Nixers Book Club - Book #3: The Wayland Book - Community & Forums Related Discussions

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Here's my summary/review/notes on chapter 7 to 9.
These were more heavy and hands-on chapters, after getting to know the
basic ideas from previous chapters regarding interfaces and decoupling,
we now put it all into practical actions with actual windows and inputs.

XDG shell basics

The XDG (cross-desktop group) shell is a standard protocol extension for
Wayland which describes the semantics for application windows. Roles are
like children in OOP, or traits, defining extra methods for wl_surfaces.
This extension of wl_surface will keep reappearing over and over, that's
how everything is done in Wayland, adding capabilities, functionalities,
and features over it, new traits.

In XDG, there are 2 roles defined: toplevel and popup. To form a tree
of surfaces. It's not a standard protocol, it's defined in extensions
xdg_surfaces are surfaces in the domain of xdg-shell, new traits that
add functionalities over normal surfaces. Namely, toplevel and popup,
xdg_toplevel and xdg_popup. xdg_toplevel is finally our window.

I initially tried the example movq showed on irc by using wl_shell
get_shell_surface method but apparently, we should use xdg_wm_base
instead with get_xdg_surface.
As we said, it's not defined in the usual wayland headers that come
with the distro packages, so at this point we need to generate the files
using the wayland-scanner we've seen before and include it in our Makefile.

The actual drawing of pixels happens on the configure and is done after
acknowledging it.
To answer movq, we assume after the rountrip we have all the globals
from the registry, after wl_display_roundtrip(state.wl_display).

The example is extensive, it takes quite a lot of implementation to get
a window drawn, and without decoration. It's even more decoupled than
I thought. Yet, in the following chapter about inputs, we add even more
boiler/glue code.
Also, there's no decoration, but let's wait, maybe the book will talk
about that later.

Surfaces in depth.

This chapter dives more into the functionalities of the wl_surface. Which
as I understood correctly is extended heavily into different roles, adding
traits with methods that it can fill in extensions. From wl_surface to
xdg_surface to xdg_toplevel.

These wl_surfaces have their own lifecycle.
The wl_surface drives the atomicity through its pending, committed,
and applied states.
A lot of states can be changed before committing to the surface, such
as the wl_buffer, damage region, input events, etc..

To give the first state, you need to give the surface a role, allocating
and attaching the buffer, then commit again.
We see that in the previous example at the end of the configure request
for xdg_surface_listener.configure. I guess that's what it does.
Quote:The next question is: when should I prepare a new frame?

And how too? in the event loop? … And no, it's done preferably after
receiving an event from the wl_callback called "done", or after input
events in event-driven applications.
This is interestingly efficient and also low-level. We can manage
each frame.

To get this behavior of frame-callback, we need an object implementing
this wl_callback interface which we get from the wl_surface.frame
request. Then we set a listener for event "done".
callback_data is the current time in millisecond.

Then inside this callback, we destroy and recreate the callback, call
draw again, reattach the buffer to the wl_surface, call damage the entire
surface, and commit. The destruction and reconstruction of the callback
is a bit confusing.
So I guess internally it'll automatically only redraw what needs to
be redrawn if we damage only a certain area.
That's exactly what is done in the 8.3 section.

Overall, it seems like we need to keep all these structures, these objects
adding "traits" to the surface and manipulate them from everywhere.
The global state invades everything, passed to all events. I'm sure it
could be done otherwise though.

Surface regions
wl_compositor can be used to create an object of type region, a wl_region
by callling the request create_region.
A region is a goup of rectangles creating an arbitrary shape, by doing
operations between different rectangles.
These arbitrary regions can then be passed to wl_surface as either opaque
region, for what part of the wl_surface is visible, or as input region,
for which part can accept input.
These are interesting to control surfaces I think.

