[FORGED] recordPlot/replayPlot not working with saveRDS/readRDS

Paul Murrell <paul at stat.auckland.ac.nz>
     on Tue, 3 Apr 2018 09:41:56 +1200 writes:

     > Hi What you are doing "wrong" is loading a recordedplot
     > into the same session that it was created in.  The
     > saveRDS()/readRDS() works if you save in one R session and
     > then read in a different R session.  The assumption is
     > that if you are still in the same session that created the
     > recordedplot you still have the recordedplot (e.g., you
     > can just do replayPlot(r) instead of replayPlot(r2).  Is
     > that a bad assumption ?

Not "bad", really,  but somewhat *un*intuitive I believe to most
users, including me.
I would have done the same as Winston to check if saving and
loading of a recorded plot work fine.

Also, the error message we get is not very helpful:

     > saveRDS(r, 'recordedplot.rds')
     > r <- readRDS('recordedplot.rds')
     > png('test2.png')
     > replayPlot(r)

     Error: NULL value passed as symbol address

{coming from deep down: checkValidSymbolId() in src/main/dotcode.c }

I've pondered a bit and in the end found it easy enough to
tryCatch() the error, and "do the right thing" instead.
As that should typically only happen in exactly such a use case,
the change should be "uniformly >=" the previous state.

Ideally, maybe we (R core) would provide an R level API to a
"TRUE/FALSE" valued version of checkValidSymbolId() or its
caller, so that *from R code* one could check if the
recordedplot 'x' passed to replayPlot() with its external
pointers needs to be fixed up by
      x <- restoreRecordedPlot(x)
or not.

But that would be another topic I'd not get envolved deeply just now.

Martin

Paul

On 03/04/18 05:23, Winston Chang wrote:

The documentation for recordPlot says the following:

As of R 3.3.0, it is possible (again) to replay a plot from another R session using, for example, saveRDS and readRDS.

However, I haven't been able to save and restore a plot displaylist
and have it work within the same R session, using R 3.4.3 or 3.3.3.
Here's an example:

# Save displaylist for a simple plot
png('test.png')
dev.control(displaylist ="enable")
plot(1:5, 1:5)
r <- recordPlot()
dev.off()

# Replay plot. This works.
png('test1.png')
replayPlot(r)
dev.off()

#  Save the plot and load it, then try to replay it. This does not work.
saveRDS(r, 'recordedplot.rds')
r2 <- readRDS('recordedplot.rds')
png('test2.png')
replayPlot(r2)  ## Error: NULL value passed as symbol address
dev.off()


Is there something that I'm doing wrong here?

-- 
Dr Paul Murrell
Department of Statistics
The University of Auckland