widget: [API] rename Bool.Changed to Update and move state update to it

Similar to a previous change for Clickable, this change separates Bool
state changes to its renamed method Update. This allows access to
the most recent state before calling Layout.

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

widget/bool.go

@@ -11,15 +11,15 @@ type Bool struct {
 	Value bool
 
 	clk Clickable
-
-	changed bool
 }
 
-// Changed reports whether Value has changed since the last
-// call to Changed.
-func (b *Bool) Changed() bool {
-	changed := b.changed
-	b.changed = false
+// Update the widget state and report whether Value was changed.
+func (b *Bool) Update(gtx layout.Context) bool {
+	changed := false
+	for b.clk.Clicked(gtx) {
+		b.Value = !b.Value
+		changed = true
+	}
 	return changed
 }
 
@@ -43,10 +43,7 @@ func (b *Bool) History() []Press {
 }
 
 func (b *Bool) Layout(gtx layout.Context, w layout.Widget) layout.Dimensions {
-	for b.clk.Clicked(gtx) {
-		b.Value = !b.Value
-		b.changed = true
-	}
+	b.Update(gtx)
 	dims := b.clk.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
 		semantic.SelectedOp(b.Value).Add(gtx.Ops)
 		semantic.EnabledOp(gtx.Queue != nil).Add(gtx.Ops)