ui/app/internal/input: schedule an extra frame after event delivery

A program might choose to process events that affect UI state already laid out.
For example, a button click might switch to a completely different UI page, but
the click might be processed during the drawing of the current page.

Avoiding that require either processing events very early during layout or
adding InvalidateOps whenever events are handled late.

Early event processing is awkward and InvalidateOps are easy to forget and
their absence masked by any other cause of redraw.

This change adds an implicit InvalidateOp for each frame where events have been
delivered to the program, allowing late event handling without use of
InvalidateOps.

In the worst case we waste a frame, increasing power use. I hope that future
optimizations will detect and discard the duplicate frame before it reaches
the GPU.

A similar situation applies to the delayed delivery of Editor events, but
since Editor.Layout flushes remaining events, extra InvalidateOps are not
required. Add a comment.

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

ui/app/internal/input/router.go

@@ -24,6 +24,15 @@ type Router struct {
 
 	reader ops.Reader
 
+	// deliveredEvents tracks whether events has been returned to the
+	// user from Events. If so, another frame is scheduled to flush
+	// half-updated state. This is important when a an event changes
+	// UI state that has already been laid out. In the worst case, we
+	// waste a frame, increasing power usage.
+	// Gio is expected to grow the ability to construct frame-to-frame
+	// differences and only render to changed areas. In that case, the
+	// waste of a spurious frame should be minimal.
+	deliveredEvents bool
 	// InvalidateOp summary.
 	wakeup     bool
 	wakeupTime time.Time
@@ -33,12 +42,14 @@ type Router struct {
 }
 
 type handlerEvents struct {
-	handlers map[ui.Key][]ui.Event
-	updated  bool
+	handlers  map[ui.Key][]ui.Event
+	hadEvents bool
 }
 
 func (q *Router) Events(k ui.Key) []ui.Event {
-	return q.handlers.Events(k)
+	events := q.handlers.Events(k)
+	q.deliveredEvents = q.deliveredEvents || len(events) > 0
+	return events
 }
 
 func (q *Router) Frame(ops *ui.Ops) {
@@ -50,7 +61,8 @@ func (q *Router) Frame(ops *ui.Ops) {
 
 	q.pqueue.Frame(ops, &q.handlers)
 	q.kqueue.Frame(ops, &q.handlers)
-	if q.handlers.Updated() {
+	if q.deliveredEvents || q.handlers.HadEvents() {
+		q.deliveredEvents = false
 		q.wakeup = true
 		q.wakeupTime = time.Time{}
 	}
@@ -63,7 +75,7 @@ func (q *Router) Add(e ui.Event) bool {
 	case key.EditEvent, key.Event, key.FocusEvent:
 		q.kqueue.Push(e, &q.handlers)
 	}
-	return q.handlers.Updated()
+	return q.handlers.HadEvents()
 }
 
 func (q *Router) TextInputState() TextInputState {
@@ -109,18 +121,18 @@ func (h *handlerEvents) init() {
 func (h *handlerEvents) Set(k ui.Key, evts []ui.Event) {
 	h.init()
 	h.handlers[k] = evts
-	h.updated = true
+	h.hadEvents = true
 }
 
 func (h *handlerEvents) Add(k ui.Key, e ui.Event) {
 	h.init()
 	h.handlers[k] = append(h.handlers[k], e)
-	h.updated = true
+	h.hadEvents = true
 }
 
-func (h *handlerEvents) Updated() bool {
-	u := h.updated
-	h.updated = false
+func (h *handlerEvents) HadEvents() bool {
+	u := h.hadEvents
+	h.hadEvents = false
 	return u
 }
 

ui/text/editor.go

@@ -175,6 +175,7 @@ func (e *Editor) Focus() {
 	e.requestFocus = true
 }
 
+// Layout flushes any remaining events and lays out the editor.
 func (e *Editor) Layout(gtx *layout.Context) {
 	cs := gtx.Constraints
 	for _, ok := e.Event(gtx); ok; _, ok = e.Event(gtx) {