Add Transform field to Scene

engine/scene.go

@@ -24,12 +24,15 @@ type ZPositioner interface {
 // Scene manages drawing and updating a bunch of components.
 type Scene struct {
 	Components []interface{}
-	needsSort  bool
+	Transform  ebiten.GeoM
+
+	needsSort bool
 }
 
 // Draw draws all components in order.
-func (l *Scene) Draw(screen *ebiten.Image, geom ebiten.GeoM) {
-	for _, i := range l.Components {
+func (s *Scene) Draw(screen *ebiten.Image, geom ebiten.GeoM) {
+	geom.Concat(s.Transform)
+	for _, i := range s.Components {
 		if d, ok := i.(Drawer); ok {
 			d.Draw(screen, geom)
 		}
@@ -37,18 +40,18 @@ func (l *Scene) Draw(screen *ebiten.Image, geom ebiten.GeoM) {
 }
 
 // SetNeedsSort informs l that its layers may be out of order.
-func (l *Scene) SetNeedsSort() {
-	l.needsSort = true
+func (s *Scene) SetNeedsSort() {
+	s.needsSort = true
 }
 
 // sortByZ sorts the components by Z position.
 // Stable sort is used to avoid Z-fighting among layers without a Z, or
 // among those with equal Z. All non-ZPositioners are sorted first.
-func (l *Scene) sortByZ() {
-	l.needsSort = false
-	sort.SliceStable(l.Components, func(i, j int) bool {
-		a, aok := l.Components[i].(ZPositioner)
-		b, bok := l.Components[j].(ZPositioner)
+func (s *Scene) sortByZ() {
+	s.needsSort = false
+	sort.SliceStable(s.Components, func(i, j int) bool {
+		a, aok := s.Components[i].(ZPositioner)
+		b, bok := s.Components[j].(ZPositioner)
 		if aok && bok {
 			return a.Z() < b.Z()
 		}
@@ -57,16 +60,16 @@ func (l *Scene) sortByZ() {
 }
 
 // Update calls Update on all Updater components.
-func (l *Scene) Update() error {
-	for _, c := range l.Components {
+func (s *Scene) Update() error {
+	for _, c := range s.Components {
 		if u, ok := c.(Updater); ok {
 			if err := u.Update(); err != nil {
 				return err
 			}
 		}
 	}
-	if l.needsSort {
-		l.sortByZ()
+	if s.needsSort {
+		s.sortByZ()
 	}
 	return nil
 }