spplot() does not work properly when called via a function

Question

I have spatial data in R, let's say an object of formal class SpatialPointsDataFrame but the below problem is encountered with Raster too.

I can plot easily the data with spplot() and save a .png file. If I want to create and store a function to recall this operation (since I have to repeat this many times in a simulation), spplot() save an empty .pgn file UNLESS I use dev.off() outside the function.

# example data
trees <- data.frame(x=runif(100, 1,100), y=runif(100, 1,100), value=runif(100, 20,60))
trees <- SpatialPointsDataFrame(trees[,c("x","y")], trees)
proj4string(trees) <- CRS("+proj=utm +zone=30 ellps=WGS84")

# simple plotting, it works
png(filename="spplot0.png", width = 480, height = 480, units = "px")
spplot(trees, "value")
dev.off()

# create functions
map1 <- function(){
  png(filename="spplot1.png", width = 480, height = 480, units = "px")
  spplot(trees, "value")
  dev.off()
}

map2 <- function(){
  png(filename="spplot2.png", width = 480, height = 480, units = "px")
  spplot(trees, "value")
}

# carry out functions. map1() save an empty image.
map1()
map2();dev.off()

Why map1() does not work and map2()+dev.off() outside does?

I know there are alternative mapping options (plot, spplot, ggplot) but I wish to stick with spplot() for this job. I have already found a work-around solution (see function map2) but it baffles me, my code gets more cluttered and I fear to encounter more problems in the future.

Answer 1

You have to call print() inside the function:

map1 <- function(){
  png(filename="spplot1.png", width = 480, height = 480, units = "px")
  print(spplot(trees, "value"))
  dev.off()
}

map1() # works