added SetMode for windows

go.mod

@@ -4,6 +4,7 @@ go 1.16
 
 require (
 	github.com/smartystreets/goconvey v1.6.4 // indirect
+	golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a
 	gopkg.in/ini.v1 v1.62.0
 	gopkg.in/yaml.v2 v2.4.0
 )

go.sum

@@ -8,6 +8,7 @@ github.com/smartystreets/goconvey v1.6.4 h1:fv0U8FUIMPNf1L9lnHLvLhgicrIVChEkdzIK
 github.com/smartystreets/goconvey v1.6.4/go.mod h1:syvi0/a8iFYH4r/RixwvyeAJjdLS9QV7WQ/tjFTllLA=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/net v0.0.0-20190311183353-d8887717615a/go.mod h1:t9HGtf8HONx5eT2rtn7q6eTqICYqUVnKs3thJo3Qplg=
+golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a h1:1BGLXjeY4akVXGgbC9HugT3Jv3hCI0z56oJR5vAMgBU=
 golang.org/x/sys v0.0.0-20190215142949-d0b11bdaac8a/go.mod h1:STP8DvDyc/dI5b8T5hshtkjS+E42TnysNCUPdjciGhY=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/tools v0.0.0-20190328211700-ab21143f2384/go.mod h1:LCzVGOaR6xXOjkQ3onu1FJEFr0SW1gC7cKk1uF8kGRs=

main.go

@@ -8,6 +8,17 @@ import (
 	"path/filepath"
 )
 
+type Mode int
+
+const (
+	Center Mode = iota
+	Crop
+	Fit
+	Span
+	Stretch
+	Tile
+)
+
 // Desktop contains the current desktop environment on Linux.
 // Empty string on all other operating systems.
 var Desktop = os.Getenv("XDG_CURRENT_DESKTOP")

windows.go

@@ -3,6 +3,7 @@
 package wallpaper
 
 import (
+	"golang.org/x/sys/windows/registry"
 	"os"
 	"strings"
 	"syscall"
@@ -57,6 +58,54 @@ func SetFromFile(filename string) error {
 	return nil
 }
 
+// SetMode sets the wallpaper mode.
+func SetMode(mode Mode) error {
+	key, _, err := registry.CreateKey(registry.CURRENT_USER, "Control Panel\\Desktop", registry.SET_VALUE)
+	if err != nil {
+		return err
+	}
+	defer key.Close()
+
+	var tile string
+	if mode == Tile {
+		tile = "1"
+	} else {
+		tile = "0"
+	}
+	err = key.SetStringValue("TileWallpaper", tile)
+	if err != nil {
+		return err
+	}
+
+	var style string
+	switch mode {
+	case Center, Tile:
+		style = "0"
+	case Fit:
+		style = "6"
+	case Span:
+		style = "22"
+	case Stretch:
+		style = "2"
+	case Crop:
+		style = "10"
+	default:
+		panic("invalid wallpaper mode")
+	}
+	err = key.SetStringValue("WallpaperStyle", style)
+	if err != nil {
+		return err
+	}
+
+	// updates wallpaper
+	path, err := Get()
+	if err != nil {
+		return err
+	}
+
+	return SetFromFile(path)
+}
+
 func getCacheDir() (string, error) {
 	return os.TempDir(), nil
 }