diff --git a/.gitignore b/.gitignore
index 538c8c553..4392f62e4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,4 @@
 .DS_Store
 *~
+*.aar
+*.apk
diff --git a/internal/loop/run.go b/internal/loop/run.go
index 989896348..a458339a8 100644
--- a/internal/loop/run.go
+++ b/internal/loop/run.go
@@ -104,6 +104,7 @@ func Run(g GraphicsContext, width, height, scale int, title string, fps int) err
 	if err := ui.CurrentUI().Start(width, height, scale, title); err != nil {
 		return err
 	}
+	// TODO: Use the error value
 	defer ui.CurrentUI().Terminate()
 
 	frames := 0
@@ -143,7 +144,9 @@ func Run(g GraphicsContext, width, height, scale int, title string, fps int) err
 						return err
 					}
 				}
-				ui.CurrentUI().SwapBuffers()
+				if err := ui.CurrentUI().SwapBuffers(); err != nil {
+					return err
+				}
 				beforeForUpdate += int64(tt) * int64(time.Second) / int64(fps)
 				frames++
 			}
diff --git a/internal/ui/ui.go b/internal/ui/ui.go
new file mode 100644
index 000000000..81e6b0b33
--- /dev/null
+++ b/internal/ui/ui.go
@@ -0,0 +1,25 @@
+// Copyright 2015 Hajime Hoshi
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ui
+
+type UserInterface interface {
+	Start(width, height, scale int, title string) error
+	Update() (interface{}, error)
+	SwapBuffers() error
+	Terminate() error
+	ScreenScale() int
+	SetScreenSize(width, height int) bool
+	SetScreenScale(scale int) bool
+}
diff --git a/internal/ui/ui_glfw.go b/internal/ui/ui_glfw.go
index 0aee5adb2..9fe9a0fdf 100644
--- a/internal/ui/ui_glfw.go
+++ b/internal/ui/ui_glfw.go
@@ -27,7 +27,7 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/graphics/opengl"
 )
 
-type UserInterface struct {
+type userInterface struct {
 	window           *glfw.Window
 	width            int
 	height           int
@@ -39,9 +39,9 @@ type UserInterface struct {
 	sizeChanged      bool
 }
 
-var currentUI *UserInterface
+var currentUI *userInterface
 
-func CurrentUI() *UserInterface {
+func CurrentUI() UserInterface {
 	return currentUI
 }
 
@@ -62,7 +62,7 @@ func initialize() (*opengl.Context, error) {
 		return nil, err
 	}
 
-	u := &UserInterface{
+	u := &userInterface{
 		window:      window,
 		funcs:       make(chan func()),
 		sizeChanged: true,
@@ -98,7 +98,7 @@ func Main() error {
 	return currentUI.main()
 }
 
-func (u *UserInterface) main() error {
+func (u *userInterface) main() error {
 	// TODO: Check this is done on the main thread.
 	for f := range u.funcs {
 		f()
@@ -106,7 +106,7 @@ func (u *UserInterface) main() error {
 	return nil
 }
 
-func (u *UserInterface) runOnMainThread(f func()) {
+func (u *userInterface) runOnMainThread(f func()) {
 	if u.funcs == nil {
 		// already closed
 		return
@@ -119,7 +119,7 @@ func (u *UserInterface) runOnMainThread(f func()) {
 	<-ch
 }
 
-func (u *UserInterface) SetScreenSize(width, height int) bool {
+func (u *userInterface) SetScreenSize(width, height int) bool {
 	r := false
 	u.runOnMainThread(func() {
 		r = u.setScreenSize(width, height, u.scale)
@@ -127,7 +127,7 @@ func (u *UserInterface) SetScreenSize(width, height int) bool {
 	return r
 }
 
-func (u *UserInterface) SetScreenScale(scale int) bool {
+func (u *userInterface) SetScreenScale(scale int) bool {
 	r := false
 	u.runOnMainThread(func() {
 		r = u.setScreenSize(u.width, u.height, scale)
@@ -135,7 +135,7 @@ func (u *UserInterface) SetScreenScale(scale int) bool {
 	return r
 }
 
