UI: Add MainWindow Widget and start an example app

* MainWindow is ideal for apps that just want a UI and
  don't manage their own SDL windows.
* The example app will grow into a series of demos that
  test the UI toolkit to help fix bugs and grow features.

eg/layout/layout.go

@@ -0,0 +1,7 @@
+package layout
+
+import "fmt"
+
+func main() {
+	fmt.Println("Hello world")
+}

eg/layout/main.go

@@ -0,0 +1,12 @@
+package main
+
+import (
+	"fmt"
+
+	"git.kirsle.net/apps/doodle/lib/ui/eg/layout"
+)
+
+func main() {
+	fmt.Println("Hello world")
+	layout.main()
+}

eg/main.go

@@ -0,0 +1,47 @@
+package main
+
+import (
+	"git.kirsle.net/apps/doodle/lib/render"
+	"git.kirsle.net/apps/doodle/lib/ui"
+)
+
+func main() {
+	mw, err := ui.NewMainWindow("UI Toolkit Demo")
+	if err != nil {
+		panic(err)
+	}
+
+	leftFrame := ui.NewFrame("Left Frame")
+	leftFrame.Configure(ui.Config{
+		Width:       200,
+		BorderSize:  1,
+		BorderStyle: ui.BorderRaised,
+		Background:  render.Grey,
+	})
+	mw.Pack(leftFrame, ui.Pack{
+		Anchor: ui.W,
+		FillY:  true,
+	})
+
+	mainFrame := ui.NewFrame("Main Frame")
+	mainFrame.Configure(ui.Config{
+		Background: render.RGBA(255, 255, 255, 180),
+	})
+	mw.Pack(mainFrame, ui.Pack{
+		Anchor: ui.W,
+		Expand: true,
+		PadX:   10,
+	})
+
+	label := ui.NewLabel(ui.Label{
+		Text: "Hello world",
+	})
+	leftFrame.Pack(label, ui.Pack{
+		Anchor: ui.SE,
+	})
+
+	err = mw.MainLoop()
+	if err != nil {
+		panic("MainLoop:" + err.Error())
+	}
+}

main_window.go

@@ -0,0 +1,126 @@
+package ui
+
+import (
+	"fmt"
+	"time"
+
+	"git.kirsle.net/apps/doodle/lib/render"
+	"git.kirsle.net/apps/doodle/lib/render/sdl"
+)
+
+// Target frames per second for the MainWindow to render at.
+var (
+	FPS = 60
+)
+
+// MainWindow is the parent window of a UI application.
+type MainWindow struct {
+	engine     render.Engine
+	supervisor *Supervisor
+	frame      *Frame
+	w          int
+	h          int
+}
+
+// NewMainWindow initializes the MainWindow. You should probably only have one
+// of these per application.
+func NewMainWindow(title string) (*MainWindow, error) {
+	mw := &MainWindow{
+		w:          800,
+		h:          600,
+		supervisor: NewSupervisor(),
+	}
+
+	mw.engine = sdl.New(
+		title,
+		mw.w,
+		mw.h,
+	)
+	if err := mw.engine.Setup(); err != nil {
+		return nil, err
+	}
+
+	// Add a default frame to the window.
+	mw.frame = NewFrame("MainWindow Body")
+	mw.frame.SetBackground(render.RGBA(0, 153, 255, 100))
+	mw.Add(mw.frame)
+
+	// Compute initial window size.
+	mw.resized()
+
+	return mw, nil
+}
+
+// Add a child widget to the window.
+func (mw *MainWindow) Add(w Widget) {
+	mw.supervisor.Add(w)
+}
+
+// Pack a child widget into the window's default frame.
+func (mw *MainWindow) Pack(w Widget, pack Pack) {
+	mw.Add(w)
+	mw.frame.Pack(w, pack)
+}
+
+// resized handles the window being resized.
+func (mw *MainWindow) resized() {
+	mw.frame.Resize(render.Rect{
+		W: int32(mw.w),
+		H: int32(mw.h),
+	})
+}
+
+// Present the window.
+func (mw *MainWindow) Present() {
+	mw.supervisor.Present(mw.engine)
+}
+
+// MainLoop starts the main event loop and blocks until there's an error.
+func (mw *MainWindow) MainLoop() error {
+	for true {
+		if err := mw.Loop(); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// Loop does one loop of the UI.
+func (mw *MainWindow) Loop() error {
+	mw.engine.Clear(render.White)
+
+	// Record how long this loop took.
+	start := time.Now()
+
+	// Poll for events.
+	ev, err := mw.engine.Poll()
+	if err != nil {
+		return fmt.Errorf("event poll error: %s", err)
+	}
+
+	if ev.Resized.Now {
+		w, h := mw.engine.WindowSize()
+		if w != mw.w || h != mw.h {
+			mw.w = w
+			mw.h = h
+			mw.resized()
+		}
+	}
+
+	mw.frame.Compute(mw.engine)
+
+	// Render the child widgets.
+	mw.supervisor.Present(mw.engine)
+	mw.engine.Present()
+
+	// Delay to maintain target frames per second.
+	var delay uint32
+	var targetFPS = 1000 / FPS
+	elapsed := time.Now().Sub(start) / time.Millisecond
+	if targetFPS-int(elapsed) > 0 {
+		delay = uint32(targetFPS - int(elapsed))
+	}
+	mw.engine.Delay(delay)
+
+	return nil
+}

