[Rd] getGraphicsEvent on X11 and event queuing

[frederik at ofb.net]

Hi R-Devel,

I've been working on an oscilloscope project using an Arduino
microcontroller board. I found that it's quite easy to get realtime
updates, e.g. 30+ frames per second, if I read data from the board in
a little Rcpp library. I have to use dev.hold() and dev.flush() to
keep the plot from flickering, which restricts me to the "cairo" X11
device.

I'd like to be able to add interactivity to the oscilloscope display,
for instance to bind a key to save the current plot to a file, or to
bind keys for adjusting the time scale etc.

However, I ran into two problems:

(1) setGraphicsEventHandlers only works on the "Xlib" X11 device,
which doesn't support buffering via dev.hold() - it flickers.

(2) getGraphicsEvent and friends lack some interface features which
are needed to use the functions in an asynchronous fashion. Typically,
event listener library functions have a "timeout" parameter, and the
ability to return already-queued events. But getGraphicsEvent() has
neither - it waits indefinitely (you can't set a timeout), and it
seems to ignore events which occurred before it was called (I can't
figure out why, from the code, I guess the normal R event processing
grabs events which occur between calls to getGraphicsEvent?).

It seems like it should be possible to set a handler for keyboard
events and have it execute in between plot updates when the user
presses a key - yet without blocking further updates if no key has
been pressed.

Is anyone interested in fixing (1) and (2)? Or is there some other
library or workaround to solve my problems?

Here is some code I used to play around with these functions:

    X11(type="Xlib");

    keydown = function(key) {
      cat("Got key: ",key);
        lastkey <<- key
        }

    setGraphicsEventHandlers(onKeybd = keydown);

    plot(c(0,0));
    getGraphicsEvent();

Well, I think it's great that getGraphicsEvent exists at all, and want
to salute Duncan Murdoch who is listed as the author. I hope I may
have helped by describing some new possible uses for these functions.

Thank you,

Frederick