[Rd] Re: [R] Problem going back to a viewport with gridBase

[Gabor Grothendieck]

Here is the code once again.  This time I have supplied two 
names methods and a getChildren.viewport function to 
encapsulate the corresponding grid internals.  It would
be easiest if grid provided these itself but in the absence
of that this does encapsulate dependencies on grid
internals to a well defined set of functions.   Note that
names is only used in 'with' and 'with' will be eliminated
once Deepayan adds the use.viewport= (or whatever
its called) to print.   I am not sure from your response
whether or not you intend to add these items to the grid
API but in any case this provides an intermediate
level of safety.

library(grid)
library(lattice)

pushLayout <- function(nr, nc, name="layout") {
  pushViewport(viewport(layout=grid.layout(nr, nc), name=name))
  for (i in 1:nr) {
    for (j in 1:nc) {
      pushViewport(viewport(layout.pos.row=i, layout.pos.col=j))
      upViewport()
    }
  }
  upViewport()
}

names.vpPath <- names.viewport <- function(x) x$name

with.vpPath <- with.viewport <- function(data, expr, ...) {
      # if data is a vpPath it cannot be ROOT since  
      # NULL will never dispatch here
      depth <- if (data$name == "ROOT") 0 else downViewport(names(data))
      result <- eval.parent(substitute(expr))
      upViewport(depth)
      invisible(result)
}

getChildren.viewport <- function(x) x$children

grid.newpage()

# specify number of cells to fill and number of rows
n <- 5; nr <- 3

nc <- ceiling(n/nr)
downViewport(pushLayout(nr, nc))

vpt <- current.vpTree(all = FALSE)
for(k in 1:n) with(getChildren.viewport(vpt)[[k]],
      print( xyplot(v ~ v, list(v = 1:k)), newpage = FALSE )
)