WIP: Scan now uses visitor pattern

engine/billboard.go

@@ -52,7 +52,10 @@ func (b *Billboard) Prepare(g *Game) error {
 }
 
 // Scan returns a slice containing Src.
-func (b *Billboard) Scan() []interface{} { return []interface{}{&b.Src} }
+//func (b *Billboard) Scan() []interface{} { return []interface{}{&b.Src} }
+func (b *Billboard) Scan(visit func(interface{}) error) error {
+	return visit(&b.Src)
+}
 
 func (b *Billboard) String() string {
 	return fmt.Sprintf("Billboard@%v", b.Pos)

engine/camera.go

@@ -86,7 +86,10 @@ func (c *Camera) Prepare(game *Game) error {
 }
 
 // Scan returns s.Child.
-func (c *Camera) Scan() []interface{} { return []interface{}{c.Child} }
+//func (c *Camera) Scan() []interface{} { return []interface{}{c.Child} }
+func (c *Camera) Scan(visit func(interface{}) error) error {
+	return visit(c.Child)
+}
 
 // Transform returns the camera transform.
 func (c *Camera) Transform() (opts ebiten.DrawImageOptions) {

engine/container.go

@@ -1,10 +1,23 @@
 package engine
 
+var _ interface {
+	Registrar
+	Scanner
+} = &Container{}
+
 // Container contains many components.
 type Container []interface{}
 
 // Scan returns c.
-func (c Container) Scan() []interface{} { return c }
+//func (c Container) Scan() []interface{} { return c }
+func (c Container) Scan(visit func(interface{}) error) error {
+	for _, x := range c {
+		if err := visit(x); err != nil {
+			return err
+		}
+	}
+	return nil
+}
 
 // Register records component in the slice, if parent is the container.
 func (c *Container) Register(component, parent interface{}) error {

engine/drawdag.go

@@ -114,7 +114,10 @@ func (d *DrawDAG) Prepare(game *Game) error {
 	return d.Register(d, nil)
 }
 
-func (d *DrawDAG) Scan() []interface{} { return []interface{}{d.Child} }
+//func (d *DrawDAG) Scan() []interface{} { return []interface{}{d.Child} }
+func (d *DrawDAG) Scan(visit func(interface{}) error) error {
+	return visit(d.Child)
+}
 
 // Update checks for any changes to descendants, and updates its internal
 // data structures accordingly.
@@ -160,11 +163,15 @@ func (d *DrawDAG) Register(component, _ interface{}) error {
 		return nil
 	}
 	if sc, ok := component.(Scanner); ok {
-		for _, x := range sc.Scan() {
+		/*for _, x := range sc.Scan() {
 			if err := d.Register(x, nil); err != nil {
 				return err
 			}
+		}*/
+		scv := func(x interface{}) error {
+			return d.Register(x, nil)
 		}
+		return sc.Scan(scv)
 	}
 	return nil
 }
@@ -232,9 +239,14 @@ func (d *DrawDAG) Unregister(component interface{}) {
 		return
 	}
 	if sc, ok := component.(Scanner); ok {
-		for _, x := range sc.Scan() {
+		/*for _, x := range sc.Scan() {
 			d.Unregister(x)
+		}*/
+		scv := func(x interface{}) error {
+			d.Unregister(x)
+			return nil
 		}
+		sc.Scan(scv)
 	}
 }
 

engine/drawdfs.go

@@ -46,10 +46,18 @@ func (d *DrawDFS) draw(component interface{}, screen *ebiten.Image, opts ebiten.
 	}
 	// Has subcomponents? recurse
 	if sc, ok := component.(Scanner); ok {
-		for _, ch := range sc.Scan() {
+		/*for _, ch := range sc.Scan() {
 			d.draw(ch, screen, opts)
+		}*/
+		scv := func(x interface{}) error {
+			d.draw(x, screen, opts)
+			return nil
 		}
+		sc.Scan(scv)
 	}
 }
 
-func (d *DrawDFS) Scan() []interface{} { return []interface{}{d.Child} }
+//func (d *DrawDFS) Scan() []interface{} { return []interface{}{d.Child} }
+func (d *DrawDFS) Scan(visit func(interface{}) error) error {
+	return visit(d.Child)
+}

engine/game.go

@@ -75,11 +75,15 @@ func (g *Game) updateRecursive(c interface{}) error {
 		return nil
 	}
 	if sc, ok := c.(Scanner); ok {
-		for _, x := range sc.Scan() {
+		/*for _, x := range sc.Scan() {
 			if err := g.updateRecursive(x); err != nil {
 				return err
 			}
 		}
+		*/
+		if err := sc.Scan(g.updateRecursive); err != nil {
+			return err
+		}
 	}
 	if c == g { // prevent infinite recursion
 		return nil
@@ -167,7 +171,10 @@ func (g *Game) Query(ancestor interface{}, behaviour reflect.Type) map[interface
 }
 
