wlr_surface **surface, double *sx, double *sy) {
+	/* This iterates over all of our surfaces and attempts to find one under the
+	 * cursor. This relies on server->views being ordered from top-to-bottom. */
 	struct tinywl_view *view;
 	wl_list_for_each(view, &server->views, link) {
 		if (view_at(view, lx, ly, surface, sx, sy)) {
@@ -280,11 +346,22 @@ static struct tinywl_view *desktop_view_at(
 }
 
 static void process_cursor_move(struct tinywl_server *server, uint32_t time) {
+	/* Move the grabbed view to the new position. */
 	server->grabbed_view->x = server->cursor->x - server->grab_x;
 	server->grabbed_view->y = server->cursor->y - server->grab_y;
 }
 
 static void process_cursor_resize(struct tinywl_server *server, uint32_t time) {
+	/*
+	 * Resizing the grabbed view can be a little bit complicated, because we
+	 * could be resizing from any corner or edge. This not only resizes the view
+	 * on one or two axes, but can also move the view if you resize from the top
+	 * or left edges (or top-left corner).
+	 *
+	 * Note that I took some shortcuts here. In a more fleshed-out compositor,
+	 * you'd wait for the client to prepare a buffer at the new size, then
+	 * commit any movement that was prepared.
+	 */
 	struct tinywl_view *view = server->grabbed_view;
 	double dx = server->cursor->x - server->grab_x;
 	double dy = server->cursor->y - server->grab_y;
@@ -316,6 +393,7 @@ static void process_cursor_resize(struct tinywl_server *server, uint32_t time) {
 }
 
 static void process_cursor_motion(struct tinywl_server *server, uint32_t time) {
+	/* If the mode is non-passthrough, delegate to those functions. */
 	if (server->cursor_mode == TINYWL_CURSOR_MOVE) {
 		process_cursor_move(server, time);
 		return;
@@ -324,30 +402,53 @@ static void process_cursor_motion(struct tinywl_server *server, uint32_t time) {
 		return;
 	}
 
+	/* Otherwise, find the view under the pointer and send the event along. */
 	double sx, sy;
 	struct wlr_seat *seat = server->seat;
 	struct wlr_surface *surface = NULL;
 	struct tinywl_view *view = desktop_view_at(server,
 			server->cursor->x, server->cursor->y, &surface, &sx, &sy);
 	if (!view) {
+		/* If there's no view under the cursor, set the cursor image to a
+		 * default. This is what makes the cursor image appear when you move it
+		 * around the screen, not over any views. */
 		wlr_xcursor_manager_set_cursor_image(
 				server->cursor_mgr, "left_ptr", server->cursor);
 	}
 	if (surface) {
 		bool focus_changed = seat->pointer_state.focused_surface != surface;
+		/*
+		 * "Enter" the surface if necessary. This lets the client know that the
+		 * cursor has entered one of its surfaces.
+		 *
+		 * Note that this gives the surface "pointer focus", which is distinct
+		 * from keyboard focus. You get pointer focus by moving the pointer over
+		 * a window.
+		 */
 		wlr_seat_pointer_notify_enter(seat, surface, sx, sy);
 		if (!focus_changed) {
+			/* The enter event contains coordinates, so we only need to notify
+			 * on motion if the focus did not change. */
 			wlr_seat_pointer_notify_motion(seat, time, sx, sy);
 		}
 	} else {
+		/* Clear pointer focus so future button events and such are not sent to
+		 * the last client to have the cursor over it. */
 		wlr_seat_pointer_clear_focus(seat);
 	}
 }
 
 static void server_cursor_motion(struct wl_listener *listener, void *data) {
+	/* This event is forwarded by the cursor when a pointer emits a _relative_
+	 * pointer motion event (i.e. a delta) */
 	struct tinywl_server *server =
 		wl_container_of(listener, server, cursor_motion);
 	struct wlr_event_pointer_motion *event = data;
+	/* The cursor doesn't move unless we tell it to. The cursor automatically
+	 * handles constraining the motion to the output layout, as well as any
+	 * special configuration applied for the specific input device which
+	 * generated the event. You can pass NULL for the device if you want to move
+	 * the cursor around without any input. */
 	wlr_cursor_move(server->cursor, event->device,
 			event->delta_x, event->delta_y);
 	process_cursor_motion(server, event->time_msec);
@@ -355,6 +456,12 @@ static void server_cursor_motion(struct wl_listener *listener, void *data) {
 
