apps/hello: add hello world example

apps/go.mod

@@ -1,6 +1,6 @@
 module gioui.org/apps
 
-go 1.13
+go 1.12
 
 require (
 	gioui.org/ui v0.0.0-20190331090026-ca5204fcb8b3

apps/hello/hello.go

@@ -0,0 +1,63 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package main
+
+import (
+	"image"
+	"image/color"
+	"log"
+
+	"gioui.org/ui"
+	"gioui.org/ui/app"
+	"gioui.org/ui/layout"
+	"gioui.org/ui/measure"
+	"gioui.org/ui/text"
+
+	"golang.org/x/image/font/gofont/goregular"
+	"golang.org/x/image/font/sfnt"
+)
+
+func main() {
+	err := app.CreateWindow(app.WindowOptions{
+		Width:  ui.Dp(400),
+		Height: ui.Dp(800),
+		Title:  "Hello World",
+	})
+	if err != nil {
+		log.Fatal(err)
+	}
+	app.Main()
+}
+
+func init() {
+	go func() {
+		for w := range app.Windows() {
+			go loop(w)
+		}
+	}()
+}
+
+func loop(w *app.Window) {
+	regular, err := sfnt.Parse(goregular.TTF)
+	if err != nil {
+		panic("failed to load font")
+	}
+	var faces measure.Faces
+	black := &image.Uniform{color.Black}
+	face := faces.For(regular, ui.Dp(50))
+	for w.IsAlive() {
+		e := <-w.Events()
+		switch e := e.(type) {
+		case app.Draw:
+			faces.Cfg = e.Config
+			cs := layout.ExactConstraints(w.Size())
+			root, _ := (text.Label{Src: black, Face: face, Text: "Hello, World!"}).Layout(cs)
+			w.Draw(root)
+			faces.Frame()
+		}
+		w.Ack()
+	}
+	if w.Err() != nil {
+		log.Fatal(err)
+	}
+}