why/main.go

package main

import (
	_ "embed"
	"io"
	"log"
	"os"
	"path/filepath"
	"slices"

	tk "modernc.org/tk9.0"
)

// this is a default image
//
//go:embed noise.png
var noise []byte

var validFileTypes = []tk.FileType{
	{TypeName: "BMP", Extensions: []string{".bmp"}},
	{TypeName: "JPEG", Extensions: []string{".jpg", ".jpeg"}},
	{TypeName: "GIF", Extensions: []string{".gif"}},
	{TypeName: "PNG", Extensions: []string{".png"}},
	{TypeName: "SVG", Extensions: []string{".svg"}},
	{TypeName: "TGA", Extensions: []string{".tga"}},
	{TypeName: "TIFF", Extensions: []string{".tiff"}},
}

var metaActive bool

var directoryState struct {
	currentDirectory string
	currentFile      string
	images           []string
}

func isImage(entry os.DirEntry) bool {
	if entry.IsDir() {
		return false
	}
	ext := filepath.Ext(entry.Name())
	for _, ft := range validFileTypes {
		if slices.Contains(ft.Extensions, ext) {
			return true
		}
	}
	return false
}

func newFileInDirectory(img *tk.LabelWidget) func() {
	return func() {
		files := tk.GetOpenFile(tk.Filetypes(validFileTypes))
		if len(files) < 1 || files[0] == "" {
			log.Println("no file chosen")
			return
		}
		directoryState.currentFile = filepath.Base(files[0])
		directoryState.currentDirectory = filepath.Dir(files[0])
		dirfiles, err := os.ReadDir(directoryState.currentDirectory)
		if err != nil {
			log.Println(err)
			return
		}
		directoryState.images = []string{}
		for _, v := range dirfiles {
			if isImage(v) {
				directoryState.images = append(directoryState.images, v.Name())
			}
		}
		updateImage(files[0], img)
	}
}

func newDirectory(img *tk.LabelWidget) func() {
	return func() {
		dir := tk.ChooseDirectory()
		if dir == "" {
			log.Println("no directory chosen")
			return
		}
		directoryState.currentDirectory = dir
		dirfiles, err := os.ReadDir(directoryState.currentDirectory)
		if err != nil {
			log.Println(err)
			return
		}
		directoryState.images = []string{}
		for _, v := range dirfiles {
			if isImage(v) {
				directoryState.images = append(directoryState.images, v.Name())
			}
		}
		updateImage(filepath.Join(directoryState.currentDirectory, directoryState.images[0]), img)
	}
}

func updateImage(file string, img *tk.LabelWidget) {
	f, err := os.Open(file)
	if err != nil {
		log.Println(err.Error())
		return
	}
	i, err := io.ReadAll(f)
	if err != nil {
		log.Println(err.Error())
		return
	}
	img.Configure(tk.Image(tk.NewPhoto(tk.Data(i))))
	directoryState.currentFile = filepath.Base(file)
}

func main() {
	img := tk.Label(tk.Image(tk.NewPhoto(tk.Data(noise))))

	menubar := tk.Menu()
	fileMenu := menubar.Menu()
	fileMenu.AddCommand(tk.Lbl("Open File"), tk.Underline(0), tk.Accelerator("Meta+O"), tk.Command(newFileInDirectory(img)))
	fileMenu.AddCommand(tk.Lbl("Open Directory"), tk.Underline(0), tk.Command(newDirectory(img)))
	menubar.AddCascade(tk.Lbl("File"), tk.Underline(0), tk.Mnu(fileMenu))

	// TODO: if someone presses the Meta key again after the openfile dialog box closes,
	// it triggers a _release_ event instead of a press event. The second time afterward, it works correctly.
	tk.Bind(tk.App, "<KeyPress>", tk.Command(func(e *tk.Event) {
		curr := slices.Index(directoryState.images, directoryState.currentFile)
		switch e.Keysym {
		case ".":
			log.Printf("state: %+v", directoryState)
		case "Meta_L", "Meta_R":
			metaActive = true
		case "o":
			if metaActive {
				newFileInDirectory(img)()
			}
		case "a":
			if metaActive {
				tk.Toplevel()
			}
		case "Up":
			if curr > 0 {
				updateImage(filepath.Join(directoryState.currentDirectory, directoryState.images[curr-1]), img)
			}
		case "Down":
			if curr < len(directoryState.images)-1 && curr != -1 {
				updateImage(filepath.Join(directoryState.currentDirectory, directoryState.images[curr+1]), img)
			}
		}
	}))
	tk.Bind(tk.App, "<KeyRelease>", tk.Command(func(e *tk.Event) {
		switch e.Keysym {
		case "Meta_L", "Meta_R":
			metaActive = false
		}
	}))
	// todo: resize image based on scroll events
	// tk.Bind(tk.App, "<TouchpadScroll>", tk.Command(func(e *tk.Event) {
	// 	log.Printf("%v, %v", int16(e.Delta>>16), int16(e.Delta&0xFFFF))
	// }))

	tk.Pack(img)
	tk.App.Configure(tk.Mnu(menubar)).Center().Wait()
}