 // Scan returns g.Root.
-func (g *Game) Scan() []interface{} { return []interface{}{g.Root} }
+//func (g *Game) Scan() []interface{} { return []interface{}{g.Root} }
+func (g *Game) Scan(visit func(interface{}) error) error {
+	return visit(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
@@ -185,12 +192,15 @@ func preorderWalk(component, parent interface{}, visit func(component, parent in
 	if !ok {
 		return nil
 	}
-	for _, c := range sc.Scan() {
+	/*for _, c := range sc.Scan() {
 		if err := preorderWalk(c, component, visit); err != nil {
 			return err
 		}
+	}*/
+	scv := func(c interface{}) error {
+		return preorderWalk(c, component, visit)
 	}
-	return nil
+	return sc.Scan(scv)
 }
 
 // PostorderWalk calls visit with every component and its parent, reachable from
@@ -203,10 +213,16 @@ func PostorderWalk(component interface{}, visit func(component, parent interface
 
 func postorderWalk(component, parent interface{}, visit func(component, parent interface{}) error) error {
 	if sc, ok := component.(Scanner); ok {
-		for _, c := range sc.Scan() {
+		scv := func(c interface{}) error {
+			return postorderWalk(c, component, visit)
+		}
+		/*for _, c := range sc.Scan() {
 			if err := postorderWalk(c, component, visit); err != nil {
 				return err
 			}
+		}*/
+		if err := sc.Scan(scv); err != nil {
+			return err
 		}
 	}
 	return visit(component, parent)

engine/interface.go

@@ -144,10 +144,12 @@ type Saver interface {
 }
 
 // Scanner components can be scanned. It is called when the game tree is walked
-// (such as when the game component database is constructed).
-// Scan should return a slice containing all immediate subcomponents.
+// (such as when the game component database is constructed) and various times
+// afterward (such as during Update).
+// Scan should visit each immediate subcomponent, aborting early if an error is
+// returned from the visitor.
 type Scanner interface {
-	Scan() []interface{}
+	Scan(visit func(interface{}) error) error
 }
 
 // Transformer components can provide draw options to apply to themselves and

engine/parallax.go

@@ -39,7 +39,10 @@ func (p *Parallax) Prepare(game *Game) error {
 }
 
 // Scan returns the child component.
-func (p *Parallax) Scan() []interface{} { return []interface{}{p.Child} }
+//func (p *Parallax) Scan() []interface{} { return []interface{}{p.Child} }
+func (p *Parallax) Scan(visit func(interface{}) error) error {
+	return visit(p.Child)
+}
 
 // Transform returns a GeoM translation of Factor * camera.Centre.
 func (p *Parallax) Transform() (opts ebiten.DrawImageOptions) {

engine/prisms.go

@@ -113,13 +113,24 @@ func (m *PrismMap) Prepare(g *Game) error {
 }
 
 // Scan returns the Sheet and all the Prisms.
-func (m *PrismMap) Scan() []interface{} {
+/*func (m *PrismMap) Scan() []interface{} {
 	c := make([]interface{}, 1, len(m.Map)+1)
 	c[0] = &m.Sheet
 	for _, prism := range m.Map {
 		c = append(c, prism)
 	}
 	return c
+}*/
+func (m *PrismMap) Scan(visit func(interface{}) error) error {
+	if err := visit(&m.Sheet); err != nil {
+		return err
+	}
+	for _, prism := range m.Map {
+		if err := visit(prism); err != nil {
+			return err
+		}
+	}
+	return nil
 }
 
 // Transform retrurns a translation by the draw offset.

engine/scene.go

@@ -38,7 +38,10 @@ type Scene struct {
 	Hides
 }
 
-func (s *Scene) Scan() []interface{} { return []interface{}{s.Child} }
+//func (s *Scene) Scan() []interface{} { return []interface{}{s.Child} }
+func (s *Scene) Scan(visit func(interface{}) error) error {
+	return visit(s.Child)
+}
 
 // SceneRef loads a gzipped, gob-encoded Scene from the asset FS.
 // After Load, Scene is usable.

engine/sheet.go

@@ -47,7 +47,10 @@ func (s *Sheet) Prepare(*Game) error {
 }
 
 // Scan returns the Src.
-func (s *Sheet) Scan() []interface{} { return []interface{}{&s.Src} }
+//func (s *Sheet) Scan() []interface{} { return []interface{}{&s.Src} }
+func (s *Sheet) Scan(visit func(interface{}) error) error {
+	return visit(&s.Src)
+}
 
