ui/pointer: don't drop handlers that whose events haven't been read

We're about to merge the pointer and key event streams in a single
input queue. To do that, we need to simplify Queue.For to just returning
events for the given handler.

First step is the handler active flag.

Dropping handlers that haven't had their events read doesn't seem
worth it. Drop that special case and only determine a handler's activeness
from its presence in the ops list.

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

ui/pointer/queue.go

@@ -81,9 +81,13 @@ func (q *Queue) collectHandlers(r *ui.OpsReader, t ui.Transform, layer int) {
 			q.hitTree = append(q.hitTree, hitNode{level: layer, key: op.Key})
 			h, ok := q.handlers[op.Key]
 			if !ok {
-				h = new(handler)
+				h = &handler{
+					// Reset the handler on (each) first appearance.
+					events: []Event{Event{Type: Cancel}},
+				}
 				q.handlers[op.Key] = h
 			}
+			h.active = true
 			h.area = q.areas.intersection()
 			h.transform = t
 			h.wantsGrab = h.wantsGrab || op.Grab
@@ -126,36 +130,29 @@ func (q *Queue) init() {
 
 func (q *Queue) Frame(root *ui.Ops) {
 	q.init()
-	for k, h := range q.handlers {
-		if !h.active {
-			q.dropHandler(k)
-		} else {
-			// Reset handler.
-			h.events = h.events[:0]
-		}
+	for _, h := range q.handlers {
+		// Reset handler.
+		h.active = false
+		h.events = h.events[:0]
 	}
 	q.hitTree = q.hitTree[:0]
 	q.areas.reset()
 	q.reader.Reset(root)
 	q.collectHandlers(&q.reader, ui.Transform{}, 0)
+	for k, h := range q.handlers {
+		if !h.active {
+			q.dropHandler(k)
+		}
+	}
 }
 
 func (q *Queue) For(k Key) []Event {
 	if k == nil {
 		panic("nil handler")
 	}
-	q.init()
-	h, ok := q.handlers[k]
-	if !ok {
-		h = new(handler)
-		q.handlers[k] = h
-	}
-	if !h.active {
-		h.active = true
-		// Prepend a Cancel.
-		h.events = append(h.events, Event{})
-		copy(h.events[1:], h.events)
-		h.events[0] = Event{Type: Cancel}
+	h := q.handlers[k]
+	if h == nil {
+		return nil
 	}
 	return h.events
 }