internal/ui: refactoring

internal/ui/context.go

@@ -59,13 +59,13 @@ type context struct {
 
 	setContextOnce sync.Once
 
-	deferred  []func()
-	deferredM sync.Mutex
+	funcsInFrameCh chan func()
 }
 
 func newContext(game Game) *context {
 	return &context{
-		game: game,
+		game:           game,
+		funcsInFrameCh: make(chan func()),
 	}
 }
 
@@ -118,8 +118,8 @@ func (c *context) updateFrameImpl(graphicsDriver graphicsdriver.Graphics, update
 	}()
 
 	// Flush deferred functions, like reading pixels from GPU.
-	if err := c.flushDeferredFuncs(ui); err != nil {
-		return err
+	if err := c.processFuncsInFrame(ui); err != nil {
+		return nil
 	}
 
 	// ForceUpdate can be invoked even if the context is not initialized yet (#1591).
@@ -297,30 +297,31 @@ func (u *UserInterface) LogicalPositionToClientPosition(x, y float64) (float64,
 	return u.context.logicalPositionToClientPosition(x, y, u.DeviceScaleFactor())
 }
 
-// appendDeferredFunc appends a function.
-// An appended function is called at the next frame.
-func (c *context) appendDeferredFunc(f func()) {
-	c.deferredM.Lock()
-	defer c.deferredM.Unlock()
-	c.deferred = append(c.deferred, f)
-}
-
-func (c *context) flushDeferredFuncs(ui *UserInterface) error {
-	c.deferredM.Lock()
-	fs := c.deferred
-	c.deferred = nil
-	c.deferredM.Unlock()
-
-	for _, f := range fs {
+func (c *context) runInFrame(f func()) {
+	ch := make(chan struct{})
+	c.funcsInFrameCh <- func() {
+		defer close(ch)
 		f()
 	}
+	<-ch
+	return
+}
 
-	if len(fs) > 0 {
-		// Catch the error that happened at (*Image).At.
-		if err := ui.error(); err != nil {
-			return err
+func (c *context) processFuncsInFrame(ui *UserInterface) error {
+	var processed bool
+	for {
+		select {
+		case f := <-c.funcsInFrameCh:
+			f()
+			processed = true
+		default:
+			if processed {
+				// Catch the error that happened at (*Image).At.
+				if err := ui.error(); err != nil {
+					return err
+				}
+			}
+			return nil
 		}
 	}
-
-	return nil
 }

internal/ui/ui.go

@@ -128,12 +128,16 @@ func (u *UserInterface) readPixels(mipmap *mipmap.Mipmap, pixels []byte, region
 	// ReadPixels failed since this was called in between two frames.
 	// Try this again at the next frame.
 	if !ok {
-		ch := make(chan error)
-		u.context.appendDeferredFunc(func() {
-			defer close(ch)
+		// If this function is called from the same sequence as a game's Update and Draw,
+		// this causes a dead lock.
+		// This never happens so far, but if handling inputs after EndFrame is implemented,
+		// this might be possible (#1704).
+
+		var err1 error
+		u.context.runInFrame(func() {
 			ok, err := mipmap.ReadPixels(u.graphicsDriver, pixels, region)
 			if err != nil {
-				ch <- err
+				err1 = err
 				return
 			}
 			if !ok {
@@ -141,12 +145,7 @@ func (u *UserInterface) readPixels(mipmap *mipmap.Mipmap, pixels []byte, region
 				panic("ui: ReadPixels unexpectedly failed")
 			}
 		})
-
-		// If this function is called from the game (Update/Draw) goroutine in between two frames,
-		// this channel never returns and causes a dead lock.
-		// This never happens so far, but if handling inputs after EndFrame is implemented,
-		// this might be possible (#1704).
-		return <-ch
+		return err1
 	}
 
 	return nil