-func (u *UserInterface) ScreenScale() int {
+func (u *userInterface) ScreenScale() int {
 	s := 0
 	u.runOnMainThread(func() {
 		s = u.scale
@@ -143,7 +143,7 @@ func (u *UserInterface) ScreenScale() int {
 	return s
 }
 
-func (u *UserInterface) Start(width, height, scale int, title string) error {
+func (u *userInterface) Start(width, height, scale int, title string) error {
 	var err error
 	u.runOnMainThread(func() {
 		m := glfw.GetPrimaryMonitor()
@@ -174,20 +174,20 @@ func (u *UserInterface) Start(width, height, scale int, title string) error {
 	return err
 }
 
-func (u *UserInterface) windowScale() int {
+func (u *userInterface) windowScale() int {
 	return u.scale * int(u.deviceScale)
 }
 
-func (u *UserInterface) actualScreenScale() int {
+func (u *userInterface) actualScreenScale() int {
 	return u.windowScale() * u.framebufferScale
 }
 
-func (u *UserInterface) pollEvents() error {
+func (u *userInterface) pollEvents() error {
 	glfw.PollEvents()
 	return currentInput.update(u.window, u.windowScale())
 }
 
-func (u *UserInterface) Update() (interface{}, error) {
+func (u *userInterface) Update() (interface{}, error) {
 	shouldClose := false
 	u.runOnMainThread(func() {
 		shouldClose = u.window.ShouldClose()
@@ -237,21 +237,23 @@ func (u *UserInterface) Update() (interface{}, error) {
 	return RenderEvent{}, nil
 }
 
-func (u *UserInterface) Terminate() {
+func (u *userInterface) Terminate() error {
 	u.runOnMainThread(func() {
 		glfw.Terminate()
 	})
 	close(u.funcs)
 	u.funcs = nil
+	return nil
 }
 
-func (u *UserInterface) SwapBuffers() {
+func (u *userInterface) SwapBuffers() error {
 	u.runOnMainThread(func() {
 		u.swapBuffers()
 	})
+	return nil
 }
 
-func (u *UserInterface) swapBuffers() {
+func (u *userInterface) swapBuffers() {
 	// The bound framebuffer must be the default one (0) before swapping buffers.
 	u.context.BindZeroFramebuffer()
 	u.context.RunOnContextThread(func() error {
@@ -260,7 +262,7 @@ func (u *UserInterface) swapBuffers() {
 	})
 }
 
-func (u *UserInterface) setScreenSize(width, height, scale int) bool {
+func (u *userInterface) setScreenSize(width, height, scale int) bool {
 	if u.width == width && u.height == height && u.scale == scale {
 		return false
 	}
diff --git a/internal/ui/ui_js.go b/internal/ui/ui_js.go
index 801215911..74a0740ae 100644
--- a/internal/ui/ui_js.go
+++ b/internal/ui/ui_js.go
@@ -24,36 +24,19 @@ import (
 	"github.com/hajimehoshi/ebiten/internal/graphics/opengl"
 )
 
-func (u *UserInterface) SetScreenSize(width, height int) bool {
-	return u.setScreenSize(width, height, u.scale)
-}
-
-func (u *UserInterface) SetScreenScale(scale int) bool {
-	width, height := u.size()
-	return u.setScreenSize(width, height, scale)
-}
-
-func (u *UserInterface) ScreenScale() int {
-	return u.scale
-}
-
-func (u *UserInterface) ActualScreenScale() int {
-	return u.scale * int(u.deviceScale)
-}
-
 var canvas *js.Object
 
-type UserInterface struct {
+type userInterface struct {
 	scale       int
 	deviceScale float64
 	sizeChanged bool
 }
 
-var currentUI = &UserInterface{
+var currentUI = &userInterface{
 	sizeChanged: true,
 }
 
