plot handler called too late when evaluating a loop

[jankatins]

In the IRkernel, we encountered a problem were we couldn't explain the order of the output of a evaluated loop. The situation is basically this:

library(evaluate)

code <- "
for (i in 1:5) {
txt(paste('call:', i),'call')
cars <- c(i, 3, 6, 4, 9)
plot(cars)   
}
"

l <- list()
txt <- function(o, type) {
  t <- paste(o, collapse = '\n')
  l[length(l)+1] <<- t
}
oh <- new_output_handler(source = identity, 
                         text = function(o) txt(o, "text"), 
                         graphics = function(o) txt("plot", "plot"),
                         message = identity, 
                         warning = identity, 
                         error = identity, 
                         value = identity)


x <- evaluate(code, output_handler = oh)
cat(paste(l, collapse="\n"))

-> The resulting list is both filled by the handler and by a call from the evaluated code.

The problem is that it seems that the in some cases, the call happens too early:

> cat(paste(l, collapse="\n"))
call: 1               # No plot has triggered the handler -> call is first
call: 2               # second loop ...
plot                  # but the plot from the first loop is only now being handled
call: 3               # again a too early call
plot                  # plot from the second loop
call: 4
plot
call: 5
plot
plot

When "unrolling" the loop, it looks fine:

code <- "
{i <- 1
txt(paste('call:', i),'call')
cars <- c(i, 3, 6, 4, 9)
plot(cars)   
}
{i <- 2
txt(paste('call:', i),'call')
cars <- c(i, 3, 6, 4, 9)
plot(cars)   
}
{i <- 3
txt(paste('call:', i),'call')
cars <- c(i, 3, 6, 4, 9)
plot(cars)   
}
# [...]
"

> cat(paste(l, collapse="\n"))
call: 1
plot
call: 2
plot
call: 3
plot
# [...]

Any idea why this happens and if there is any way to get this in the right order?

[hadley]

I think it's very likely that problem is that we can't tell when a plot is "done" until a new plot starts. I don't see any way around that.