widget: [API] move Clickable state update from Layout to Clicks

Before this change, Clickable state updates would happen in Layout.
However, that is too late in cases where clicks affects layout that
contiains the Clickable.

This change removes state changes from Layout and moves them to Clicks,
to allow users pre-layout access. Note that Layout itself processes
events, which means users can no longer access clicks after Layout.

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

widget/button.go

@@ -17,18 +17,16 @@ import (
 
 // Clickable represents a clickable area.
 type Clickable struct {
-	click  gesture.Click
-	clicks []Click
-	// prevClicks is the index into clicks that marks the clicks
-	// from the most recent Layout call. prevClicks is used to keep
-	// clicks bounded.
-	prevClicks int
-	history    []Press
+	click gesture.Click
+	// clicks is for saved clicks to support Clicked.
+	clicks  []Click
+	history []Press
 
-	keyTag       struct{}
-	requestFocus bool
-	focused      bool
-	pressedKey   string
+	keyTag        struct{}
+	requestFocus  bool
+	requestClicks int
+	focused       bool
+	pressedKey    string
 }
 
 // Click represents a click.
@@ -50,26 +48,24 @@ type Press struct {
 	Cancelled bool
 }
 
-// Click executes a simple programmatic click
+// Click executes a simple programmatic click.
 func (b *Clickable) Click() {
-	b.clicks = append(b.clicks, Click{
-		Modifiers: 0,
-		NumClicks: 1,
-	})
+	b.requestClicks++
 }
 
 // Clicked reports whether there are pending clicks as would be
 // reported by Clicks. If so, Clicked removes the earliest click.
-func (b *Clickable) Clicked() bool {
-	if len(b.clicks) == 0 {
-		return false
+func (b *Clickable) Clicked(gtx layout.Context) bool {
+	if len(b.clicks) > 0 {
+		b.clicks = b.clicks[1:]
+		return true
 	}
-	n := copy(b.clicks, b.clicks[1:])
-	b.clicks = b.clicks[:n]
-	if b.prevClicks > 0 {
-		b.prevClicks--
+	b.clicks = b.Update(gtx)
+	if len(b.clicks) > 0 {
+		b.clicks = b.clicks[1:]
+		return true
 	}
-	return true
+	return false
 }
 
 // Hovered reports whether a pointer is over the element.
@@ -92,23 +88,15 @@ func (b *Clickable) Focused() bool {
 	return b.focused
 }
 
-// Clicks returns and clear the clicks since the last call to Clicks.
-func (b *Clickable) Clicks() []Click {
-	clicks := b.clicks
-	b.clicks = nil
-	b.prevClicks = 0
-	return clicks
-}
-
 // History is the past pointer presses useful for drawing markers.
 // History is retained for a short duration (about a second).
 func (b *Clickable) History() []Press {
 	return b.history
 }
 
-// Layout and update the button state
+// Layout and update the button state.
 func (b *Clickable) Layout(gtx layout.Context, w layout.Widget) layout.Dimensions {
-	b.update(gtx)
+	b.Update(gtx)
 	m := op.Record(gtx.Ops)
 	dims := w(gtx)
 	c := m.Stop()
@@ -122,14 +110,22 @@ func (b *Clickable) Layout(gtx layout.Context, w layout.Widget) layout.Dimension
 			keys = ""
 		}
 		key.InputOp{Tag: &b.keyTag, Keys: keys}.Add(gtx.Ops)
-		if b.requestFocus {
-			key.FocusOp{Tag: &b.keyTag}.Add(gtx.Ops)
-			b.requestFocus = false
-		}
-	} else {
-		b.focused = false
 	}
 	c.Add(gtx.Ops)
+	return dims
+}
+
+// Update the button state by processing events, and return the resulting
+// clicks, if any.
+func (b *Clickable) Update(gtx layout.Context) []Click {
+	b.clicks = nil
+	if gtx.Queue == nil {
+		b.focused = false
+	}
+	if b.requestFocus {
+		key.FocusOp{Tag: &b.keyTag}.Add(gtx.Ops)
+		b.requestFocus = false
+	}
 	for len(b.history) > 0 {
 		c := b.history[0]
 		if c.End.IsZero() || gtx.Now.Sub(c.End) < 1*time.Second {
@@ -138,26 +134,23 @@ func (b *Clickable) Layout(gtx layout.Context, w layout.Widget) layout.Dimension
 		n := copy(b.history, b.history[1:])
 		b.history = b.history[:n]
 	}
-	return dims
-}
-
-// update the button state by processing events.
-func (b *Clickable) update(gtx layout.Context) {
-	// Flush clicks from before the last update.
-	n := copy(b.clicks, b.clicks[b.prevClicks:])
-	b.clicks = b.clicks[:n]
-	b.prevClicks = n
-
+	var clicks []Click
+	if c := b.requestClicks; c > 0 {
+		b.requestClicks = 0
+		clicks = append(clicks, Click{
+			NumClicks: c,
+		})
+	}
 	for _, e := range b.click.Events(gtx) {
 		switch e.Kind {
 		case gesture.KindClick:
-			b.clicks = append(b.clicks, Click{
-				Modifiers: e.Modifiers,
-				NumClicks: e.NumClicks,
-			})
 			if l := len(b.history); l > 0 {
 				b.history[l-1].End = gtx.Now
 			}
+			clicks = append(clicks, Click{
+				Modifiers: e.Modifiers,
+				NumClicks: e.NumClicks,
+			})
 		case gesture.KindCancel:
 			for i := range b.history {
 				b.history[i].Cancelled = true
@@ -198,11 +191,12 @@ func (b *Clickable) update(gtx layout.Context) {
 				}
 				// only register a key as a click if the key was pressed and released while this button was focused
 				b.pressedKey = ""
-				b.clicks = append(b.clicks, Click{
+				clicks = append(clicks, Click{
 					Modifiers: e.Modifiers,
 					NumClicks: 1,
 				})
 			}
 		}
 	}
+	return clicks
 }