restorable: Add an explicit way to detect context-lost

isTexture was used to detect context-lost and called every frame.
This was not good for performance.

This change adds a way to notify context-lost from the WebGL
handlers directly, and the package restorable uses it instead of
calling (*Image).isInvalaidated.

Fixes #1175

internal/graphicsdriver/opengl/context_js.go

@@ -309,9 +309,8 @@ func (c *context) deleteTexture(t textureNative) {
 }
 
 func (c *context) isTexture(t textureNative) bool {
-	c.ensureGL()
-	gl := c.gl
-	return gl.Call("isTexture", js.Value(t)).Bool()
+	// isTexture should not be called to detect context-lost since this performance is not good (#1175).
+	panic("opengl: isTexture is not implemented")
 }
 
 func (c *context) newFramebuffer(t textureNative) (framebufferNative, error) {

internal/restorable/images.go

@@ -39,9 +39,10 @@ func EnableRestoringForTesting() {
 
 // images is a set of Image objects.
 type images struct {
-	images     map[*Image]struct{}
-	shaders    map[*Shader]struct{}
-	lastTarget *Image
+	images      map[*Image]struct{}
+	shaders     map[*Shader]struct{}
+	lastTarget  *Image
+	contextLost bool
 }
 
 // theImages represents the images for the current process.
@@ -73,21 +74,27 @@ func RestoreIfNeeded() error {
 	}
 
 	if !forceRestoring {
-		r := false
-		// As isInvalidated() is expensive, call this only for one image.
-		// This assumes that if there is one image that is invalidated, all images are invalidated.
-		for img := range theImages.images {
-			// The screen image might not have a texture. Skip this.
-			if img.screen {
-				continue
+		var r bool
+
+		if canDetectContextLostExplicitly {
+			r = theImages.contextLost
+		} else {
+			// As isInvalidated() is expensive, call this only for one image.
+			// This assumes that if there is one image that is invalidated, all images are invalidated.
+			for img := range theImages.images {
+				// The screen image might not have a texture. Skip this.
+				if img.screen {
+					continue
+				}
+				var err error
+				r, err = img.isInvalidated()
+				if err != nil {
+					return err
+				}
+				break
 			}
-			var err error
-			r, err = img.isInvalidated()
-			if err != nil {
-				return err
-			}
-			break
 		}
+
 		if !r {
 			return nil
 		}
@@ -257,6 +264,9 @@ func (i *images) restore() error {
 			return err
 		}
 	}
+
+	i.contextLost = false
+
 	return nil
 }
 
@@ -264,3 +274,11 @@ func (i *images) restore() error {
 func InitializeGraphicsDriverState() error {
 	return graphicscommand.ResetGraphicsDriverState()
 }
+
+// OnContextLost is called when the context lost is detected in an explicit way.
+func OnContextLost() {
+	if !canDetectContextLostExplicitly {
+		panic("restorable: OnContextLost cannot be called in this environment")
+	}
+	theImages.contextLost = true
+}

internal/restorable/js.go

@@ -16,7 +16,11 @@
 
 package restorable
 
+// needsDisposingWhenRestoring reports whether disposing resources is necessary or not when restoring.
+//
 // On browsers, disposing resources is not required since the objects are already managed by JavaScript GC and they
 // are already invalidated. Rather, disposing them when restoring causes warnings on the console.
-
 const needsDisposingWhenRestoring = false
+
+// canDetectContextLostExplicitly reports whether the context lost can be detected by handlers in an explicit way.
+const canDetectContextLostExplicitly = true

internal/restorable/notjs.go

@@ -17,3 +17,5 @@
 package restorable
 
 const needsDisposingWhenRestoring = true
+
+const canDetectContextLostExplicitly = false

internal/uidriver/js/ui.go

@@ -27,6 +27,7 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/graphicsdriver/opengl"
 	"github.com/hajimehoshi/ebiten/internal/hooks"
 	"github.com/hajimehoshi/ebiten/internal/jsutil"
+	"github.com/hajimehoshi/ebiten/internal/restorable"
 )
 
 type UserInterface struct {
@@ -400,6 +401,7 @@ func init() {
 		e := args[0]
 		e.Call("preventDefault")
 		theUI.contextLost = true
+		restorable.OnContextLost()
 		return nil
 	}))
 	canvas.Call("addEventListener", "webglcontextrestored", js.FuncOf(func(this js.Value, args []js.Value) interface{} {