re-un-re-refactor?

engine/billboard.go

@@ -52,7 +52,7 @@ func (b *Billboard) Prepare(g *Game) error {
 }
 
 // Scan returns a slice containing Src.
-func (b *Billboard) Scan() Components { return Components{&b.Src} }
+func (b *Billboard) Scan() []interface{} { return []interface{}{&b.Src} }
 
 func (b *Billboard) String() string {
 	return fmt.Sprintf("Billboard@%v", b.Pos)

engine/camera.go

@@ -86,7 +86,7 @@ func (c *Camera) Prepare(game *Game) error {
 }
 
 // Scan returns s.Child.
-func (c *Camera) Scan() Components { return Components{c.Child} }
+func (c *Camera) Scan() []interface{} { return []interface{}{c.Child} }
 
 // Transform returns the camera transform.
 func (c *Camera) Transform() (opts ebiten.DrawImageOptions) {

engine/container.go

@@ -0,0 +1,45 @@
+package engine
+
+// Container contains many components.
+type Container []interface{}
+
+// Scan returns c.
+func (c Container) Scan() []interface{} { return c }
+
+// Register records component in the slice, if parent is the container.
+func (c *Container) Register(component, parent interface{}) error {
+	if parent == c {
+		*c = append(*c, component)
+	}
+	return nil
+}
+
+// Unregister searches the slice for the component, and removes it by setting
+// to nil. If the number of nil items is greater than half the slice, the slice
+// is compacted.
+func (c *Container) Unregister(component interface{}) {
+	free := 0
+	for i, x := range *c {
+		switch x {
+		case component:
+			(*c)[i] = nil
+			free++
+		case nil:
+			free++
+		}
+	}
+	if free > len(*c)/2 {
+		c.compact()
+	}
+}
+
+func (c *Container) compact() {
+	i := 0
+	for _, x := range *c {
+		if x != nil {
+			(*c)[i] = x
+			i++
+		}
+	}
+	*c = (*c)[:i]
+}

engine/drawdag.go

@@ -25,7 +25,7 @@ var _ interface {
 // the number of tests between components.
 type DrawDAG struct {
 	ChunkSize int
-	Components
+	Child     interface{}
 	Hides
 
 	*dag
@@ -114,6 +114,8 @@ func (d *DrawDAG) Prepare(game *Game) error {
 	return d.Register(d, nil)
 }
 
+func (d *DrawDAG) Scan() []interface{} { return []interface{}{d.Child} }
+
 // Update checks for any changes to descendants, and updates its internal
 // data structures accordingly.
 func (d *DrawDAG) Update() error {

engine/drawdfs.go

@@ -13,7 +13,7 @@ var _ interface {
 // through the game tree, without any extra sorting based on Z values or
 // consideration for DrawOrderer).
 type DrawDFS struct {
-	Components
+	Child interface{}
 	Hides
 }
 
@@ -21,9 +21,7 @@ func (d *DrawDFS) Draw(screen *ebiten.Image, opts *ebiten.DrawImageOptions) {
 	if d.Hidden() {
 		return
 	}
-	for _, component := range d.Components {
+	d.draw(d.Child, screen, *opts)
-		d.draw(component, screen, *opts)
-	}
 }
 
 // exists to satisfy interface
@@ -53,3 +51,5 @@ func (d *DrawDFS) draw(component interface{}, screen *ebiten.Image, opts ebiten.
 		}
 	}
 }
+
+func (d *DrawDFS) Scan() []interface{} { return []interface{}{d.Child} }

engine/game.go

@@ -167,7 +167,7 @@ func (g *Game) Query(ancestor interface{}, behaviour reflect.Type) map[interface
 }
 
 // Scan returns g.Root.
-func (g *Game) Scan() Components { return Components{g.Root} }
+func (g *Game) Scan() []interface{} { return []interface{}{g.Root} }
 
 // PreorderWalk calls visit with every component and its parent, reachable from
 // the  given component via Scan, for as long as visit returns nil. The parent

engine/interface.go

@@ -147,7 +147,7 @@ type Saver interface {
 // (such as when the game component database is constructed).
 // Scan should return a slice containing all immediate subcomponents.
 type Scanner interface {
-	Scan() Components
+	Scan() []interface{}
 }
 
 // Transformer components can provide draw options to apply to themselves and

engine/misc.go

@@ -39,9 +39,3 @@ func (h *Hides) Hide() { *h = true }
 
 // Show sets h to false.
 func (h *Hides) Show() { *h = false }
-
-// Components implements Scan directly (as itself!)
-type Components []interface{}
-
-// Scan returns c.
-func (c Components) Scan() Components { return c }

engine/parallax.go

@@ -39,7 +39,7 @@ func (p *Parallax) Prepare(game *Game) error {
 }
 
 // Scan returns the child component.
-func (p *Parallax) Scan() Components { return Components{p.Child} }
+func (p *Parallax) Scan() []interface{} { return []interface{}{p.Child} }
 
 // Transform returns a GeoM translation of Factor * camera.Centre.
 func (p *Parallax) Transform() (opts ebiten.DrawImageOptions) {

engine/prisms.go

@@ -113,8 +113,8 @@ func (m *PrismMap) Prepare(g *Game) error {
 }
 
 // Scan returns the Sheet and all the Prisms.
-func (m *PrismMap) Scan() Components {
+func (m *PrismMap) Scan() []interface{} {
-	c := make(Components, 1, len(m.Map)+1)
+	c := make([]interface{}, 1, len(m.Map)+1)
 	c[0] = &m.Sheet
 	for _, prism := range m.Map {
 		c = append(c, prism)

engine/scene.go

@@ -21,7 +21,6 @@ type scener interface {
 	Disabler
 	Hider
 	Identifier
-	Registrar
 	Scanner
 }
 
