Refactor Layers into separate type so that Layers can nest

engine/game.go

@@ -1,55 +1,17 @@
 package engine
 
-import (
+import "github.com/hajimehoshi/ebiten/v2"
-	"sort"
-
-	"github.com/hajimehoshi/ebiten/v2"
-)
-
-// Updater is a component that can update. Update is called repeatedly.
-type Updater interface {
-	Update() error
-}
-
-// Drawer is a component that can draw itself. Draw is called often.
-// Z is used to reorder components.
-type Drawer interface {
-	Draw(screen *ebiten.Image, geom ebiten.GeoM)
-	Z() float64
-}
 
 // Game implements the ebiten methods using a collection of components.
 type Game struct {
 	ScreenWidth  int
 	ScreenHeight int
-	Components   []interface{}
+	Layers       *Layers
-
-	needsSort bool
 }
 
-// Update calls Update on all Updater components.
+// Draw draws the entire thing.
-func (g *Game) Update() error {
-	for _, c := range g.Components {
-		if u, ok := c.(Updater); ok {
-			if err := u.Update(); err != nil {
-				return err
-			}
-		}
-	}
-	if g.needsSort {
-		g.needsSort = false
-		g.sortDrawers()
-	}
-	return nil
-}
-
-// Draw calls Draw on all Drawer components.
 func (g *Game) Draw(screen *ebiten.Image) {
-	for _, c := range g.Components {
+	g.Layers.Draw(screen, ebiten.GeoM{})
-		if d, ok := c.(Drawer); ok {
-			d.Draw(screen, ebiten.GeoM{})
-		}
-	}
 }
 
 // Layout returns the configured screen width/height.
@@ -57,22 +19,7 @@ func (g *Game) Layout(outsideWidth, outsideHeight int) (w, h int) {
 	return g.ScreenWidth, g.ScreenHeight
 }
 
-// SetNeedsSort tells the game that the Drawers need sorting.
+// Update just passes the call onto Layers.
-// This will be done in the current update.
+func (g *Game) Update() error {
-func (g *Game) SetNeedsSort() {
+	return g.Layers.Update()
-	g.needsSort = true
-}
-
-// sortDrawers sorts the components by Z position.
-// Non-Drawers are sorted before all Drawers.
-func (g *Game) sortDrawers() {
-	// Stable sort to avoid z-fighting (among Non-Drawers and equal Drawers)
-	sort.SliceStable(g.Components, func(i, j int) bool {
-		a, aok := g.Components[i].(Drawer)
-		b, bok := g.Components[j].(Drawer)
-		if aok && bok {
-			return a.Z() < b.Z()
-		}
-		return !aok && bok
-	})
 }

engine/layers.go

@@ -0,0 +1,71 @@
+package engine
+
+import (
+	"sort"
+
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+// Drawer is a component that can draw itself. Draw is called often.
+type Drawer interface {
+	Draw(screen *ebiten.Image, geom ebiten.GeoM)
+}
+
+// Updater is a component that can update. Update is called repeatedly.
+type Updater interface {
+	Update() error
+}
+
+// ZPositioner is used to reorder layers.
+type ZPositioner interface {
+	Z() float64
+}
+
+// Layers
+type Layers struct {
+	Components []interface{}
+	needsSort  bool
+}
+
+// Draw draws all layers in order.
+func (l *Layers) Draw(screen *ebiten.Image, geom ebiten.GeoM) {
+	for _, i := range l.Components {
+		if d, ok := i.(Drawer); ok {
+			d.Draw(screen, geom)
+		}
+	}
+}
+
+// SetNeedsSort informs l that its layers may be out of order.
+func (l *Layers) SetNeedsSort() {
+	l.needsSort = true
+}
+
+// sortByZ sorts the components by Z position.
+// Stable sort is used to avoid z-fighting among layers without a Z.
+func (l *Layers) sortByZ() {
+	sort.SliceStable(l.Components, func(i, j int) bool {
+		a, aok := l.Components[i].(ZPositioner)
+		b, bok := l.Components[j].(ZPositioner)
+		if aok && bok {
+			return a.Z() < b.Z()
+		}
+		return !aok && bok
+	})
+	l.needsSort = false
+}
+
+// Update calls Update on all Updater components.
+func (l *Layers) Update() error {
+	for _, c := range l.Components {
+		if u, ok := c.(Updater); ok {
+			if err := u.Update(); err != nil {
+				return err
+			}
+		}
+	}
+	if l.needsSort {
+		l.sortByZ()
+	}
+	return nil
+}

main.go

@@ -76,15 +76,19 @@ func main() {
 		TileSize: 16,
 	}
 
+	components := []interface{}{
+		tilemap,
+		engine.PerfDisplay{},
+	}
+
 	game := &engine.Game{
 		ScreenHeight: screenHeight,
 		ScreenWidth:  screenWidth,
-		Components: []interface{}{
+		Layers: &engine.Layers{
-			tilemap,
+			Components: components,
-			engine.PerfDisplay{},
 		},
 	}
-	game.SetNeedsSort()
+	game.Layers.SetNeedsSort()
 
 	if err := ebiten.RunGame(game); err != nil {
 		log.Fatalf("Game error: %v", err)