[Rd] pausing between plots - waiting for graphics input

Duncan Murdoch murdoch at stats.uwo.ca
Tue Nov 30 22:00:36 CET 2004

On Wed, 01 Dec 2004 08:55:27 +1300, Paul Murrell
<p.murrell at auckland.ac.nz> wrote :

>This sounds like the general problem of being able to capture keyboard 
>input on a graphics device (a key-stroke equivalent of dev_locator). 
>Robert has been keen on this for a while too.
>It would presumably be not too difficult to implement something modal 
>(like dev_locator) - in effect, a dev_eventloop, which blocks the 
>command line and processes events (both mouse clicks and key strokes) in 
>a particular graphics window until a prearranged event to quit.  Nasty 
>modal behaviour, but doable and obviously useful in some ways.  Any 
>interest in that?

You mean something like this?

The user sees a function 

 graphevents <- function(handler, events = c('mousedown', 'mouseup',
'mousemove', 'keydown', 'keyup') , prompt = 'Please do something')

which calls the handler function with a standard set of args
indicating what event just happened and keeps going until the handler
returns some non-NULL value.  So locating a single point could be
implemented as

 onmousedown <- function(event, button, x, y) {

 graphevents(onmousedown, events='mousedown', prompt='Click on the

and waiting for the user to hit a key could be implemented as

 onkeyup <- function(event, key) {

 graphevents(onkeyup, events='keyup', prompt='Hit any key')

If we wanted both, maybe we could have

 graphevents( list(onmousedown, onkeyup), c('mousedown', 'keyup'))

That would be fairly easy to implement in the windows() device, but I
have no idea if it would make sense in other interactive devices.
Issues would be defining what sort of values "key" would take, what
events to allow handlers for, what the event handler arg lists would
look like, and so on.  A test of whether it was good enough might be
whether locator() could be rewritten in R.

>A much nicer solution of course would be asynchronous event handling in 
>the graphics window (i.e., you don't block the command line), but that 
>depends on the event loop integration problem being solved and that does 
>not look like happening soon.

Yes, definitely harder.


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