2 files changed, 80 insertions, 7 deletions
diff --git a/lib/ui/grid.go b/lib/ui/grid.go
index ede7d0c..3c375ee 100644
--- a/lib/ui/grid.go
+++ b/lib/ui/grid.go
@@ -10,7 +10,7 @@ type Grid struct {
 	rowLayout    []gridLayout
 	columns      []GridSpec
 	columnLayout []gridLayout
-	Cells        []*GridCell
+	cells        []*GridCell
 	onInvalidate func(d Drawable)
 	invalid      bool
 }
@@ -76,7 +76,7 @@ func (grid *Grid) Draw(ctx *Context) {
 	if invalid {
 		grid.reflow(ctx)
 	}
-	for _, cell := range grid.Cells {
+	for _, cell := range grid.cells {
 		if !cell.invalid && !invalid {
 			continue
 		}
@@ -140,7 +140,7 @@ func (grid *Grid) invalidateLayout() {
 
 func (grid *Grid) Invalidate() {
 	grid.invalidateLayout()
-	for _, cell := range grid.Cells {
+	for _, cell := range grid.cells {
 		cell.Content.Invalidate()
 	}
 }
@@ -156,7 +156,7 @@ func (grid *Grid) AddChild(content Drawable) *GridCell {
 		Content: content,
 		invalid: true,
 	}
-	grid.Cells = append(grid.Cells, cell)
+	grid.cells = append(grid.cells, cell)
 	cell.Content.OnInvalidate(grid.cellInvalidated)
 	cell.invalid = true
 	grid.invalidateLayout()
@@ -164,9 +164,9 @@ func (grid *Grid) AddChild(content Drawable) *GridCell {
 }
 
 func (grid *Grid) RemoveChild(cell *GridCell) {
-	for i, _cell := range grid.Cells {
+	for i, _cell := range grid.cells {
 		if _cell == cell {
-			grid.Cells = append(grid.Cells[:i], grid.Cells[i+1:]...)
+			grid.cells = append(grid.cells[:i], grid.cells[i+1:]...)
 			break
 		}
 	}
@@ -175,7 +175,7 @@ func (grid *Grid) RemoveChild(cell *GridCell) {
 
 func (grid *Grid) cellInvalidated(drawable Drawable) {
 	var cell *GridCell
-	for _, cell = range grid.Cells {
+	for _, cell = range grid.cells {
 		if cell.Content == drawable {
 			break
 		}
diff --git a/lib/ui/stack.go b/lib/ui/stack.go
new file mode 100644
index 0000000..9f81db8
--- /dev/null
+++ b/lib/ui/stack.go
@@ -0,0 +1,73 @@
+package ui
+
+import (
+	"fmt"
+
+	tb "github.com/nsf/termbox-go"
+)
+
+type Stack struct {
+	children []Drawable
+	onInvalidate func(d Drawable)
+}
+
+func NewStack() *Stack {
+	return &Stack{}
+}
+
+func (stack *Stack) OnInvalidate(onInvalidate func (d Drawable)) {
+	stack.onInvalidate = onInvalidate
+}
+
+func (stack *Stack) Invalidate() {
+	if stack.onInvalidate != nil {
+		stack.onInvalidate(stack)
+	}
+}
+
+func (stack *Stack) Draw(ctx *Context) {
+	if len(stack.children) > 0 {
+		stack.Peek().Draw(ctx)
+	} else {
+		cell := tb.Cell{
+			Fg: tb.ColorDefault,
+			Bg: tb.ColorDefault,
+			Ch: ' ',
+		}
+		ctx.Fill(0, 0, ctx.Width(), ctx.Height(), cell)
+	}
+}
+
+func (stack *Stack) Push(d Drawable) {
+	if len(stack.children) != 0 {
+		stack.Peek().OnInvalidate(nil)
+	}
+	stack.children = append(stack.children, d)
+	d.OnInvalidate(stack.invalidateFromChild)
+	stack.Invalidate()
+}
+
+func (stack *Stack) Pop() Drawable {
+	if len(stack.children) == 0 {
+		panic(fmt.Errorf("Tried to pop from an empty UI stack"))
+	}
+	d := stack.children[len(stack.children)-1]
+	stack.children = stack.children[:len(stack.children)-1]
+	stack.Invalidate()
+	d.OnInvalidate(nil)
+	if len(stack.children) != 0 {
+		stack.Peek().OnInvalidate(stack.invalidateFromChild)
+	}
+	return d
+}
+
+func (stack *Stack) Peek() Drawable {
+	if len(stack.children) == 0 {
+		panic(fmt.Errorf("Tried to peek from an empty stack"))
+	}
+	return stack.children[len(stack.children)-1]
+}
+
+func (stack *Stack) invalidateFromChild(d Drawable) {
+	stack.Invalidate()
+}