ui: Implement restoring context lost on browsers correctly

Fixes #734

examples/contextlost/main.go

@@ -23,6 +23,7 @@ import (
 	_ "image/jpeg"
 	"log"
 	"math"
+	"time"
 
 	"github.com/gopherjs/gopherwasm/js"
 
@@ -41,8 +42,39 @@ var (
 	count        = 0
 	gophersImage *ebiten.Image
 	extraImages  []*ebiten.Image
+	lost         = false
 )
 
+func loseAndRestoreContext(context js.Value) {
+	if lost {
+		return
+	}
+
+	// Edge might not support the extension. See
+	// https://developer.mozilla.org/en-US/docs/Web/API/WEBGL_lose_context
+	ext := context.Call("getExtension", "WEBGL_lose_context")
+	if ext == js.Null() {
+		fmt.Println("Fail to force context lost. Edge might not support the extension yet.")
+		return
+	}
+
+	ext.Call("loseContext")
+	fmt.Println("Lost the context!")
+	fmt.Println("The context is automatically restored after 3 seconds.")
+	lost = true
+
+	// If and only if the context is lost by loseContext, you need to call restoreContext. Note that in usual
+	// case of context lost, you cannot call restoreContext but the context should be restored automatically.
+	//
+	// After the context is lost, update will not be called. Instead, fire the goroutine to restore the context.
+	go func() {
+		time.Sleep(3 * time.Second)
+		ext.Call("restoreContext")
+		fmt.Println("Restored the context!")
+		lost = false
+	}()
+}
+
 func update(screen *ebiten.Image) error {
 	if inpututil.IsKeyJustPressed(ebiten.KeySpace) && js.Global() != js.Null() {
 		doc := js.Global().Get("document")
@@ -51,14 +83,8 @@ func update(screen *ebiten.Image) error {
 		if context == js.Null() {
 			context = canvas.Call("getContext", "experimental-webgl")
 		}
-		// Edge might not support the extension. See
-		// https://developer.mozilla.org/en-US/docs/Web/API/WEBGL_lose_context
-		if ext := context.Call("getExtension", "WEBGL_lose_context"); ext != js.Null() {
-			ext.Call("loseContext")
-			fmt.Println("Context Lost!")
-		} else {
-			fmt.Println("Fail to force context lost. Edge might not support the extension yet.")
-		}
+
+		loseAndRestoreContext(context)
 		return nil
 	}
 
@@ -75,7 +101,7 @@ func update(screen *ebiten.Image) error {
 	op.GeoM.Translate(screenWidth/2, screenHeight/2)
 	screen.DrawImage(gophersImage, op)
 
-	ebitenutil.DebugPrint(screen, "Press Space to force GL context lost!\n(Browser only)")
+	ebitenutil.DebugPrint(screen, "Press Space to force to lose/restore the GL context!\n(Browser only)")
 
 	return nil
 }

graphicscontext.go

@@ -23,7 +23,6 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/hooks"
 	"github.com/hajimehoshi/ebiten/internal/shareable"
 	"github.com/hajimehoshi/ebiten/internal/ui"
-	"github.com/hajimehoshi/ebiten/internal/web"
 )
 
 func newGraphicsContext(f func(*Image) error) *graphicsContext {
@@ -155,9 +154,6 @@ func (c *graphicsContext) Update(afterFrameUpdate func()) error {
 }
 
 func (c *graphicsContext) needsRestoring() (bool, error) {
-	if web.IsBrowser() {
-		return c.invalidated, nil
-	}
 	return c.offscreen.mipmap.original().IsInvalidated()
 }
 

internal/graphicsdriver/opengl/context_js.go

@@ -90,7 +90,6 @@ var (
 
 type contextImpl struct {
 	gl            js.Value
-	loseContext   js.Value
 	lastProgramID programID
 }
 
@@ -116,10 +115,6 @@ func (c *context) ensureGL() {
 	}
 
 	c.gl = gl
-
-	// Getting an extension might fail after the context is lost, so
-	// it is required to get the extension here.
-	c.loseContext = gl.Call("getExtension", "WEBGL_lose_context")
 }
 
 func (c *context) reset() error {
@@ -130,7 +125,11 @@ func (c *context) reset() error {
 	c.lastViewportHeight = 0
 	c.lastCompositeMode = graphics.CompositeModeUnknown
 
+	c.gl = js.Value{}
 	c.ensureGL()
+	if c.gl.Call("isContextLost").Bool() {
+		return fmt.Errorf("opengl: the context is lost")
+	}
 	gl := c.gl
 	gl.Call("enable", blend)
 	c.blendFunc(graphics.CompositeModeSourceOver)
@@ -469,15 +468,3 @@ func (c *context) flush() {
 	gl := c.gl
 	gl.Call("flush")
 }
-
-func (c *context) isContextLost() bool {
-	c.ensureGL()
-	gl := c.gl
-	return gl.Call("isContextLost").Bool()
-}
-
-func (c *context) restoreContext() {
-	if c.loseContext != js.Null() {
-		c.loseContext.Call("restoreContext")
-	}
-}

internal/graphicsdriver/opengl/driver_js.go

@@ -1,25 +0,0 @@
-// Copyright 2018 The Ebiten Authors
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-// +build js
-
-package opengl
-
-func (d *Driver) IsContextLost() bool {
-	return d.context.isContextLost()
-}
-
-func (d *Driver) RestoreContext() {
-	d.context.restoreContext()
-}

internal/ui/ui_js.go

@@ -25,7 +25,6 @@ import (
 	"github.com/gopherjs/gopherwasm/js"
 
 	"github.com/hajimehoshi/ebiten/internal/devicescale"
-	"github.com/hajimehoshi/ebiten/internal/graphicsdriver/opengl"
 	"github.com/hajimehoshi/ebiten/internal/hooks"
 	"github.com/hajimehoshi/ebiten/internal/input"
 )
@@ -43,6 +42,7 @@ type userInterface struct {
 	sizeChanged bool
 	windowFocus bool
 	pageVisible bool
+	contextLost bool
 
 	lastActualScale float64
 }
@@ -216,15 +216,6 @@ func (u *userInterface) update(g GraphicsContext) error {
 	}
 	hooks.ResumeAudio()
 
-	if opengl.Get().IsContextLost() {
-		opengl.Get().RestoreContext()
-		g.Invalidate()
-
-		// Need to return once to wait restored (#526)
-		// TODO: Is it necessary to handle webglcontextrestored event?
-		return nil
-	}
-
 	input.Get().UpdateGamepads()
 	u.updateGraphicsContext(g)
 	if err := g.Update(func() {
@@ -241,6 +232,11 @@ func (u *userInterface) loop(g GraphicsContext) <-chan error {
 	ch := make(chan error)
 	var cf js.Callback
 	f := func([]js.Value) {
+		if u.contextLost {
+			requestAnimationFrame.Invoke(cf)
+			return
+		}
+
 		if err := u.update(g); err != nil {
 			ch <- err
 			close(ch)
@@ -359,11 +355,12 @@ func init() {
 		// Do nothing.
 	}))
 
+	// Context
 	canvas.Call("addEventListener", "webglcontextlost", js.NewEventCallback(js.PreventDefault, func(js.Value) {
-		// Do nothing.
+		currentUI.contextLost = true
 	}))
 	canvas.Call("addEventListener", "webglcontextrestored", js.NewCallback(func(e []js.Value) {
-		// Do nothing.
+		currentUI.contextLost = false
 	}))
 }