 static void server_cursor_motion_absolute(
 		struct wl_listener *listener, void *data) {
+	/* This event is forwarded by the cursor when a pointer emits an _absolute_
+	 * motion event, from 0..1 on each axis. This happens, for example, when
+	 * wlroots is running under a Wayland window rather than KMS+DRM, and you
+	 * move the mouse over the window. You could enter the window from any edge,
+	 * so we have to warp the mouse there. There is also some hardware which
+	 * emits these events. */
 	struct tinywl_server *server =
 		wl_container_of(listener, server, cursor_motion_absolute);
 	struct wlr_event_pointer_motion_absolute *event = data;
@@ -363,9 +470,12 @@ static void server_cursor_motion_absolute(
 }
 
 static void server_cursor_button(struct wl_listener *listener, void *data) {
+	/* This event is forwarded by the cursor when a pointer emits a button
+	 * event. */
 	struct tinywl_server *server =
 		wl_container_of(listener, server, cursor_button);
 	struct wlr_event_pointer_button *event = data;
+	/* Notify the client with pointer focus that a button press has occured */
 	wlr_seat_pointer_notify_button(server->seat,
 			event->time_msec, event->button, event->state);
 	double sx, sy;
@@ -374,21 +484,28 @@ static void server_cursor_button(struct wl_listener *listener, void *data) {
 	struct tinywl_view *view = desktop_view_at(server,
 			server->cursor->x, server->cursor->y, &surface, &sx, &sy);
 	if (event->state == WLR_BUTTON_RELEASED) {
+		/* If you released any buttons, we exit interactive move/resize mode. */
 		server->cursor_mode = TINYWL_CURSOR_PASSTHROUGH;
 	} else {
+		/* Focus that client if the button was _pressed_ */
 		focus_view(view, surface);
 	}
 }
 
 static void server_cursor_axis(struct wl_listener *listener, void *data) {
+	/* This event is forwarded by the cursor when a pointer emits an axis event,
+	 * for example when you move the scroll wheel. */
 	struct tinywl_server *server =
 		wl_container_of(listener, server, cursor_axis);
 	struct wlr_event_pointer_axis *event = data;
+	/* Notify the client with pointer focus of the axis event. */
 	wlr_seat_pointer_notify_axis(server->seat,
 			event->time_msec, event->orientation, event->delta,
 			event->delta_discrete, event->source);
 }
 
+/* Used to move all of the data necessary to render a surface from the top-level
+ * frame handler to the per-surface render function. */
 struct render_data {
 	struct wlr_output *output;
 	struct wlr_renderer *renderer;
@@ -398,20 +515,32 @@ struct render_data {
 
 static void render_surface(struct wlr_surface *surface,
 		int sx, int sy, void *data) {
+	/* This function is called for every surface that needs to be rendered. */
 	struct render_data *rdata = data;
 	struct tinywl_view *view = rdata->view;
 	struct wlr_output *output = rdata->output;
 
+	/* We first obtain a wlr_texture, which is a GPU resource. wlroots
+	 * automatically handles negotiating these with the client. The underlying
+	 * resource could be an opaque handle passed from the client, or the client
+	 * could have sent a pixel buffer which we copied to the GPU, or a few other
+	 * means. You don't have to worry about this, wlroots takes care of it. */
 	struct wlr_texture *texture = wlr_surface_get_texture(surface);
 	if (texture == NULL) {
 		return;
 	}
 
+	/* The view has a position in layout coordinates. If you have two displays,
+	 * one next to the other, both 1080p, a view on the rightmost display might
+	 * have layout coordinates of 2000,100. We need to translate that to
+	 * output-local coordinates, or (2000 - 1920). */
 	double ox = 0, oy = 0;
 	wlr_output_layout_output_coords(
 			view->server->output_layout, output, &ox, &oy);
 	ox += view->x + sx, oy += view->y + sy;
 
