(原) ebiten，用golang做一个桌面宠物

ebiten 是golang的跨平台2D游戏引擎。以下是它自带的示例，运行程序后，会在屏幕上显示一个来回走动的golang宠物。
代码中有一点特殊的是，它居然用了两个init，不过看起来它只是顺序运行了而已，并没有什么问题。
不一定真的用它做游戏，可以把它的原理作为软件的启动界面是比较酷的。

//go:build example
// +build example

package main

import (
	"bytes"
	"image"
	_ "image/png"
	"log"
	"math/rand"
	"time"

	"github.com/hajimehoshi/ebiten/v2"
	rmascot "github.com/hajimehoshi/ebiten/v2/examples/resources/images/mascot"
)

const (
	width  = 200
	height = 200
)

var (
	gopher1 *ebiten.Image
	gopher2 *ebiten.Image
	gopher3 *ebiten.Image
)

func init() {
	// Decode an image from the image file's byte slice.
	// Now the byte slice is generated with //go:generate for Go 1.15 or older.
	// If you use Go 1.16 or newer, it is strongly recommended to use //go:embed to embed the image file.
	// See https://pkg.go.dev/embed for more details.
	img1, _, err := image.Decode(bytes.NewReader(rmascot.Out01_png))
	if err != nil {
		log.Fatal(err)
	}
	gopher1 = ebiten.NewImageFromImage(img1)

	img2, _, err := image.Decode(bytes.NewReader(rmascot.Out02_png))
	if err != nil {
		log.Fatal(err)
	}
	gopher2 = ebiten.NewImageFromImage(img2)

	img3, _, err := image.Decode(bytes.NewReader(rmascot.Out03_png))
	if err != nil {
		log.Fatal(err)
	}
	gopher3 = ebiten.NewImageFromImage(img3)
}

func init() {
	rand.Seed(time.Now().UnixNano())
}

type mascot struct {
	x16  int
	y16  int
	vx16 int
	vy16 int

	count int
}

func (m *mascot) Layout(outsideWidth, outsideHeight int) (int, int) {
	return width, height
}

func (m *mascot) Update() error {
	m.count++

	sw, sh := ebiten.ScreenSizeInFullscreen()
	ebiten.SetWindowPosition(m.x16/16, m.y16/16+sh-height)

	if m.vx16 == 0 {
		m.vx16 = 64
	}
	m.x16 += m.vx16
	if m.x16/16 > sw-width && m.vx16 > 0 {
		m.vx16 = -64
	}
	if m.x16 <= 0 && m.vx16 < 0 {
		m.vx16 = 64
	}

	// Accelarate the mascot in the Y direction.
	m.vy16 += 8
	m.y16 += m.vy16

	// If the mascot is on the ground, stop it in the Y direction.
	if m.y16 >= 0 {
		m.y16 = 0
		m.vy16 = 0
	}

	// If the mascto is on the ground, cause an action in random.
	if rand.Intn(60) == 0 && m.y16 == 0 {
		switch rand.Intn(2) {
		case 0:
			// Jump.
			m.vy16 = -240
		case 1:
			// Turn.
			m.vx16 = -m.vx16
		}
	}
	return nil
}

func (m *mascot) Draw(screen *ebiten.Image) {
	img := gopher1
	if m.y16 == 0 {
		switch (m.count / 3) % 4 {
		case 0:
			img = gopher1
		case 1, 3:
			img = gopher2
		case 2:
			img = gopher3
		}
	}
	op := &ebiten.DrawImageOptions{}
	if m.vx16 < 0 {
		op.GeoM.Scale(-1, 1)
		op.GeoM.Translate(width, 0)
	}
	screen.DrawImage(img, op)
}

func main() {
	ebiten.SetScreenTransparent(true)
	ebiten.SetWindowDecorated(false)
	ebiten.SetWindowFloating(true)
	ebiten.SetWindowSize(width, height)
	if err := ebiten.RunGame(&mascot{}); err != nil {
		log.Fatal(err)
	}
}