Add Stack ref class

DESCRIPTION

@@ -54,6 +54,7 @@ Collate:
     'reactives.R'
     'run-url.R'
     'server.R'
+    'stack.R'
     'shiny.R'
     'shinyui.R'
     'shinywrappers.R'

R/stack.R

@@ -0,0 +1,28 @@
+Stack <- setRefClass(
+  'Stack',
+  fields = list(
+    .stack = 'list'
+  ),
+  methods = list(
+    push = function(obj) {
+      .stack <<- c(.stack, list(obj))
+      invisible(.self)
+    },
+    pop = function() {
+      len <- length(.stack) 
+      if (len == 0)
+        return(NULL)
+      obj <- .stack[[len]]
+      .stack <<- .stack[-len]
+      obj
+    },
+    peek = function() {
+      len <- length(.stack) 
+      if (len == 0) return(NULL)
+      .stack[[len]]
+    },
+    size = function() {
+      length(.stack)
+    }
+  )
+)