+	/* We also have to apply the scale factor for HiDPI outputs. This is only
+	 * part of the puzzle, TinyWL does not fully support HiDPI. */
 	struct wlr_box box = {
 		.x = ox * output->scale,
 		.y = oy * output->scale,
@@ -419,18 +548,35 @@ static void render_surface(struct wlr_surface *surface,
 		.height = surface->current.height * output->scale,
 	};
 
+	/*
+	 * Those familiar with OpenGL are also familiar with the role of matricies
+	 * in graphics programming. We need to prepare a matrix to render the view
+	 * with. wlr_matrix_project_box is a helper which takes a box with a desired
+	 * x, y coodrinates, width and height, and an output geometry, then
+	 * prepares an orthographic projection and multiplies the necessary
+	 * transforms to produce a model-view-projection matrix.
+	 *
+	 * Naturally you can do this any way you like, for example to make a 3D
+	 * compositor.
+	 */
 	float matrix[9];
 	enum wl_output_transform transform =
 		wlr_output_transform_invert(surface->current.transform);
 	wlr_matrix_project_box(matrix, &box, transform, 0,
 		output->transform_matrix);
 
+	/* This takes our matrix, the texture, and an alpha, and performs the actual
+	 * rendering on the GPU. */
 	wlr_render_texture_with_matrix(rdata->renderer, texture, matrix, 1);
 
+	/* This lets the client know that we've displayed that frame and it can
+	 * prepare another one now if it likes. */
 	wlr_surface_send_frame_done(surface, rdata->when);
 }
 
 static void output_frame(struct wl_listener *listener, void *data) {
+	/* This function is called every time an output is ready to display a frame,
+	 * generally at the output's refresh rate (e.g. 60Hz). */
 	struct tinywl_output *output =
 		wl_container_of(listener, output, frame);
 	struct wlr_renderer *renderer = output->server->renderer;
@@ -438,19 +584,25 @@ static void output_frame(struct wl_listener *listener, void *data) {
 	struct timespec now;
 	clock_gettime(CLOCK_MONOTONIC, &now);
 
+	/* wlr_output_make_current makes the OpenGL context current. */
 	if (!wlr_output_make_current(output->wlr_output, NULL)) {
 		return;
 	}
+	/* The "effective" resolution can change if you rotate your outputs. */
 	int width, height;
 	wlr_output_effective_resolution(output->wlr_output, &width, &height);
+	/* Begin the renderer (calls glViewport and some other GL sanity checks) */
 	wlr_renderer_begin(renderer, width, height);
 
 	float color[4] = {0.3, 0.3, 0.3, 1.0};
 	wlr_renderer_clear(renderer, color);
 
+	/* Each subsequent window we render is rendered on top of the last. Because
+	 * our view list is ordered front-to-back, we iterate over it backwards. */
 	struct tinywl_view *view;
 	wl_list_for_each_reverse(view, &output->server->views, link) {
 		if (!view->mapped) {
+			/* An unmapped view should not be rendered. */
 			continue;
 		}
 		struct render_data rdata = {
@@ -459,50 +611,73 @@ static void output_frame(struct wl_listener *listener, void *data) {
 			.renderer = renderer,
 			.when = &now,
 		};
+		/* This calls our render_surface function for each surface among the
+		 * xdg_surface's toplevel and popups. */
 		wlr_xdg_surface_for_each_surface(view->xdg_surface,
 				render_surface, &rdata);
 	}
 
+	/* Conclude rendering and swap the buffers, showing the final frame
+	 * on-screen. */
 	wlr_renderer_end(renderer);
 	wlr_output_swap_buffers(output->wlr_output, NULL, NULL);
 }
 
 static void server_new_output(struct wl_listener *listener, void *data) {
+	/* This event is rasied by the backend when a new output (aka a display or
+	 * monitor) becomes available. */
 	struct tinywl_server *server =
 		wl_container_of(listener, server, new_output);
 	struct wlr_output *wlr_output = data;
 