-func CurrentUI() *UserInterface {
+func CurrentUI() UserInterface {
 	return currentUI
 }
 
@@ -80,7 +63,24 @@ func vsync() {
 	<-ch
 }
 
-func (u *UserInterface) Update() (interface{}, error) {
+func (u *userInterface) SetScreenSize(width, height int) bool {
+	return u.setScreenSize(width, height, u.scale)
+}
+
+func (u *userInterface) SetScreenScale(scale int) bool {
+	width, height := u.size()
+	return u.setScreenSize(width, height, scale)
+}
+
+func (u *userInterface) ScreenScale() int {
+	return u.scale
+}
+
+func (u *userInterface) ActualScreenScale() int {
+	return u.scale * int(u.deviceScale)
+}
+
+func (u *userInterface) Update() (interface{}, error) {
 	currentInput.UpdateGamepads()
 	if u.sizeChanged {
 		u.sizeChanged = false
@@ -96,15 +96,17 @@ func (u *UserInterface) Update() (interface{}, error) {
 	return RenderEvent{}, nil
 }
 
-func (u *UserInterface) Terminate() {
+func (u *userInterface) Terminate() error {
 	// Do nothing.
+	return nil
 }
 
-func (u *UserInterface) SwapBuffers() {
+func (u *userInterface) SwapBuffers() error {
 	vsync()
 	for !shown() {
 		vsync()
 	}
+	return nil
 }
 
 func initialize() (*opengl.Context, error) {
@@ -252,7 +254,7 @@ func Main() error {
 	return nil
 }
 
-func (u *UserInterface) Start(width, height, scale int, title string) error {
+func (u *userInterface) Start(width, height, scale int, title string) error {
 	doc := js.Global.Get("document")
 	doc.Set("title", title)
 	u.setScreenSize(width, height, scale)
@@ -260,7 +262,7 @@ func (u *UserInterface) Start(width, height, scale int, title string) error {
 	return nil
 }
 
-func (u *UserInterface) size() (width, height int) {
+func (u *userInterface) size() (width, height int) {
 	a := int(u.ActualScreenScale())
 	if a == 0 {
 		// a == 0 only on the initial state.
@@ -271,7 +273,7 @@ func (u *UserInterface) size() (width, height int) {
 	return
 }
 
-func (u *UserInterface) setScreenSize(width, height, scale int) bool {
+func (u *userInterface) setScreenSize(width, height, scale int) bool {
 	w, h := u.size()
 	s := u.scale
 	if w == width && h == height && s == scale {
diff --git a/internal/ui/ui_mobile.go b/internal/ui/ui_mobile.go
index f675ff6ba..40485378e 100644
--- a/internal/ui/ui_mobile.go
+++ b/internal/ui/ui_mobile.go
@@ -29,37 +29,37 @@ func Main() error {
 	return nil
 }
 
-type UserInterface struct {
+type userInterface struct {
 }
 
-func CurrentUI() *UserInterface {
+func CurrentUI() UserInterface {
 	return nil
 }
 
-func (u *UserInterface) Start(width, height, scale int, title string) error {
+func (u *userInterface) Start(width, height, scale int, title string) error {
 	return nil
 }
 
-func (u *UserInterface) Terminate() error {
+func (u *userInterface) Terminate() error {
 	return nil
 }
 
-func (u *UserInterface) Update() (interface{}, error) {
+func (u *userInterface) Update() (interface{}, error) {
 	return nil, nil
 }
 
-func (u *UserInterface) SwapBuffers() error {
+func (u *userInterface) SwapBuffers() error {
 	return nil
 }
 
-func (u *UserInterface) SetScreenSize(width, height int) bool {
+func (u *userInterface) SetScreenSize(width, height int) bool {
 	return false
 }
 
-func (u *UserInterface) SetScreenScale(scale int) bool {
+func (u *userInterface) SetScreenScale(scale int) bool {
 	return false
 }
 
-func (u *UserInterface) ScreenScale() int {
+func (u *userInterface) ScreenScale() int {
 	return 1
 }