Composable z-position

engine/debug.go

@@ -2,6 +2,7 @@ package engine
 
 import (
 	"fmt"
+	"math"
 
 	"github.com/hajimehoshi/ebiten/v2"
 	"github.com/hajimehoshi/ebiten/v2/ebitenutil"
@@ -14,7 +15,7 @@ func (TPSDisplay) Draw(screen *ebiten.Image) {
 	ebitenutil.DebugPrint(screen, fmt.Sprintf("TPS: %0.2f", ebiten.CurrentTPS()))
 }
 
-func (TPSDisplay) DrawAfter(Drawer) bool {
-	// Always draw last
-	return true
+func (TPSDisplay) Z() float64 {
+	// Always draw on top
+	return math.MaxFloat64
 }

engine/game.go

@@ -15,7 +15,7 @@ type Updater interface {
 // DrawAfter is used to reorder components.
 type Drawer interface {
 	Draw(*ebiten.Image)
-	DrawAfter(Drawer) bool
+	Z() float64
 }
 
 // Game implements the ebiten methods using a collection of components.
@@ -52,13 +52,14 @@ func (g *Game) Layout(outsideWidth, outsideHeight int) (w, h int) {
 }
 
 // Sort sorts the components by draw order.
-// Non-Drawers are sorted first.
+// Non-Drawers are sorted before all Drawers.
 func (g *Game) Sort() {
-	sort.Slice(g.Components, func(i, j int) bool {
+	// SliceStable 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 b.DrawAfter(a)
+			return a.Z() < b.Z()
 		}
 		return !aok && bok
 	})

engine/tiles.go

@@ -0,0 +1,16 @@
+package engine
+
+import "github.com/hajimehoshi/ebiten/v2"
+
+type Tilemap struct {
+	Map      [][]int
+	Src      *ebiten.Image
+	TileSize int
+	GeoM     *ebiten.GeoM
+
+	ZPos
+}
+
+func (t *Tilemap) Draw(screen *ebiten.Image) {
+
+}

engine/z.go

@@ -0,0 +1,5 @@
+package engine
+
+type ZPos float64
+
+func (z ZPos) Z() float64 { return float64(z) }