widget: add Button.Clicks for retrieving clicks

An earlier change unexported the Button.Update method that exposed raw pointer
input not available from the boolean Button.Clicked method. Introduce Click
and Button.Clicks to replace it, and implement Clicked in terms of it.

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

widget/button.go

@@ -8,6 +8,7 @@ import (
 
 	"gioui.org/f32"
 	"gioui.org/gesture"
+	"gioui.org/io/key"
 	"gioui.org/io/pointer"
 	"gioui.org/layout"
 	"gioui.org/op"
@@ -15,10 +16,18 @@ import (
 
 // Clickable represents a clickable area.
 type Clickable struct {
-	click gesture.Click
-	// clicks tracks the number of unreported clicks.
-	clicks  int
-	history []Press
+	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 represents a click.
+type Click struct {
+	Modifiers key.Modifiers
 }
 
 // Press represents a past pointer press.
@@ -27,14 +36,26 @@ type Press struct {
 	Time     time.Time
 }
 
-// Clicked and reports whether the button was clicked since the last
-// call to Clicked. Clicked returns true once per click.
+// 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 b.clicks > 0 {
-		b.clicks--
-		return true
+	if len(b.clicks) == 0 {
+		return false
 	}
-	return false
+	n := copy(b.clicks, b.clicks[1:])
+	b.clicks = b.clicks[:n]
+	if b.prevClicks > 0 {
+		b.prevClicks--
+	}
+	return true
+}
+
+// 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.
@@ -63,10 +84,17 @@ func (b *Clickable) Layout(gtx layout.Context) layout.Dimensions {
 
 // 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
+
 	for _, e := range b.click.Events(gtx) {
 		switch e.Type {
 		case gesture.TypeClick:
-			b.clicks++
+			b.clicks = append(b.clicks, Click{
+				Modifiers: e.Modifiers,
+			})
 		case gesture.TypePress:
 			b.history = append(b.history, Press{
 				Position: e.Position,