diff --git a/cmd/view-obj/main.go b/cmd/view-obj/main.go
index 6b363e1..e7a1915 100644
--- a/cmd/view-obj/main.go
+++ b/cmd/view-obj/main.go
@@ -9,7 +9,6 @@ import (
 	"path/filepath"
 
 	"github.com/hajimehoshi/ebiten"
-	"github.com/hajimehoshi/ebiten/inpututil"
 
 	"ur.gs/ordoor/internal/conv"
 	"ur.gs/ordoor/internal/data"
@@ -22,14 +21,15 @@ var (
 )
 
 type env struct {
-	obj   *conv.Object
-	state *state
+	obj  *conv.Object
+	step int
+
+	state     state
+	lastState state
 }
 
 type state struct {
-	step      int
 	spriteIdx int
-	maxSprite int
 
 	zoom   float64
 	origin image.Point
@@ -53,18 +53,30 @@ func main() {
 		log.Fatalf("Failed to convert %s: %v", *objFile, err)
 	}
 
-	state := &state{
-		zoom:      6.0,
-		origin:    image.Point{0, 0},
-		maxSprite: len(obj.Sprites) - 1,
+	state := state{
+		zoom:   6.0,
+		origin: image.Point{0, 0},
+	}
+
+	env := &env{
+		obj:       obj,
+		state:     state,
+		lastState: state,
 	}
-	env := &env{obj: obj, state: state}
 
 	win, err := ui.NewWindow("View Object: " + *objFile)
 	if err != nil {
 		log.Fatal(err)
 	}
 
+	win.OnKeyUp(ebiten.KeyMinus, env.changeSprite(-1))
+	win.OnKeyUp(ebiten.KeyEqual, env.changeSprite(+1))
+	win.OnKeyUp(ebiten.KeyLeft, env.changeOrigin(+4, +0))
+	win.OnKeyUp(ebiten.KeyRight, env.changeOrigin(-4, +0))
+	win.OnKeyUp(ebiten.KeyUp, env.changeOrigin(+0, +4))
+	win.OnKeyUp(ebiten.KeyDown, env.changeOrigin(+0, -4))
+	win.OnMouseWheel(env.changeZoom)
+
 	// The main thread now belongs to ebiten
 	if err := win.Run(env.Update, env.Draw); err != nil {
 		log.Fatal(err)
@@ -72,32 +84,23 @@ func main() {
 }
 
 func (e *env) Update() error {
-	oldState := e.state
-	state := oldState.runStep()
-
-	if oldState.step == 0 || *oldState != *state {
+	if e.step == 0 || e.lastState != e.state {
 		log.Printf(
 			"new state: numSprites=%d sprite=%d zoom=%.2f, origin=%+v",
 			len(e.obj.Sprites),
-			state.spriteIdx,
-			state.zoom,
-			state.origin,
+			e.state.spriteIdx,
+			e.state.zoom,
+			e.state.origin,
 		)
 	}
 
-	state.step += 1
-	e.state = state
+	// This should be the final action
+	e.step += 1
+	e.lastState = e.state
 
 	return nil
 }
 
-func (s *state) runStep() *state {
-	newState := *s
-	newState.handleKeys()
-
-	return &newState
-}
-
 func (e *env) Draw(screen *ebiten.Image) error {
 	sprite := e.obj.Sprites[e.state.spriteIdx]
 
@@ -108,36 +111,28 @@ func (e *env) Draw(screen *ebiten.Image) error {
 	return screen.DrawImage(sprite.Image, &ebiten.DrawImageOptions{GeoM: cam})
 }
 
-func (s *state) handleKeys() {
-	if inpututil.IsKeyJustReleased(ebiten.KeyMinus) {
-		if s.spriteIdx > 0 {
-			s.spriteIdx -= 1
+func (e *env) changeSprite(by int) func() {
+	return func() {
+		e.state.spriteIdx += by
+
+		if e.state.spriteIdx < 0 {
+			e.state.spriteIdx = 0
+		}
+
+		if e.state.spriteIdx > len(e.obj.Sprites)-1 {
+			e.state.spriteIdx = len(e.obj.Sprites) - 1
 		}
 	}
-
-	if inpututil.IsKeyJustReleased(ebiten.KeyEqual) {
-		if s.spriteIdx < s.maxSprite {
-			s.spriteIdx += 1
-		}
-	}
-
-	if ebiten.IsKeyPressed(ebiten.KeyLeft) {
-		s.origin.X += 4
-	}
-
-	if ebiten.IsKeyPressed(ebiten.KeyRight) {
-		s.origin.X -= 4
-	}
-
-	if ebiten.IsKeyPressed(ebiten.KeyUp) {
-		s.origin.Y += 4
-	}
-
-	if ebiten.IsKeyPressed(ebiten.KeyDown) {
-		s.origin.Y -= 4
-	}
-
-	// Zoom in and out with the mouse wheel
-	_, wheelY := ebiten.Wheel()
-	s.zoom *= math.Pow(1.2, wheelY)
+}
+
+func (e *env) changeOrigin(byX, byY int) func() {
+	return func() {
+		e.state.origin.X += byX
+		e.state.origin.Y += byY
+	}
+}
+
+func (e *env) changeZoom(_, y float64) {
+	// Zoom in and out with the mouse wheel
+	e.state.zoom *= math.Pow(1.2, y)
 }
diff --git a/internal/ui/window.go b/internal/ui/window.go
index 976c671..c6e3ea2 100644
--- a/internal/ui/window.go
+++ b/internal/ui/window.go
@@ -4,6 +4,7 @@ import (
 	"flag"
 
 	"github.com/hajimehoshi/ebiten"
+	"github.com/hajimehoshi/ebiten/inpututil"
 )
 
 var (
@@ -12,8 +13,9 @@ var (
 )
 
 type Window struct {
-	Title            string
-	KeyEventHandlers map[ebiten.Key][]func()
+	Title             string
+	KeyUpHandlers     map[ebiten.Key]func()
+	MouseWheelHandler func(float64, float64)
 
 	// User-provided update actions
 	updateFn func() error
@@ -25,13 +27,18 @@ type Window struct {
 // ebiten assumes a single window, so only call this once...
 func NewWindow(title string) (*Window, error) {
 	return &Window{
-		Title:            title,
-		KeyEventHandlers: make(map[ebiten.Key][]func()),
+		Title:         title,
+		KeyUpHandlers: make(map[ebiten.Key]func()),
 	}, nil
 }
 
+// TODO: multiple handlers for the same key?
 func (w *Window) OnKeyUp(key ebiten.Key, f func()) {
+	w.KeyUpHandlers[key] = f
+}
 
+func (w *Window) OnMouseWheel(f func(x, y float64)) {
+	w.MouseWheelHandler = f
 }
 
 func (w *Window) run(screen *ebiten.Image) error {
@@ -40,6 +47,20 @@ func (w *Window) run(screen *ebiten.Image) error {
 	}
 
 	// Process keys
+	// TODO: efficient set operations
+
+	for key, cb := range w.KeyUpHandlers {
+		if inpututil.IsKeyJustReleased(key) {
+			cb()
+		}
+	}
+
+	if w.MouseWheelHandler != nil {
+		x, y := ebiten.Wheel()
+		if x != 0 || y != 0 {
+			w.MouseWheelHandler(x, y)
+		}
+	}
 
 	if !ebiten.IsDrawingSkipped() {
 		if err := w.drawFn(screen); err != nil {