+	/* Some backends don't have modes. DRM+KMS does, and we need to set a mode
+	 * before we can use the output. The mode is a tuple of (width, height,
+	 * refresh rate), and each monitor supports only a specific set of modes. We
+	 * just pick the first, a more sophisticated compositor would let the user
+	 * configure it or pick the mode the display advertises as preferred. */
 	if (!wl_list_empty(&wlr_output->modes)) {
 		struct wlr_output_mode *mode =
 			wl_container_of(wlr_output->modes.prev, mode, link);
 		wlr_output_set_mode(wlr_output, mode);
 	}
 
+	/* Allocates and configures our state for this output */
 	struct tinywl_output *output =
 		calloc(1, sizeof(struct tinywl_output));
 	output->wlr_output = wlr_output;
 	output->server = server;
+	/* Sets up a listener for the frame notify event. */
 	output->frame.notify = output_frame;
 	wl_signal_add(&wlr_output->events.frame, &output->frame);
 	wl_list_insert(&server->outputs, &output->link);
 
+	/* Adds this to the output layout. The add_auto function arranges outputs
+	 * from left-to-right in the order they appear. A more sophisticated
+	 * compositor would let the user configure the arrangement of outputs in the
+	 * layout. */
 	wlr_output_layout_add_auto(server->output_layout, wlr_output);
 
+	/* Creating the global adds a wl_output global to the display, which Wayland
+	 * clients can see to find out information about the output (such as
+	 * DPI, scale factor, manufacturer, etc). */
 	wlr_output_create_global(wlr_output);
 }
 
 static void xdg_surface_map(struct wl_listener *listener, void *data) {
+	/* Called when the surface is mapped, or ready to display on-screen. */
 	struct tinywl_view *view = wl_container_of(listener, view, map);
 	view->mapped = true;
 	focus_view(view, view->xdg_surface->surface);
 }
 
 static void xdg_surface_unmap(struct wl_listener *listener, void *data) {
+	/* Called when the surface is unmapped, and should no longer be shown. */
 	struct tinywl_view *view = wl_container_of(listener, view, unmap);
 	view->mapped = false;
 }
 
 static void xdg_surface_destroy(struct wl_listener *listener, void *data) {
+	/* Called when the surface is destroyed and should never be shown again. */
 	struct tinywl_view *view = wl_container_of(listener, view, destroy);
 	wl_list_remove(&view->link);
 	free(view);
@@ -510,10 +685,14 @@ static void xdg_surface_destroy(struct wl_listener *listener, void *data) {
 
 static void begin_interactive(struct tinywl_view *view,
 		enum tinywl_cursor_mode mode, uint32_t edges) {
+	/* This function sets up an interactive move or resize operation, where the
+	 * compositor stops propegating pointer events to clients and instead
+	 * consumes them itself, to move or resize windows. */
 	struct tinywl_server *server = view->server;
 	struct wlr_surface *focused_surface =
 		server->seat->pointer_state.focused_surface;
 	if (view->xdg_surface->surface != focused_surface) {
+		/* Deny move/resize requests from unfocused clients. */
 		return;
 	}
 	server->grabbed_view = view;
@@ -534,18 +713,30 @@ static void begin_interactive(struct tinywl_view *view,
 
 static void xdg_toplevel_request_move(
 		struct wl_listener *listener, void *data) {
+	/* This event is raised when a client would like to begin an interactive
+	 * move, typically because the user clicked on their client-side
+	 * decorations. Note that a more sophisticated compositor should check the
+	 * provied serial against a list of button press serials sent to this
+	 * client, to prevent the client from requesting this whenver they want. */
 	struct tinywl_view *view = wl_container_of(listener, view, request_move);
 	begin_interactive(view, TINYWL_CURSOR_MOVE, 0);
 }
 
 static void xdg_toplevel_request_resize(
 		struct wl_listener *listener, void *data) {
+	/* This event is raised when a client would like to begin an interactive
+	 * resize, typically because the user clicked on their client-side
+	 * decorations. Note that a more sophisticated compositor should check the
+	 * provied serial against a list of button press serials sent to this
+	 * client, to prevent the client from requesting this whenver they want. */
 	struct wlr_xdg_toplevel_resize_event *event = data;
 	struct tinywl_view *view = wl_container_of(listener, view, request_resize);
 	begin_interactive(view, TINYWL_CURSOR_RESIZE, event->edges);
 }
 
 static void server_new_xdg_surface(struct wl_listener *listener, void *data) {
+	/* This event is raised when wlr_xdg_shell receives a new xdg surface from a
+	 * client, either a toplevel (application window) or popup. */
 	struct tinywl_server *server =
 		wl_container_of(listener, server, new_xdg_surface);
 	struct wlr_xdg_surface *xdg_surface = data;
@@ -553,11 +744,13 @@ static void server_new_xdg_surface(struct wl_listener *listener, void *data) {
 		return;
 	}
 
