[Xcb] My X toolkit plan

Barton C Massey bart at cs.pdx.edu
Sun May 7 22:09:56 PDT 2006

Vincent: Hah.

Let me just type my "toolkit plans" into this email.  Then I
will at least have discharged my immediate obligation to
you.  So here's the current scoop, based on conversations
with Keithp, Jamey, and others.  Actually, I'll probably
also blog it at some point, but I should let XCB listers
have first crack at it.

This, BTW, is a pretty different plan from what it was two
months ago.  This may mean that it's also different from
what I'll eventually actually do.  MMMV.

Premise: Current toolkits (meaning practically everything)
have lousy solutions to several important problems.  As an
example, let's imagine I'm going to build a chess program
with an integrated GUI. Problems:

  1. The chess engine wants to do its search without paying
     attention to the rest of the program, except when it
     has calculated some kind of update.  In a
     single-threaded app, this means that the user may be
     locked out.

  2. The user inputs are serialized by the window system
     into an event stream.  In most toolkits, all other
     kinds of application inputs are serialized as well.
     This means that activities can't easily be performed
     out-of-order with the event stream, or concurrently,
     even when it would simplify or improve the application
     to do so.

  3. Toolkits are typically set up to support a modularity
     module in which individual windows or drawing areas are
     mapped 1::1 to individual objects or structures, and in
     which handling I/O is mapped onto these structures.
     This makes it hard to support metaphors other than the
     standard desktop metaphor, and hard to get emergent

  4. Toolkits typically make it hard to get robust,
     high-quality layout on the screen.  Indeed, IMHO
     Keithp's and my Kgames Xt layout widget from the 80s is
     easier to use and works better than any of the standard
     solutions around today.

  5. Toolkits typically make it hard to support direct
     manipulation; yet direct manipulation is arguably the
     easiest, most powerful GUI style possible.  It is
     embarassing that the user interfaces for word
     processing and vector drawing today are IMHO uniformly
     so much worse than those of MacWrite and MacDraw for
     the original Mac, but there we are.

So what's my solution?  Four phases---you've seen the first
two already.

  I.   Get the Xserver and client libraries to support
       reasonable rendering and typography.  Done by Keithp
       and friends: see Xft/fontconfig, Render, Pango and

  II.  Replace Xlib with a protocol binding that offers
       sensible direct protocol support, thread
       transparency, and latency hiding.  Done: see XCB.

  III. Build an X toolkit that supports a reasonably wide
       range of GUI structures robustly and simply.  Current
       item, addressing 1-3 above.

  IV.  Build X GUI libraries atop this toolkit to support a
       direct-manipulation non-desktop-metaphor graphical
       environment (while retaining compatibility with
       current X apps).  Future work addressing 4-5 above.

Specifically, phase III involves trying new architectures
for X application structuring.  My first candidate is based
on these observations:

  A. Model-View-Controller is *almost* right.  Certainly,
     the Model idea seems to be a good one.  In our chess
     program, the engine would be responsible for
     maintaining the model.  But in practice, MVC tends to
     degenerate into Model + ViewControllerThingy, and the
     communication between them tends to be a mess.  We need
     some way of coupling the engine to GUI input and output
     that is easy to use and robust.

  B. Concurrency and dynamic modularity means threads, but
     programmers have a hard time handling standard thread
     programming; debugging is hard, and it's easy to
     introduce races, deadlocks and livelocks and hard to
     detect them.  If GUI apps are to be threaded, the
     thread processing and communication model must be so
     simple that it's easier than programming
     single-threaded event-driven applications.
     Fortunately, in my experience this isn't a very high

OK, enough blather.  Here's my proposed toolkit architecture,
version 0.

     event thread ---> |            | -> drawing thread 
	               | blackboard | -> other output
     one thread per -> |            |    threads
      other input      |            |
                           |   ^
	                   v   |

It is important that all threads are completely lockless,
block waiting for inputs ("pull model"), and communicate
only via channels and the blackboard.

What's a "blackboard"?  It's an architectural data structure
idea that has been around, especially in AI circles, for 40
years, and IMHO has been grossly underutilized.  Basically,
a blackboard is a large data structure segmented into
components.  Applications can block waiting to be triggered
by combinations of data appearing on the blackboard.  They
can also poll the blackboard, and post data to the
blackboard.  Thus, the blackboard fills simultaneous roles
of synchronization/rendezvous of threads and data sharing.

(Mental exercise---60 seconds: What blackboard has been a
stock part of the X Window System since the beginning, and
is used much as above?  [Note that this does *not* mean that
I'm planning to use the existing thingy as a blackboard;
we'll build a custom blackboard from scratch.])

The theory is that with blackboard and library magic, I can
provide a library that makes the threads essentially
invisible (as e.g. Java AWT/Swing does), but still prevents
or detects common threading bugs.  1 above is taken care of
trivially.  2 can be handled by not serializing or even by
de-serializing where appropriate, and by getting rid of the
locks, condition variables, shared structures etc that make
thread programming hard.  Debugging becomes much more fun,
because you can stop one or more threads at any point and
examine the blackboard to figure out what's going on.  3 is
handled by the fact that there's no necessary or real
correspondence between threads or objects in the program and
objects on the screen; this allows much more flexibility in
tackling 4 and 5 in Phase IV.

At least this is the current theory.  It awaits time and/or
volunteers to try building it out.


In message <Pine.LNX.4.51.0605060733110.4748 at cartan.iecn.u-nancy.fr> you wrote:
> Hey :)
> > Vincent: I still owe you a toolkit plan.  Now that
> > XCB is stable I'm gonna get started---Keithp and I
> > hashed out a bunch of the critical remaining issues last
> > night and I'll blog something in the next couple of days
> > and post a link here.
> do you have a link about your plans ?
> Vincent

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