@@ -30,30 +29,16 @@ func init() {
 	gob.Register(&SceneRef{})
 }
 
-// Scene gives an identity, bounds, and other properties to a collection of
+// Scene is a component for adding an identity, bounds, and other properties.
-// components.
 type Scene struct {
 	ID
 	Bounds // world coordinates
-	Components
+	Child  interface{}
 	Disables
 	Hides
 }
 
-func (s *Scene) Register(component, parent interface{}) error {
+func (s *Scene) Scan() []interface{} { return []interface{}{s.Child} }
-	if parent == s {
-		s.Components = append(s.Components, component)
-	}
-	return nil
-}
-
-func (s *Scene) Unregister(component interface{}) {
-	for i, c := range s.Components {
-		if c == component {
-			s.Components[i] = nil
-		}
-	}
-}
 
 // SceneRef loads a gzipped, gob-encoded Scene from the asset FS.
 // After Load, Scene is usable.

engine/sheet.go

@@ -47,7 +47,7 @@ func (s *Sheet) Prepare(*Game) error {
 }
 
 // Scan returns the Src.
-func (s *Sheet) Scan() Components { return Components{&s.Src} }
+func (s *Sheet) Scan() []interface{} { return []interface{}{&s.Src} }
 
 // SubImage returns an *ebiten.Image corresponding to the given cell index.
 func (s *Sheet) SubImage(i int) *ebiten.Image {

engine/sprite.go

@@ -41,8 +41,8 @@ func (s *Sprite) Draw(screen *ebiten.Image, opts *ebiten.DrawImageOptions) {
 }
 
 // Scan returns the Actor and the Sheet.
-func (s *Sprite) Scan() Components {
+func (s *Sprite) Scan() []interface{} {
-	return Components{
+	return []interface{}{
 		&s.Actor,
 		&s.Sheet,
 	}

engine/tiles.go

@@ -100,8 +100,8 @@ func (t *Tilemap) Load(fs.FS) error {
 }
 
 // Scan returns a slice containing Src and all non-nil tiles.
-func (t *Tilemap) Scan() Components {
+func (t *Tilemap) Scan() []interface{} {
-	c := make(Components, 1, len(t.Map)+1)
+	c := make([]interface{}, 1, len(t.Map)+1)
 	c[0] = &t.Sheet
 	for _, tile := range t.Map {
 		c = append(c, tile)
@@ -152,4 +152,4 @@ type AnimatedTile struct {
 func (a *AnimatedTile) Cell() int { return a.anim.Cell() }
 
 // Scan returns a.anim.
-func (a *AnimatedTile) Scan() Components { return Components{a.anim} }
+func (a *AnimatedTile) Scan() []interface{} { return []interface{}{a.anim} }

engine/walls.go

@@ -61,8 +61,8 @@ func (w *Wall) CollidesWith(b geom.Box) bool {
 }
 
 // Scan returns the Sheet and all WallUnits.
-func (w *Wall) Scan() Components {
+func (w *Wall) Scan() []interface{} {
-	c := make(Components, 1, len(w.Units)+1)
+	c := make([]interface{}, 1, len(w.Units)+1)
 	c[0] = &w.Sheet
 	for _, unit := range w.Units {
 		c = append(c, unit)
@@ -103,7 +103,7 @@ func (u *WallUnit) Draw(screen *ebiten.Image, opts *ebiten.DrawImageOptions) {
 }
 
 // Scan returns the Tile.
-func (u *WallUnit) Scan() Components { return Components{u.Tile} }
+func (u *WallUnit) Scan() []interface{} { return []interface{}{u.Tile} }
 
 func (u *WallUnit) Transform() (opts ebiten.DrawImageOptions) {
 	opts.GeoM.Translate(geom.CFloat(

game/aw.go

@@ -270,8 +270,8 @@ func (aw *Awakeman) Prepare(game *engine.Game) error {
 	return nil
 }
 
-func (aw *Awakeman) Scan() engine.Components {
+func (aw *Awakeman) Scan() []interface{} {
-	return engine.Components{&aw.Sprite}
+	return []interface{}{&aw.Sprite}
 }
 
 func (aw *Awakeman) String() string {

game/bubble.go

@@ -55,8 +55,8 @@ func NewBubble(pos geom.Int3) *Bubble {
 	}
 }
 
-func (b *Bubble) Scan() engine.Components {
+func (b *Bubble) Scan() []interface{} {
-	return engine.Components{&b.Sprite}
+	return []interface{}{&b.Sprite}
 }
 
 func (b *Bubble) String() string {

game/level1.go

@@ -12,7 +12,7 @@ func Level1() *engine.Scene {
 	return &engine.Scene{
 		ID:     "level_1",
 		Bounds: engine.Bounds(image.Rect(-32, -32, 320+32, 240+32)),
-		Components: engine.Components{
+		Child: &engine.Container{
 			&engine.Parallax{
 				CameraID: "game_camera",
 				Child: &engine.Billboard{

main.go

@@ -68,18 +68,16 @@ func main() {
 			Z: math.Sqrt(3),
 		},
 		Root: &engine.DrawDFS{
-			Components: engine.Components{
+			Child: &engine.Container{
 				&engine.Fill{
 					ID:    "bg_fill",
 					Color: color.Gray{100},
 				},
 				&engine.DrawDAG{
 					ChunkSize: 16,
-					Components: engine.Components{
+					Child: &engine.Camera{
-						&engine.Camera{
+						ID:    "game_camera",
-							ID:    "game_camera",
+						Child: lev1,
-							Child: lev1,
-						},
 					},
 				},
 				&engine.DebugToast{ID: "toast", Pos: image.Pt(0, 15)},