 // SubImage returns an *ebiten.Image corresponding to the given cell index.
 func (s *Sheet) SubImage(i int) *ebiten.Image {

engine/sprite.go

@@ -41,11 +41,17 @@ func (s *Sprite) Draw(screen *ebiten.Image, opts *ebiten.DrawImageOptions) {
 }
 
 // Scan returns the Actor and the Sheet.
-func (s *Sprite) Scan() []interface{} {
+/*func (s *Sprite) Scan() []interface{} {
 	return []interface{}{
 		&s.Actor,
 		&s.Sheet,
 	}
+}*/
+func (s *Sprite) Scan(visit func(interface{}) error) error {
+	if err := visit(&s.Actor); err != nil {
+		return err
+	}
+	return visit(&s.Sheet)
 }
 
 // SetAnim sets the Anim to use for the sprite. If it is not the same as the

engine/tiles.go

@@ -100,13 +100,24 @@ func (t *Tilemap) Load(fs.FS) error {
 }
 
 // Scan returns a slice containing Src and all non-nil tiles.
-func (t *Tilemap) Scan() []interface{} {
+/*func (t *Tilemap) Scan() []interface{} {
 	c := make([]interface{}, 1, len(t.Map)+1)
 	c[0] = &t.Sheet
 	for _, tile := range t.Map {
 		c = append(c, tile)
 	}
 	return c
+}*/
+func (t *Tilemap) Scan(visit func(interface{}) error) error {
+	if err := visit(&t.Sheet); err != nil {
+		return err
+	}
+	for _, tile := range t.Map {
+		if err := visit(tile); err != nil {
+			return err
+		}
+	}
+	return nil
 }
 
 // Transform returns a translation by t.Offset.
@@ -152,4 +163,7 @@ type AnimatedTile struct {
 func (a *AnimatedTile) Cell() int { return a.anim.Cell() }
 
 // Scan returns a.anim.
-func (a *AnimatedTile) Scan() []interface{} { return []interface{}{a.anim} }
+//func (a *AnimatedTile) Scan() []interface{} { return []interface{}{a.anim} }
+func (a *AnimatedTile) Scan(visit func(interface{}) error) error {
+	return visit(a.anim)
+}

engine/walls.go

@@ -61,6 +61,7 @@ func (w *Wall) CollidesWith(b geom.Box) bool {
 }
 
 // Scan returns the Sheet and all WallUnits.
+/*
 func (w *Wall) Scan() []interface{} {
 	c := make([]interface{}, 1, len(w.Units)+1)
 	c[0] = &w.Sheet
@@ -69,6 +70,18 @@ func (w *Wall) Scan() []interface{} {
 	}
 	return c
 }
+*/
+func (w *Wall) Scan(visit func(interface{}) error) error {
+	if err := visit(&w.Sheet); err != nil {
+		return err
+	}
+	for _, unit := range w.Units {
+		if err := visit(unit); err != nil {
+			return err
+		}
+	}
+	return nil
+}
 
 // Prepare makes sure all WallUnits know about Wall and where they are, for
 // drawing.
@@ -103,7 +116,10 @@ func (u *WallUnit) Draw(screen *ebiten.Image, opts *ebiten.DrawImageOptions) {
 }
 
 // Scan returns the Tile.
-func (u *WallUnit) Scan() []interface{} { return []interface{}{u.Tile} }
+//func (u *WallUnit) Scan() []interface{} { return []interface{}{u.Tile} }
+func (u *WallUnit) Scan(visit func(interface{}) error) error {
+	return visit(u.Tile)
+}
 
 func (u *WallUnit) Transform() (opts ebiten.DrawImageOptions) {
 	opts.GeoM.Translate(geom.CFloat(

game/aw.go

@@ -270,8 +270,9 @@ func (aw *Awakeman) Prepare(game *engine.Game) error {
 	return nil
 }
 
-func (aw *Awakeman) Scan() []interface{} {
-	return []interface{}{&aw.Sprite}
+//func (aw *Awakeman) Scan() []interface{} { return []interface{}{&aw.Sprite} }
+func (aw *Awakeman) Scan(visit func(interface{}) error) error {
+	return visit(&aw.Sprite)
 }
 
 func (aw *Awakeman) String() string {

game/bubble.go

@@ -55,8 +55,9 @@ func NewBubble(pos geom.Int3) *Bubble {
 	}
 }
 
-func (b *Bubble) Scan() []interface{} {
-	return []interface{}{&b.Sprite}
+//func (b *Bubble) Scan() []interface{} { return []interface{}{&b.Sprite} }
+func (b *Bubble) Scan(visit func(interface{}) error) error {
+	return visit(&b.Sprite)
 }
 
 func (b *Bubble) String() string {