Lazy loading with ref

engine/asset.go

@@ -1,6 +1,18 @@
 package engine
 
-import "github.com/hajimehoshi/ebiten/v2"
+import (
+	"fmt"
+	"image"
+	"io/fs"
+
+	"github.com/hajimehoshi/ebiten/v2"
+)
+
+var (
+	AssetFS fs.FS
+
+	imageCache = make(map[string]*ebiten.Image)
+)
 
 type ImageRef struct {
 	Path string
@@ -8,9 +20,24 @@ type ImageRef struct {
 	image *ebiten.Image
 }
 
-func (r *ImageRef) Image() *ebiten.Image {
-	if r.image == nil {
-		// TODO
+func (r *ImageRef) Image() (*ebiten.Image, error) {
+	if r.image != nil {
+		return r.image, nil
 	}
-	return r.image
+	r.image = imageCache[r.Path]
+	if r.image != nil {
+		return r.image, nil
+	}
+	f, err := AssetFS.Open(r.Path)
+	if err != nil {
+		return nil, fmt.Errorf("open asset: %w", err)
+	}
+	defer f.Close()
+	i, _, err := image.Decode(f)
+	if err != nil {
+		return nil, fmt.Errorf("decode asset: %w", err)
+	}
+	r.image = ebiten.NewImageFromImage(i)
+	imageCache[r.Path] = r.image
+	return r.image, nil
 }

engine/tiles.go

@@ -3,6 +3,7 @@ package engine
 import (
 	"encoding/gob"
 	"image"
+	"log"
 
 	"github.com/hajimehoshi/ebiten/v2"
 )
@@ -15,7 +16,7 @@ func init() {
 
 type Tilemap struct {
 	Map       [][]Tile
-	Src       *ebiten.Image // must be a horizontal tile set
+	Src       ImageRef // must be a horizontal tile set
 	TileSize  int
 	Transform ebiten.GeoM
 	ZPos
@@ -30,7 +31,11 @@ func (t *Tilemap) Draw(screen *ebiten.Image, geom ebiten.GeoM) {
 			op.GeoM.Concat(geom)
 
 			sx := tile.TileIndex() * t.TileSize
-			src := t.Src.SubImage(image.Rect(sx, 0, sx+t.TileSize, t.TileSize)).(*ebiten.Image)
+			im, err := t.Src.Image()
+			if err != nil {
+				log.Fatalf("Loading image from reference: %v", err)
+			}
+			src := im.SubImage(image.Rect(sx, 0, sx+t.TileSize, t.TileSize)).(*ebiten.Image)
 			screen.DrawImage(src, &op)
 		}
 	}

main.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"embed"
-	"image"
 	_ "image/png"
 	"log"
 
@@ -19,15 +18,7 @@ func main() {
 	ebiten.SetWindowSize(screenWidth*2, screenHeight*2)
 	ebiten.SetWindowTitle("TODO")
 
-	boxesFile, err := assets.Open("assets/boxes.png")
-	if err != nil {
-		log.Fatalf("Couldn't open asset: %v", err)
-	}
-	boxesPNG, _, err := image.Decode(boxesFile)
-	if err != nil {
-		log.Fatalf("Couldn't decode asset: %v", err)
-	}
-	boxesFile.Close()
+	engine.AssetFS = assets
 
 	staticTiles := [][]engine.StaticTile{
 		{0, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 1},
@@ -73,7 +64,7 @@ func main() {
 
 	tilemap := &engine.Tilemap{
 		Map:      tiles,
-		Src:      ebiten.NewImageFromImage(boxesPNG),
+		Src:      engine.ImageRef{Path: "assets/boxes.png"},
 		TileSize: 16,
 	}