+	/* Allocate a tinywl_view for this surface */
 	struct tinywl_view *view =
 		calloc(1, sizeof(struct tinywl_view));
 	view->server = server;
 	view->xdg_surface = xdg_surface;
 
+	/* Listen to the various events it can emit */
 	view->map.notify = xdg_surface_map;
 	wl_signal_add(&xdg_surface->events.map, &view->map);
 	view->unmap.notify = xdg_surface_unmap;
@@ -565,12 +758,14 @@ static void server_new_xdg_surface(struct wl_listener *listener, void *data) {
 	view->destroy.notify = xdg_surface_destroy;
 	wl_signal_add(&xdg_surface->events.destroy, &view->destroy);
 
+	/* cotd */
 	struct wlr_xdg_toplevel *toplevel = xdg_surface->toplevel;
 	view->request_move.notify = xdg_toplevel_request_move;
 	wl_signal_add(&toplevel->events.request_move, &view->request_move);
 	view->request_resize.notify = xdg_toplevel_request_resize;
 	wl_signal_add(&toplevel->events.request_resize, &view->request_resize);
 
+	/* Add it to the list of views. */
 	wl_list_insert(&server->views, &view->link);
 }
 
@@ -595,34 +790,83 @@ int main(int argc, char *argv[]) {
 	}
 
 	struct tinywl_server server;
+	/* The Wayland display is managed by libwayland. It handles accepting
+	 * clients from the Unix socket, manging Wayland globals, and so on. */
 	server.wl_display = wl_display_create();
+	/* The backend is a wlroots feature which abstracts the underlying input and
+	 * output hardware. The autocreate option will choose the most suitable
+	 * backend based on the current environment, such as opening an X11 window
+	 * if an X11 server is running. The NULL argument here optionally allows you
+	 * to pass in a custom renderer if wlr_renderer doesn't meet your needs. The
+	 * backend uses the renderer, for example, to fall back to software cursors
+	 * if the backend does not support hardware cursors (some older GPUs
+	 * don't). */
 	server.backend = wlr_backend_autocreate(server.wl_display, NULL);
 
+	/* If we don't provide a renderer, autocreate makes a GLES2 renderer for us.
+	 * The renderer is responsible for defining the various pixel formats it
+	 * supports for shared memory, this configures that for clients. */
 	server.renderer = wlr_backend_get_renderer(server.backend);
 	wlr_renderer_init_wl_display(server.renderer, server.wl_display);
 
+	/* This creates some hands-off wlroots interfaces. The compositor is
+	 * necessary for clients to allocate surfaces, dmabuf allows them to use
+	 * opaque GPU handles for buffers to avoid copying pixels on the CPU, and
+	 * the data device manager handles the clipboard. Each of these wlroots
+	 * interfaces has room for you to dig your fingers in and play with their
+	 * behavior if you want. */
 	wlr_compositor_create(server.wl_display, server.renderer);
 	wlr_linux_dmabuf_v1_create(server.wl_display, server.renderer);
 	wlr_data_device_manager_create(server.wl_display);
 
+	/* Creates an output layout, which a wlroots utility for working with an
+	 * arrangement of screens in a physical layout. */
 	server.output_layout = wlr_output_layout_create();
 
+	/* Configure a listener to be notified when new outputs are available on the
+	 * backend. */
 	wl_list_init(&server.outputs);
 	server.new_output.notify = server_new_output;
 	wl_signal_add(&server.backend->events.new_output, &server.new_output);
 
