Isolate side effects in graphics functions

[nanxstats]

Fixes #227

This PR introduces safe_par() and safe_strwidth() as alternatives to their original graphics functions. These wrappers will be evaluated in a temporary BMP device, to avoid generating Rplots.pdf when running r2rtf code under certain context - such as clicking the test buttons in RStudio environments, even for third-party packages.

@yihui I'm not sure if this is the optimal solution so I'm open to suggestions.

Using these wrappers can be be 10x slower than calling the graphics functions directly:

microbenchmark::microbenchmark(
  r2rtf:::safe_strwidth("Hello world!", units = "inches", cex = 1, font = 1, family = "sans"),
  graphics::strwidth("Hello world!", units = "inches", cex = 1, font = 1, family = "sans"),
  times = 5000
)

#>    min     lq      mean median     uq      max neval cld
#> 44.526 46.945 53.097288 47.806 49.692 4399.300  5000   a
#>  2.747  2.952  3.199353  3.198  3.362   16.482  5000   b

[elong0527]

Just for my understanding. Why would bmp() device used instead of pdf()

[elong0527]

Could we use "tempfile" to avoid a OS dependent logic?

[yihui]

I guess writing to the null device is often faster than a real file.

[yihui]

I think a more elegant and faster solution is to use the null pdf device, i.e.,

opts <- options(device = function(...) pdf(NULL, ...))
on.exit(options(opts), add = TRUE)

[yihui]

I don't think you need substitute() + eval(), but can just take advantage of R's lazy evaluation of arguments, i.e., simply put expr at the end, and R will evaluate it at that time.

Suggested change

	expr <- substitute(expr)
	grDevices::bmp(file_null())
	on.exit(
	{
	current <- grDevices::dev.cur()
	if (current > 1) {
	prev <- grDevices::dev.prev(current)
	grDevices::dev.off(current)
	if (prev != current) grDevices::dev.set(prev)
	}
	},
	add = TRUE
	)
	eval(expr, envir = parent.frame())
	grDevices::bmp(file_null())
	on.exit(
	{
	current <- grDevices::dev.cur()
	if (current > 1) {
	prev <- grDevices::dev.prev(current)
	grDevices::dev.off(current)
	if (prev != current) grDevices::dev.set(prev)
	}
	},
	add = TRUE
	)
	expr