example/tabs: add example for tabbed content

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

example/go.mod

@@ -3,7 +3,7 @@ module gioui.org/example
 go 1.13
 
 require (
-	gioui.org v0.0.0-20200402190513-da01fbdea784
+	gioui.org v0.0.0-20200416175719-b1aed3eae023
 	github.com/go-gl/gl v0.0.0-20190320180904-bf2b1f2f34d7
 	github.com/go-gl/glfw/v3.3/glfw v0.0.0-20191125211704-12ad95a8df72
 	github.com/google/go-github/v24 v24.0.1

example/go.sum

@@ -1,7 +1,7 @@
 cloud.google.com/go v0.34.0/go.mod h1:aQUYkXzVsufM+DwF1aE+0xfcU+56JwCaLick0ClmMTw=
 dmitri.shuralyov.com/gpu/mtl v0.0.0-20190408044501-666a987793e9/go.mod h1:H6x//7gZCb22OMCxBHrMx7a5I7Hp++hsVxbQ4BYO7hU=
-gioui.org v0.0.0-20200402190513-da01fbdea784 h1:gH0+2HCtuzxAdK6sd0BeCxZLAkCbhptF607odwS5QY0=
+gioui.org v0.0.0-20200416175719-b1aed3eae023 h1:I7I2412lWuAC2kQPk9TLONjCv/8dWfOBJEjFOJhwF3g=
-gioui.org v0.0.0-20200402190513-da01fbdea784/go.mod h1:AHI9rFr6AEEHCb8EPVtb/p5M+NMJRKH58IOp8O3Je04=
+gioui.org v0.0.0-20200416175719-b1aed3eae023/go.mod h1:AHI9rFr6AEEHCb8EPVtb/p5M+NMJRKH58IOp8O3Je04=
 github.com/BurntSushi/xgb v0.0.0-20160522181843-27f122750802/go.mod h1:IVnqGOEym/WlBOVXweHU+Q+/VP0lqqI8lqeDx9IjBqo=
 github.com/go-gl/gl v0.0.0-20190320180904-bf2b1f2f34d7 h1:SCYMcCJ89LjRGwEa0tRluNRiMjZHalQZrVrvTbPh+qw=
 github.com/go-gl/gl v0.0.0-20190320180904-bf2b1f2f34d7/go.mod h1:482civXOzJJCPzJ4ZOX/pwvXBWSnzD4OKMdH4ClKGbk=

example/tabs/tabs.go

@@ -0,0 +1,115 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package main
+
+import (
+	"fmt"
+	"image"
+	"image/color"
+	"log"
+
+	"gioui.org/app"
+	"gioui.org/f32"
+	"gioui.org/io/system"
+	"gioui.org/layout"
+	"gioui.org/op/paint"
+	"gioui.org/unit"
+	"gioui.org/widget"
+	"gioui.org/widget/material"
+
+	"gioui.org/font/gofont"
+)
+
+func main() {
+	go func() {
+		w := app.NewWindow()
+		if err := loop(w); err != nil {
+			log.Fatal(err)
+		}
+	}()
+	app.Main()
+}
+
+func loop(w *app.Window) error {
+	gofont.Register()
+	th := material.NewTheme()
+	gtx := layout.NewContext(w.Queue())
+	for {
+		e := <-w.Events()
+		switch e := e.(type) {
+		case system.DestroyEvent:
+			return e.Err
+		case system.FrameEvent:
+			gtx.Reset(e.Config, e.Size)
+			drawTabs(gtx, th)
+			e.Frame(gtx.Ops)
+		}
+	}
+}
+
+var tabs Tabs
+
+type Tabs struct {
+	list     layout.List
+	tabs     []Tab
+	selected int
+}
+
+type Tab struct {
+	btn   widget.Button
+	Title string
+}
+
+func init() {
+	for i := 1; i <= 100; i++ {
+		tabs.tabs = append(tabs.tabs,
+			Tab{Title: fmt.Sprintf("Tab %d", i)},
+		)
+	}
+}
+
+func drawTabs(gtx *layout.Context, th *material.Theme) {
+	layout.Flex{Axis: layout.Vertical}.Layout(gtx,
+		layout.Rigid(func() {
+			tabs.list.Layout(gtx, len(tabs.tabs), func(tabIdx int) {
+				t := &tabs.tabs[tabIdx]
+				if t.btn.Clicked(gtx) {
+					tabs.selected = tabIdx
+				}
+				var tabWidth int
+				layout.Stack{Alignment: layout.S}.Layout(gtx,
+					layout.Stacked(func() {
+						tabBtn := th.Button(t.Title)
+						tabBtn.Background = color.RGBA{}   // No background.
+						tabBtn.CornerRadius = unit.Value{} // No corners.
+						tabBtn.Color = color.RGBA{A: 0xff} // Black text.
+						tabBtn.TextSize = unit.Sp(20)
+						tabBtn.Layout(gtx, &t.btn)
+						tabWidth = gtx.Dimensions.Size.X
+					}),
+					layout.Stacked(func() {
+						if tabs.selected != tabIdx {
+							return
+						}
+						paint.ColorOp{Color: th.Color.Primary}.Add(gtx.Ops)
+						tabHeight := gtx.Px(unit.Dp(4))
+						paint.PaintOp{Rect: f32.Rectangle{
+							Max: f32.Point{
+								X: float32(tabWidth),
+								Y: float32(tabHeight),
+							},
+						}}.Add(gtx.Ops)
+						gtx.Dimensions = layout.Dimensions{
+							Size: image.Point{X: tabWidth, Y: tabHeight},
+						}
+					}),
+				)
+			})
+		}),
+		layout.Flexed(1, func() {
+			layout.Center.Layout(gtx, func() {
+				th.H1(fmt.Sprintf("Tab content #%d", tabs.selected)).Layout(gtx)
+			})
+		}),
+	)
+}