From 0b736990a92e962f23d3fce1ea87215dbc51f583 Mon Sep 17 00:00:00 2001
From: Egon Elbre <egonelbre@gmail.com>
Date: Sun, 17 Jan 2021 18:22:10 +0200
Subject: [PATCH] widget/material: add hover to CheckBox

Signed-off-by: Egon Elbre <egonelbre@gmail.com>
---
 widget/material/checkable.go   | 58 +++++++++++++++++++++++++++-------
 widget/material/checkbox.go    |  2 +-
 widget/material/radiobutton.go |  2 +-
 3 files changed, 48 insertions(+), 14 deletions(-)

diff --git a/widget/material/checkable.go b/widget/material/checkable.go
index 90f2103e..2c3dc838 100644
--- a/widget/material/checkable.go
+++ b/widget/material/checkable.go
@@ -5,10 +5,13 @@ package material
 import (
 	"image"
 	"image/color"
+	"math"
 
+	"gioui.org/f32"
 	"gioui.org/internal/f32color"
 	"gioui.org/io/pointer"
 	"gioui.org/layout"
+	"gioui.org/op/clip"
 	"gioui.org/op/paint"
 	"gioui.org/text"
 	"gioui.org/unit"
@@ -27,7 +30,7 @@ type checkable struct {
 	uncheckedStateIcon *widget.Icon
 }
 
-func (c *checkable) layout(gtx layout.Context, checked bool) layout.Dimensions {
+func (c *checkable) layout(gtx layout.Context, checked, hovered bool) layout.Dimensions {
 	var icon *widget.Icon
 	if checked {
 		icon = c.checkedStateIcon
@@ -38,19 +41,39 @@ func (c *checkable) layout(gtx layout.Context, checked bool) layout.Dimensions {
 	min := gtx.Constraints.Min
 	dims := layout.Flex{Alignment: layout.Middle}.Layout(gtx,
 		layout.Rigid(func(gtx layout.Context) layout.Dimensions {
-			return layout.Center.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
-				return layout.UniformInset(unit.Dp(2)).Layout(gtx, func(gtx layout.Context) layout.Dimensions {
-					size := gtx.Px(c.Size)
-					icon.Color = c.IconColor
-					if gtx.Queue == nil {
-						icon.Color = f32color.Disabled(icon.Color)
-					}
-					icon.Layout(gtx, unit.Px(float32(size)))
-					return layout.Dimensions{
+			return layout.Stack{Alignment: layout.Center}.Layout(gtx,
+				layout.Stacked(func(gtx layout.Context) layout.Dimensions {
+					size := gtx.Px(c.Size) * 4 / 3
+					dims := layout.Dimensions{
 						Size: image.Point{X: size, Y: size},
 					}
-				})
-			})
+					if !hovered {
+						return dims
+					}
+
+					background := f32color.MulAlpha(c.IconColor, 70)
+
+					var p clip.Path
+					p.Begin(gtx.Ops)
+					addCircle(&p, float32(size)/2)
+					paint.FillShape(gtx.Ops, background, clip.Outline{Path: p.End()}.Op())
+
+					return dims
+				}),
+				layout.Stacked(func(gtx layout.Context) layout.Dimensions {
+					return layout.UniformInset(unit.Dp(2)).Layout(gtx, func(gtx layout.Context) layout.Dimensions {
+						size := gtx.Px(c.Size)
+						icon.Color = c.IconColor
+						if gtx.Queue == nil {
+							icon.Color = f32color.Disabled(icon.Color)
+						}
+						icon.Layout(gtx, unit.Px(float32(size)))
+						return layout.Dimensions{
+							Size: image.Point{X: size, Y: size},
+						}
+					})
+				}),
+			)
 		}),
 
 		layout.Rigid(func(gtx layout.Context) layout.Dimensions {
@@ -66,3 +89,14 @@ func (c *checkable) layout(gtx layout.Context, checked bool) layout.Dimensions {
 	pointer.Rect(image.Rectangle{Max: dims.Size}).Add(gtx.Ops)
 	return dims
 }
+
+// addCircle adds the outline of a circle to a path.
+func addCircle(p *clip.Path, r float32) {
+	// https://pomax.github.io/bezierinfo/#circles_cubic.
+	const c = 4 * (math.Sqrt2 - 1) / 3 // 4*(sqrt(2)-1)/3
+	p.Move(f32.Point{X: 2 * r, Y: 2*r - r})
+	p.Cube(f32.Point{X: 0, Y: r * c}, f32.Point{X: -r + r*c, Y: r}, f32.Point{X: -r, Y: r})
+	p.Cube(f32.Point{X: -r * c, Y: 0}, f32.Point{X: -r, Y: -r + r*c}, f32.Point{X: -r, Y: -r})
+	p.Cube(f32.Point{X: 0, Y: -r * c}, f32.Point{X: r - r*c, Y: -r}, f32.Point{X: r, Y: -r})
+	p.Cube(f32.Point{X: r * c, Y: 0}, f32.Point{X: r, Y: r - r*c}, f32.Point{X: r, Y: r})
+}
diff --git a/widget/material/checkbox.go b/widget/material/checkbox.go
index 914c208e..aac98375 100644
--- a/widget/material/checkbox.go
+++ b/widget/material/checkbox.go
@@ -31,7 +31,7 @@ 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)
+	dims := c.layout(gtx, c.CheckBox.Value, c.CheckBox.Hovered())
 	gtx.Constraints.Min = dims.Size
 	c.CheckBox.Layout(gtx)
 	return dims
diff --git a/widget/material/radiobutton.go b/widget/material/radiobutton.go
index 1546fc3a..1c4f2a88 100644
--- a/widget/material/radiobutton.go
+++ b/widget/material/radiobutton.go
@@ -36,7 +36,7 @@ func RadioButton(th *Theme, group *widget.Enum, key, label string) RadioButtonSt
 
 // Layout updates enum and displays the radio button.
 func (r RadioButtonStyle) Layout(gtx layout.Context) layout.Dimensions {
-	dims := r.layout(gtx, r.Group.Value == r.Key)
+	dims := r.layout(gtx, r.Group.Value == r.Key, false)
 	gtx.Constraints.Min = dims.Size
 	r.Group.Layout(gtx, r.Key)
 	return dims