menu.go

@@ -0,0 +1,99 @@
+package ui
+
+import (
+	"fmt"
+
+	"git.kirsle.net/apps/doodle/lib/render"
+)
+
+// Menu is a rectangle that holds menu items.
+type Menu struct {
+	BaseWidget
+	Name string
+
+	body *Frame
+}
+
+// NewMenu creates a new Menu. It is hidden by default. Usually you'll
+// use it with a MenuButton or in a right-click handler.
+func NewMenu(name string) *Menu {
+	w := &Menu{
+		Name: name,
+		body: NewFrame(name + ":Body"),
+	}
+	w.body.Configure(Config{
+		Width:       150,
+		BorderSize:  12,
+		BorderStyle: BorderRaised,
+		Background:  render.Grey,
+	})
+	w.IDFunc(func() string {
+		return fmt.Sprintf("Menu<%s>", w.Name)
+	})
+	return w
+}
+
+// Compute the menu
+func (w *Menu) Compute(e render.Engine) {
+	w.body.Compute(e)
+}
+
+// Present the menu
+func (w *Menu) Present(e render.Engine, p render.Point) {
+	w.body.Present(e, p)
+}
+
+// AddItem quickly adds an item to a menu.
+func (w *Menu) AddItem(label string, command func()) *MenuItem {
+	menu := NewMenuItem(label, command)
+	w.Pack(menu)
+	return menu
+}
+
+// Pack a menu item onto the menu.
+func (w *Menu) Pack(item *MenuItem) {
+	w.body.Pack(item, Pack{
+		Anchor: NE,
+		// Expand: true,
+		// Padding: 8,
+		FillX: true,
+	})
+}
+
+// MenuItem is an item in a Menu.
+type MenuItem struct {
+	Button
+	Label       string
+	Accelerator string
+	Command     func()
+	button      *Button
+}
+
+// NewMenuItem creates a new menu item.
+func NewMenuItem(label string, command func()) *MenuItem {
+	w := &MenuItem{
+		Label:   label,
+		Command: command,
+	}
+	w.IDFunc(func() string {
+		return fmt.Sprintf("MenuItem<%s>", w.Label)
+	})
+
+	font := DefaultFont
+	font.Color = render.White
+	font.PadX = 12
+	w.Button.child = NewLabel(Label{
+		Text: label,
+		Font: font,
+	})
+	w.Button.Configure(Config{
+		Background: render.Blue,
+	})
+
+	w.Button.Handle(Click, func(p render.Point) {
+		w.Command()
+	})
+
+	// Assign the button
+	return w
+}