+	/* Set up our list of views and the xdg-shell. The xdg-shell is a Wayland
+	 * protocol which is used for application windows. For more detail on
+	 * shells, refer to my article:
+	 *
+	 * https://drewdevault.com/2018/07/29/Wayland-shells.html
+	 */
 	wl_list_init(&server.views);
 	server.xdg_shell = wlr_xdg_shell_create(server.wl_display);
 	server.new_xdg_surface.notify = server_new_xdg_surface;
 	wl_signal_add(&server.xdg_shell->events.new_surface,
 			&server.new_xdg_surface);
 
+	/*
+	 * Creates a cursor, which is a wlroots utility for tracking the cursor
+	 * image shown on screen.
+	 */
 	server.cursor = wlr_cursor_create();
 	wlr_cursor_attach_output_layout(server.cursor, server.output_layout);
 
+	/* Creates an xcursor manager, another wlroots utility which loads up
+	 * Xcursor themes to source cursor images from and makes sure that cursor
+	 * images are available at all scale factors on the screen (necessary for
+	 * HiDPI support). We add a cursor theme at scale factor 1 to begin with. */
 	server.cursor_mgr = wlr_xcursor_manager_create(NULL, 24);
 	wlr_xcursor_manager_load(server.cursor_mgr, 1);
 
+	/*
+	 * wlr_cursor *only* displays an image on screen. It does not move around
+	 * when the pointer moves. However, we can attach input devices to it, and
+	 * it will generate aggregate events for all of them. In these events, we
+	 * can choose how we want to process them, forwarding them to clients and
+	 * moving the cursor around. More detail on this process is described in my
+	 * input handling blog post:
+	 *
+	 * https://drewdevault.com/2018/07/17/Input-handling-in-wlroots.html
+	 *
+	 * And more comments are sprinkled throughout the notify functions above.
+	 */
 	server.cursor_motion.notify = server_cursor_motion;
 	wl_signal_add(&server.cursor->events.motion, &server.cursor_motion);
 	server.cursor_motion_absolute.notify = server_cursor_motion_absolute;
@@ -633,6 +877,12 @@ int main(int argc, char *argv[]) {
 	server.cursor_axis.notify = server_cursor_axis;
 	wl_signal_add(&server.cursor->events.axis, &server.cursor_axis);
 
+	/*
+	 * Configures a seat, which is a single "seat" at which a user sits and
+	 * operates the computer. This conceptually includes up to one keyboard,
+	 * pointer, touch, and drawing tablet device. We also rig up a listener to
+	 * let us know when new input devices are available on the backend.
+	 */
 	wl_list_init(&server.keyboards);
 	server.new_input.notify = server_new_input;
 	wl_signal_add(&server.backend->events.new_input, &server.new_input);
@@ -641,28 +891,38 @@ int main(int argc, char *argv[]) {
 	wl_signal_add(&server.seat->events.request_set_cursor,
 			&server.request_cursor);
 
+	/* Add a Unix socket to the Wayland display. */
 	const char *socket = wl_display_add_socket_auto(server.wl_display);
 	if (!socket) {
 		wlr_backend_destroy(server.backend);
 		return 1;
 	}
 
+	/* Start the backend. This will enumerate outputs and inputs, become the DRM
+	 * master, etc */
 	if (!wlr_backend_start(server.backend)) {
 		wlr_backend_destroy(server.backend);
 		wl_display_destroy(server.wl_display);
 		return 1;
 	}
 
+	/* Set the WAYLAND_DISPLAY environment variable to our socket and run the
+	 * startup command if requested. */
 	setenv("WAYLAND_DISPLAY", socket, true);
 	if (startup_cmd) {
 		if (fork() == 0) {
 			execl("/bin/sh", "/bin/sh", "-c", startup_cmd, (void *)NULL);
 		}
 	}
+	/* Run the Wayland event loop. This does not return until you exit the
+	 * compositor. Starting the backend rigged up all of the necessary event
+	 * loop configuration to listen to libinput events, DRM events, generate
+	 * frame events at the refresh rate, and so on. */
 	wlr_log(WLR_INFO, "Running Wayland compositor on WAYLAND_DISPLAY=%s",
 			socket);
 	wl_display_run(server.wl_display);
 
+	/* Once wl_display_run returns, we shut down the server. */
 	wl_display_destroy_clients(server.wl_display);
 	wl_display_destroy(server.wl_display);
 	return 0;