add can_undo and can_redo; fixes #5

R/undomanager.R

@@ -5,6 +5,8 @@
 #' using undo and redo operations.
 #'
 #' @field value Get the value
+#' @field can_undo Whether there are any undo operations available
+#' @field can_redo Whether there are any redo operations available
 #' @field undo_size Get the number of undo operations
 #' @field redo_size Get the number of redo operations
 #' @examples
@@ -33,6 +35,14 @@ UndoManager <- R6::R6Class(
 
     redo_size = function() {
       length(private$.redo_stack)
+    },
+
+    can_undo = function() {
+      self$undo_size > 0
+    },
+
+    can_redo = function() {
+      self$redo_size > 0
     }
 
   ),

tests/testthat/test-undomanager.R

@@ -34,6 +34,20 @@ test_that("UndoManager with multiple types", {
   expect_error(UndoManager$new(c("character", "integer", "numeric"))$do("a")$do(1)$do(1L), NA)
 })
 
+test_that("UndoManager can_undo and can_redo", {
+  expect_false(UndoManager$new()$can_undo)
+  expect_false(UndoManager$new()$do(1)$can_undo)
+  expect_true(UndoManager$new()$do(1)$do(2)$can_undo)
+  expect_false(UndoManager$new()$do(1)$do(2)$undo()$can_undo)
+  expect_true(UndoManager$new()$do(1)$do(2)$undo()$redo()$can_undo)
+
+  expect_false(UndoManager$new()$can_redo)
+  expect_false(UndoManager$new()$do(1)$can_redo)
+  expect_false(UndoManager$new()$do(1)$do(2)$can_redo)
+  expect_true(UndoManager$new()$do(1)$do(2)$undo()$can_redo)
+  expect_false(UndoManager$new()$do(1)$do(2)$undo()$redo()$can_redo)
+})
+
 test_that("UndoManager do works", {
   expect_identical(UndoManager$new()$do(5)$value, 5)
   expect_identical(UndoManager$new()$do(5)$undo_size, 0L)