In the core protocol, wayland.xml, only one surface role is defined and
that's subsurface.
They are child surfaces that are positioned relatively to a parent
surface and a z-index, kind of like transient/modal/popup/dialog windows.
This can be used to do window decoration.
Funnily, these are created from yet another global object:
wl_subcompositor, even more separation of roles, yey!
The subsurface can then be manipulated like a normal surface but has
"place_above/below" functions. It's in sync with the parent surface
lifecycle as far as the atomic operations on buffers and others go.

high density surface (hiDPI)
wl_output, which represents a display object, I guess, sends an event
saying the scale factor in place. This scaling factor can then be applied
to wl_surface via set_buffer_scale.
I think that's a really nice way to handle hiDPI, that should solve a
lot of things. However, as with everything Wayland is only the protocol,
which practically is only the XML definition of interfaces, so we have
to handle this manually.

Chapter 9: Seats, handling inputs

Finally, we're going to interact with windows.

A seat represent a user with the inputs, pointer and keyboard.
Yet another global that is accessible and that you can bind during startup.
It offers a pointer, keyboard, and touch that you can get through requests.
Each have their own interfaces defining how to interact with them.
You can know what is supported from the capabilities, which is a bitfield,
you can do bitwise operations to compare them with constants in the form:

A concept of serial IDs is introduced, each input event is associated
with an ID that needs to be sent back so that the server can decide
whether to respect it or not.

Another concept of input frame is introduced, each input event is actual
fragmented into multiple ones, which are sent separately, until a final
"frame" event is received indicating that it was a single set of states
that belong together to the same input.
We're adviced to buffer things until we actually receive that event. That
mindset goes along the drawing on wl_surface, it's efficient.

The book then dives into each input type.

First one pointer input, returning a `wl_pointer`.
It has all the usual events we would guess: from enter, to leave, to
button clicked, scrolling/axis, etc..
We can notice the serial id being included.
We can create a cursor for the pointing device using the request
set_cursor and passing a surface.
It's interesting how the axis source is well-defined.

Second one is keyboard input, returning a `wl_keyboard`.
We're getting an explanation for XKB, keymap, and scancode, this shouldn't
be new for people who have been using X11. xkbcommon is the standalone
library offering the translation from scancode to the keymap symbols.
We receive the keymap from the wl_keyboard in an event called "keymap",
which has a format and file descriptor. I would've guessed it would be
a string but no.
Quote:Bulk data like this is transferred over file descriptors.
We could simply read from the file descriptor, but in general it's
recommended to mmap it instead.
Well… that's something new!
The keyboard mmap seem to fail on my machine for the example though,
I had to use plain malloc an read the file descriptor manually instead.

The keyboard events themselves are also somewhat obvious, key, key_state,
I like that they are separated, maybe that should fix some issues I've
personally had while manipulating keys in X11 when they get modified.
Key repeat event, alright...

Third one is touch input, returning a `wl_touch`.
Now, that makes it easy to go next level on multi-touch screens!
The "frame" event makes a lot of sense in this case, when multiple
fingers press the screen, each finger having a different id.
"down", "up", "motion", yep, that's nice.

Now for the example code.
That's extensive, new globals, states, and listeners everywhere, glue
code programming.
We add the wl_seat, wl_keyboard, wl_pointer, wl_touch.
Set the wl_set in the listener for the registry, set listeners for the
capabilities that it supports.
We then create pointer events storage structures, and obviously also
add it to our global state.
After that we can check if we have the capabilities to set the pointer
and the related many listeners that will update the new structure in our
global state.
Interestingly, I've discovered that my touchpad support 3 touch input
buttons: single finger, two fingers, and three fingers (similar to
middle mouse).

I think in general this could be always present and people would just
need to handle the "frame" event instead of all this.
We do a similar thing for the keyboard, but this time to compile we need
lib xbkcommon.
We also get a glimpse at wl_array objects helpers, with their

After these chapters, I think I have a better idea of Wayland mechanisms
and way of thinking. Having things defined in a protocol defining objects
and interfaces makes it easy to know what to expect, yet it also somehow
decouples things a bit too much.
Still, if the glue code is present it's a really clear and clean way to
handle things.

Messages In This Thread
RE: Nixers Book Club - Book #3: The Wayland Book - by venam - 03-04-2021, 09:08 AM