ui/app: introduce DestroyEvent for ending the event loop

Replace the StageDead stage with DestroyEvent dedicated to ending
the event loop and, for premature window closes, the error.

Drop the error return from NewWindow; any errors in window creation
will appear as an immediate DestroyEvent with its Err field set.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/window.go

@@ -29,7 +29,6 @@ type Window struct {
 	drawStart  time.Time
 	gpu        *gpu.GPU
 	inputState key.TextInputState
-	err        error
 
 	events chan Event
 
@@ -70,7 +69,7 @@ var ackEvent Event
 // ignore or adjust them.
 // If the current program is running on iOS and Android,
 // NewWindow returns the window previously by the platform.
-func NewWindow(opts *WindowOptions) (*Window, error) {
+func NewWindow(opts *WindowOptions) *Window {
 	if opts == nil {
 		opts = &WindowOptions{
 			Width:  ui.Dp(800),
@@ -87,9 +86,11 @@ func NewWindow(opts *WindowOptions) (*Window, error) {
 		stage:  StagePaused,
 	}
 	if err := createWindow(w, opts); err != nil {
-		return nil, err
+		// For simplicity, NewWindow always succeeds. Send
+		// an immediate DestroyEvent instead of returning the error.
+		w.destroy(err)
 	}
-	return w, nil
+	return w
 }
 
 func (w *Window) Events() <-chan Event {
@@ -107,10 +108,6 @@ func (w *Window) setTextInput(s key.TextInputState) {
 	w.inputState = s
 }
 
-func (w *Window) Err() error {
-	return w.err
-}
-
 func (w *Window) Queue() input.Queue {
 	return &w.router
 }
@@ -144,12 +141,12 @@ func (w *Window) Draw(root *ui.Ops) {
 	if w.gpu == nil {
 		ctx, err := newContext(driver)
 		if err != nil {
-			w.err = err
+			w.destroy(err)
 			return
 		}
 		w.gpu, err = gpu.NewGPU(ctx)
 		if err != nil {
-			w.err = err
+			w.destroy(err)
 			return
 		}
 	}
@@ -190,6 +187,9 @@ func (w *Window) updateAnimation() {
 		w.delayedDraw.Stop()
 		w.delayedDraw = nil
 	}
+	if !w.isAlive() {
+		return
+	}
 	if w.stage >= StageRunning && w.hasNextFrame {
 		if dt := time.Until(w.nextFrame); dt <= 0 {
 			animate = true
@@ -223,7 +223,7 @@ func (w *Window) Stage() Stage {
 }
 
 func (w *Window) isAlive() bool {
-	return w.stage > StageDead && w.err == nil
+	return w.driver != nil
 }
 
 func (w *Window) contextDriver() interface{} {
@@ -236,10 +236,18 @@ func (w *Window) setDriver(d *window) {
 	w.driver = d
 }
 
+func (w *Window) destroy(err error) {
+	w.setDriver(nil)
+	go func() {
+		w.event(DestroyEvent{err})
+	}()
+}
+
 func (w *Window) event(e Event) {
 	w.eventLock.Lock()
 	defer w.eventLock.Unlock()
 	w.mu.Lock()
+	died := false
 	needAck := false
 	switch e := e.(type) {
 	case input.Event:
@@ -248,12 +256,13 @@ func (w *Window) event(e Event) {
 		}
 	case *CommandEvent:
 		needAck = true
+	case DestroyEvent:
+		w.driver = nil
+		died = true
 	case StageEvent:
 		w.stage = e.Stage
-		if w.stage > StageDead {
-			needAck = true
-			w.syncGPU = true
-		}
+		needAck = true
+		w.syncGPU = true
 	case DrawEvent:
 		if e.Size == (image.Point{}) {
 			panic(errors.New("internal error: zero-sized Draw"))
@@ -290,7 +299,7 @@ func (w *Window) event(e Event) {
 			w.gpu.Refresh()
 		}
 	}
-	if stage == StageDead {
+	if died {
 		close(w.events)
 	}
 }