widget: change Bool.Layout to follow layout protocol

Just like Clickable, Bool.Layout should respect constraints and
return its dimensions.

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

widget/bool.go

@@ -1,8 +1,12 @@
 package widget
 
 import (
+	"image"
+
 	"gioui.org/gesture"
+	"gioui.org/io/pointer"
 	"gioui.org/layout"
+	"gioui.org/op"
 )
 
 type Bool struct {
@@ -24,7 +28,7 @@ func (b *Bool) Changed() bool {
 	return changed
 }
 
-func (b *Bool) Layout(gtx layout.Context) {
+func (b *Bool) Layout(gtx layout.Context) layout.Dimensions {
 	for _, e := range b.gesture.Events(gtx) {
 		switch e.Type {
 		case gesture.TypeClick:
@@ -35,5 +39,10 @@ func (b *Bool) Layout(gtx layout.Context) {
 			b.Value = !b.Value
 		}
 	}
+	var st op.StackOp
+	st.Push(gtx.Ops)
+	defer st.Pop()
+	pointer.Rect(image.Rectangle{Max: gtx.Constraints.Min}).Add(gtx.Ops)
 	b.gesture.Add(gtx.Ops)
+	return layout.Dimensions{Size: gtx.Constraints.Min}
 }

widget/material/checkbox.go

@@ -32,6 +32,7 @@ func CheckBox(th *Theme, checkBox *widget.Bool, label string) CheckBoxStyle {
 // Layout updates the checkBox and displays it.
 func (c CheckBoxStyle) Layout(gtx layout.Context) layout.Dimensions {
 	dims := c.layout(gtx, c.CheckBox.Value)
+	gtx.Constraints.Min = dims.Size
 	c.CheckBox.Layout(gtx)
 	return dims
 }

widget/material/switch.go

@@ -104,17 +104,13 @@ func (s SwitchStyle) Layout(gtx layout.Context) layout.Dimensions {
 		Y: (float32(trackHeight)-float32(clickSize))*.5 + trackOff,
 	}
 	op.TransformOp{}.Offset(clickOff).Add(gtx.Ops)
-	pointer.Ellipse(image.Rectangle{
-		Max: image.Point{
-			X: clickSize, Y: clickSize,
-		},
-	}).Add(gtx.Ops)
+	sz := image.Pt(clickSize, clickSize)
+	pointer.Ellipse(image.Rectangle{Max: sz}).Add(gtx.Ops)
+	gtx.Constraints.Min = sz
 	s.Switch.Layout(gtx)
 	stack.Pop()
 
-	return layout.Dimensions{
-		Size: image.Point{X: trackWidth, Y: trackHeight},
-	}
+	return layout.Dimensions{Size: image.Point{X: trackWidth, Y: thumbSize}}
 }
 
 func drawDisc(ops *op.Ops, sz float32, col color.RGBA) {