DrawLayer -> DrawManager, iface assertions, etc

engine/drawdag.go

@@ -9,6 +9,15 @@ import (
 	"github.com/hajimehoshi/ebiten/v2"
 )
 
+var _ interface {
+	Drawer
+	DrawManager
+	Hider
+	Prepper
+	Scanner
+	Updater
+} = &DrawDAG{}
+
 // DrawDAG is a DrawLayer that organises DrawBoxer descendants in a directed
 // acyclic graph (DAG), in order to draw them according to ordering constraints.
 // It combines a DAG with a spatial index used when updating vertices to reduce
@@ -26,7 +35,7 @@ type DrawDAG struct {
 }
 
 // Draw draws everything in the DAG in topological order.
-func (d *DrawDAG) DrawAll(screen *ebiten.Image, opts *ebiten.DrawImageOptions) {
+func (d *DrawDAG) Draw(screen *ebiten.Image, opts *ebiten.DrawImageOptions) {
 	if d.Hidden() {
 		return
 	}
@@ -89,6 +98,9 @@ func (d *DrawDAG) DrawAll(screen *ebiten.Image, opts *ebiten.DrawImageOptions) {
 	})
 }
 
+// exists to satisfy interface
+func (DrawDAG) ManagesDrawingSubcomponents() {}
+
 // Prepare adds all subcomponents to the DAG.
 func (d *DrawDAG) Prepare(game *Game) error {
 	d.dag = newDAG()

engine/drawdfs.go

@@ -2,6 +2,12 @@ package engine
 
 import "github.com/hajimehoshi/ebiten/v2"
 
+var _ interface {
+	Drawer
+	DrawManager
+	Scanner
+} = &DrawDFS{}
+
 // DrawDFS is a DrawLayer that does not add any structure. Components are
 // drawn in the order in which they are encountered by a depth-first search
 // through the game tree, without any extra sorting based on Z values or
@@ -11,7 +17,7 @@ type DrawDFS struct {
 	Hides
 }
 
-func (d *DrawDFS) DrawAll(screen *ebiten.Image, opts *ebiten.DrawImageOptions) {
+func (d *DrawDFS) Draw(screen *ebiten.Image, opts *ebiten.DrawImageOptions) {
 	if d.Hidden() {
 		return
 	}
@@ -20,6 +26,9 @@ func (d *DrawDFS) DrawAll(screen *ebiten.Image, opts *ebiten.DrawImageOptions) {
 	}
 }
 
+// exists to satisfy interface
+func (DrawDFS) ManagesDrawingSubcomponents() {}
+
 func (d *DrawDFS) draw(component interface{}, screen *ebiten.Image, opts ebiten.DrawImageOptions) {
 	// Hidden? stop drawing
 	if h, ok := component.(Hider); ok && h.Hidden() {
@@ -29,16 +38,14 @@ func (d *DrawDFS) draw(component interface{}, screen *ebiten.Image, opts ebiten.
 	if tf, ok := component.(Transformer); ok {
 		opts = concatOpts(tf.Transform(), opts)
 	}
-	// Is it a DrawLayer? draw all and return
-	if dl, ok := component.(DrawLayer); ok {
-		dl.DrawAll(screen, &opts)
-		return
-	}
-	// Not a draw layer.
 	// Does it draw itself? Draw
 	if dr, ok := component.(Drawer); ok {
 		dr.Draw(screen, &opts)
 	}
+	// Is it a DrawManager? return early (don't recurse)
+	if _, ok := component.(DrawManager); ok {
+		return
+	}
 	// Has subcomponents? recurse
 	if sc, ok := component.(Scanner); ok {
 		for _, ch := range sc.Scan() {

engine/game.go

@@ -39,7 +39,7 @@ type Game struct {
 	Disables
 	Hides
 	ScreenSize image.Point
-	Roots      []DrawLayer
+	Root       Drawer // usually a DrawManager
 	Projection geom.Projector
 	VoxelScale geom.Float3
 
@@ -54,12 +54,7 @@ func (g *Game) Draw(screen *ebiten.Image) {
 	if g.Hidden() {
 		return
 	}
-
-	// Make all draw managers draw, in order.
-	opts := &ebiten.DrawImageOptions{}
-	for _, dm := range g.Roots {
-		dm.DrawAll(screen, opts)
-	}
+	g.Root.Draw(screen, &ebiten.DrawImageOptions{})
 }
 
 // Layout returns the configured screen width/height.
@@ -158,14 +153,8 @@ func (g *Game) Query(ancestor interface{}, behaviour reflect.Type) map[interface
 	return g.byAB[abKey{ancestor, behaviour}]
 }
 
-// Scan implements Scanner.
-func (g *Game) Scan() []interface{} {
-	rs := make([]interface{}, 0, len(g.Roots))
-	for _, r := range g.Roots {
-		rs = append(rs, r)
-	}
-	return rs
-}
+// Scan returns 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

@@ -17,7 +17,7 @@ var (
 	BoundingBoxerType  = reflect.TypeOf((*BoundingBoxer)(nil)).Elem()
 	ColliderType       = reflect.TypeOf((*Collider)(nil)).Elem()
 	DisablerType       = reflect.TypeOf((*Disabler)(nil)).Elem()
-	DrawLayerType      = reflect.TypeOf((*DrawLayer)(nil)).Elem()
+	DrawLayerType      = reflect.TypeOf((*DrawManager)(nil)).Elem()
 	DrawerType         = reflect.TypeOf((*Drawer)(nil)).Elem()
 	DrawBoxerType      = reflect.TypeOf((*DrawBoxer)(nil)).Elem()
 	DrawOrdererType    = reflect.TypeOf((*DrawOrderer)(nil)).Elem()
@@ -73,11 +73,11 @@ type Disabler interface {
 	Enable()
 }
 
-// DrawLayer is a component responsible for calling Draw on all Drawer
-// components beneath it, except those beneath another DrawLayer (it calls
-// DrawAll on those).
-type DrawLayer interface {
-	DrawAll(*ebiten.Image, *ebiten.DrawImageOptions)
+// DrawManager is a component responsible for calling Draw on all Drawer
+// components beneath it, except those beneath another DrawManager (it might
+// call Draw on the DrawManager, but that's it).
+type DrawManager interface {
+	ManagesDrawingSubcomponents()
 }
 
 // Drawer components can draw themselves. Draw is called often. Draw is not

main.go

@@ -67,8 +67,7 @@ func main() {
 			Y: 1,
 			Z: math.Sqrt(3),
 		},
-		Roots: []engine.DrawLayer{
-			&engine.DrawDFS{
+		Root: &engine.DrawDFS{
 			Components: []interface{}{
 				&engine.Fill{
 					ID:    "bg_fill",
@@ -87,7 +86,6 @@ func main() {
 				engine.PerfDisplay{},
 			},
 		},
-		},
 	}
 	if err := g.LoadAndPrepare(game.Assets); err != nil {
 		log.Fatalf("Loading/preparing error: %v", err)