internal/graphicsdriver/metal: refactoring: initialize the driver at Get

internal/graphicsdriver/metal/graphics_darwin.go

@@ -19,6 +19,7 @@ import (
 	"math"
 	"sort"
 	"strings"
+	"sync"
 	"unsafe"
 
 	"github.com/hajimehoshi/ebiten/v2/internal/graphics"
@@ -346,28 +347,31 @@ const (
 	noStencil
 )
 
-// isMetalAvailable reports whether Metal is available or not.
+var (
-var isMetalAvailable bool
+	// isMetalAvailable reports whether Metal is available or not.
-
+	isMetalAvailable     bool
-func init() {
+	isMetalAvailableOnce sync.Once
-	if !supportsMetal() {
+)
-		return
-	}
-
-	// Initialize isMetalAvailable on the main thread.
-	// On old mac devices like iMac 2011, Metal is not supported (#779).
-	// TODO: Is there a better way to check whether Metal is available or not?
-	// It seems OK to call MTLCreateSystemDefaultDevice multiple times, so this should be fine.
-	if _, ok := mtl.CreateSystemDefaultDevice(); !ok {
-		return
-	}
-
-	isMetalAvailable = true
-}
 
 var theGraphics Graphics
 
 func Get() *Graphics {
+	isMetalAvailableOnce.Do(func() {
+		if !supportsMetal() {
+			return
+		}
+
+		// Initialize isMetalAvailable on the main thread.
+		// TODO: Now ui.chooseGraphicsDriver is called on the main thread. Add an assertion.
+
+		// On old mac devices like iMac 2011, Metal is not supported (#779).
+		// TODO: Is there a better way to check whether Metal is available or not?
+		// It seems OK to call MTLCreateSystemDefaultDevice multiple times, so this should be fine.
+		if _, ok := mtl.CreateSystemDefaultDevice(); !ok {
+			return
+		}
+		isMetalAvailable = true
+	})
 	if !isMetalAvailable {
 		return nil
 	}