widget/button,widget/material: introduce Clickable for generic click areas

material.Clickable is useful for adding a click response to any widget
or area.

Rename widget.Button to widget.Clickable to reflect the wider use
spectrum.

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

widget/button.go

@@ -13,8 +13,8 @@ import (
 	"gioui.org/op"
 )
 
-// Button represents a clickable area.
-type Button struct {
+// Clickable represents a clickable area.
+type Clickable struct {
 	click gesture.Click
 	// clicks tracks the number of unreported clicks.
 	clicks  int
@@ -30,7 +30,7 @@ type Click struct {
 // Clicked calls Update and reports whether the button was
 // clicked since the last call. Multiple clicks result in Clicked
 // returning true once per click.
-func (b *Button) Clicked(gtx *layout.Context) bool {
+func (b *Clickable) Clicked(gtx *layout.Context) bool {
 	b.Update(gtx)
 	if b.clicks > 0 {
 		b.clicks--
@@ -45,11 +45,11 @@ func (b *Button) Clicked(gtx *layout.Context) bool {
 
 // History is the past clicks useful for drawing click markers.
 // Clicks are retained for a short duration (about a second).
-func (b *Button) History() []Click {
+func (b *Clickable) History() []Click {
 	return b.history
 }
 
-func (b *Button) Layout(gtx *layout.Context) {
+func (b *Clickable) Layout(gtx *layout.Context) {
 	// Flush clicks from before the previous frame.
 	b.Update(gtx)
 	var st op.StackOp
@@ -69,7 +69,7 @@ func (b *Button) Layout(gtx *layout.Context) {
 
 // Update the button state by processing events. The underlying
 // gesture events are returned for use beyond what Clicked offers.
-func (b *Button) Update(gtx *layout.Context) []gesture.ClickEvent {
+func (b *Clickable) Update(gtx *layout.Context) []gesture.ClickEvent {
 	evts := b.click.Events(gtx)
 	for _, e := range evts {
 		switch e.Type {

widget/material/button.go

@@ -77,7 +77,29 @@ func IconButton(th *Theme, icon *widget.Icon) IconButtonStyle {
 	}
 }
 
-func (b ButtonStyle) Layout(gtx *layout.Context, button *widget.Button) {
+// Clickable lays out a rectangular clickable widget without further
+// decoration.
+func Clickable(gtx *layout.Context, button *widget.Clickable, w layout.Widget) {
+	layout.Stack{}.Layout(gtx,
+		layout.Expanded(func() {
+			button.Layout(gtx)
+		}),
+		layout.Expanded(func() {
+			clip.Rect{
+				Rect: f32.Rectangle{Max: f32.Point{
+					X: float32(gtx.Constraints.Width.Min),
+					Y: float32(gtx.Constraints.Height.Min),
+				}},
+			}.Op(gtx.Ops).Add(gtx.Ops)
+			for _, c := range button.History() {
+				drawInk(gtx, c)
+			}
+		}),
+		layout.Stacked(w),
+	)
+}
+
+func (b ButtonStyle) Layout(gtx *layout.Context, button *widget.Clickable) {
 	ButtonLayoutStyle{
 		Background:   b.Background,
 		CornerRadius: b.CornerRadius,
@@ -88,7 +110,7 @@ func (b ButtonStyle) Layout(gtx *layout.Context, button *widget.Button) {
 	})
 }
 
-func (b ButtonLayoutStyle) Layout(gtx *layout.Context, button *widget.Button, w layout.Widget) {
+func (b ButtonLayoutStyle) Layout(gtx *layout.Context, button *widget.Clickable, w layout.Widget) {
 	hmin := gtx.Constraints.Width.Min
 	vmin := gtx.Constraints.Height.Min
 	layout.Stack{Alignment: layout.Center}.Layout(gtx,
@@ -121,7 +143,7 @@ func (b ButtonLayoutStyle) Layout(gtx *layout.Context, button *widget.Button, w
 	)
 }
 
-func (b IconButtonStyle) Layout(gtx *layout.Context, button *widget.Button) {
+func (b IconButtonStyle) Layout(gtx *layout.Context, button *widget.Clickable) {
 	layout.Stack{Alignment: layout.Center}.Layout(gtx,
 		layout.Expanded(func() {
 			size := gtx.Constraints.Width.Min

widget/material/doc.go

@@ -5,14 +5,14 @@
 // To maximize reusability and visual flexibility, user interface controls are
 // split into two parts: the stateful widget and the stateless drawing of it.
 //
-// For example, widget.Button encapsulates the state and event
-// handling of all buttons, while the Theme can draw a single Button
-// in various styles.
+// For example, widget.Clickable encapsulates the state and event
+// handling of all clickable areas, while the Theme is responsible to
+// draw a specific area, for example a button.
 //
 // This snippet defines a button that prints a message when clicked:
 //
 //     var gtx *layout.Context
-//     button := new(widget.Button)
+//     button := new(widget.Clickable)
 //
 //     for button.Clicked(gtx) {
 //         fmt.Println("Clicked!")
@@ -46,7 +46,7 @@
 //     btn.Layout(gtx)
 //
 // Widget variants: A widget can have several distinct representations even
-// though the underlying state is the same. A widget.Button can be drawn as a
+// though the underlying state is the same. A widget.Clickable can be drawn as a
 // round icon button:
 //
 //     icon := material.NewIcon(...)