Control bar border (color) thickness with ggplot2

Question

Is it possible to use the stroke argument introduced with ggplot2 2.0 to adjust the thickness of borders around bars? If not, is there a way to control bar-border thickness along the lines of point-border thickness? Stroke applies to borders around certain shapes -- see the second answer

A very modest MWE, showing fill only:

factor <- c("One", "Two", "Three", "Four")
value <- c(1, 2, 3, 4)
factor2 <- c("A", "B", "A", "B")

df <- data.frame(factor = factor(factor, levels = factor),
                 value = value, factor2 = factor2) 

ggplot(df, aes(x = factor, y = value, color = factor2)) +
  geom_bar(stat = "identity")

EDIT after COMMENT OK, thanks to MLavoie's comment, it was so simple. Here is the code I have ended with, and, no, I am not actually using this plot other than to teach about ggplot and its capabilities.

ggplot(df, aes(x = factor, y = value, color = factor2)) +
  scale_color_manual(values = c("darkgreen", "slateblue4")) +
  geom_bar(stat = "identity", aes(fill = "transparent", size = ifelse(factor2 == "A", 2, 1))) +
  guides(fill = FALSE) +
  guides(size = FALSE) +
  guides(color = FALSE)

Answer 1

well as I suggested by the OP, I will just recopy my comment as an answer.

you just need to set size in your geom_bar() expression:

geom_bar(stat = "identity", aes(fill = "transparent", size = ifelse(factor2 == "A", 2, 1)), size=2)