Use grid.curve to connect grobs in different viewpoints

Question

R graphics has the grid package which in theory allows one to use grid.curve to create curved arrows between shapes (see grid.curve description here https://www.stat.auckland.ac.nz/~paul/grid/curve/curve.pdf). However, it seems grid.curve cannot connect grobs across viewpoints.

I would like to change the following code to connect the two circles with a curved arrow as in the grid.curve function examples. Does anyone know how to do this?

Example code below

library(grid)
library(gridExtra)

# Layout Branches
pdf("test.pdf")
grid.newpage()
layout=grid.layout(nrow=2,ncol=2)
pushViewport(viewport(layout=layout,name="base"))

#Now add circles for states
seekViewport("base")
for (ii in 1:2) {
  for(jj in 1:2) {
   name=paste(ii,jj,sep="a")    
   name2=paste(ii,jj,sep="_")  
   pushViewport(viewport(layout.pos.col=jj, layout.pos.row=ii, name = name2)) 
   grid.circle(r=.4, name = name)       
   upViewport()
}}

seekViewport("1_1")
grid.move.to(grobX("1a1",0),grobY("1a1",0)) 
seekViewport("2_2")
grid.line.to(grobX("2a2",180),grobY("2a2",180))
dev.off()

Answer 1

This should do the trick:

## Your code
grid.newpage()
layout=grid.layout(nrow=2,ncol=2)
pushViewport(viewport(layout=layout,name="base"))
seekViewport("base")
for (ii in 1:2) {
  for(jj in 1:2) {
   name=paste(ii,jj,sep="a")
   name2=paste(ii,jj,sep="_")
   pushViewport(viewport(layout.pos.col=jj, layout.pos.row=ii, name = name2))
   grid.circle(r=.4, name = name)
   upViewport()
}}

## Define a function that returns the location of the specified
## grob-edge point, in terms of the npc coordinate system of the
## entire device
getDeviceNPC <- function(vpName, grobName, theta) {
    seekViewport(vpName)
    A <- grid.move.to(grobX(grobName, theta), grobY(grobName, theta))
    x <- convertWidth(A$x, unitTo="inches")
    y <- convertHeight(A$y, unitTo="inches")
    xy <- unit((c(x,y,1) %*% current.transform())[1:2], "inches")
    seekViewport("base")
    convertUnit(xy, unitTo="npc")
}

## Use it to extract locations of a couple of points
A <- getDeviceNPC(vpName="1_1", grobName="1a1", theta=0)
B <- getDeviceNPC(vpName="2_2", grobName="2a2", theta=180)

## Draw a curve between the points
grid.curve(A[1], A[2], B[1], B[2], gp=gpar(col="red"), arrow=arrow(),
           curvature=-1, inflect=TRUE)