// 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.

// +build darwin freebsd linux windows
// +build !js
// +build !android
// +build !ios

package glfw

import (
	"context"
	"fmt"
	"image"
	"os"
	"runtime"
	"sync"
	"time"
	"unsafe"

	"github.com/hajimehoshi/ebiten/internal/devicescale"
	"github.com/hajimehoshi/ebiten/internal/driver"
	"github.com/hajimehoshi/ebiten/internal/glfw"
	"github.com/hajimehoshi/ebiten/internal/hooks"
	"github.com/hajimehoshi/ebiten/internal/thread"
)

type UserInterface struct {
	context driver.UIContext
	title   string
	window  *glfw.Window

	// windowWidth and windowHeight represents a window size.
	// The unit is device-dependent pixels.
	windowWidth  int
	windowHeight int

	running             bool
	toChangeSize        bool
	origPosX            int
	origPosY            int
	runnableOnUnfocused bool
	vsync               bool

	lastDeviceScaleFactor float64

	initMonitor              *glfw.Monitor
	initTitle                string
	initVsync                bool
	initFullscreenWidthInDP  int
	initFullscreenHeightInDP int
	initFullscreen           bool
	initCursorMode           driver.CursorMode
	initWindowDecorated      bool
	initWindowResizable      bool
	initWindowPositionXInDP  int
	initWindowPositionYInDP  int
	initWindowWidthInDP      int
	initWindowHeightInDP     int
	initWindowFloating       bool
	initWindowMaximized      bool
	initScreenTransparent    bool
	initIconImages           []image.Image

	vsyncInited bool

	reqWidth  int
	reqHeight int

	input   Input
	iwindow window

	t *thread.Thread
	m sync.RWMutex
}

const (
	maxInt     = int(^uint(0) >> 1)
	minInt     = -maxInt - 1
	invalidPos = minInt
)

var (
	theUI = &UserInterface{
		origPosX:                invalidPos,
		origPosY:                invalidPos,
		initVsync:               true,
		initCursorMode:          driver.CursorModeVisible,
		initWindowDecorated:     true,
		initWindowPositionXInDP: invalidPos,
		initWindowPositionYInDP: invalidPos,
		initWindowWidthInDP:     640,
		initWindowHeightInDP:    480,
	}
)

func init() {
	theUI.input.ui = theUI
	theUI.iwindow.ui = theUI
}

func Get() *UserInterface {
	return theUI
}

func init() {
	hideConsoleWindowOnWindows()
	if err := initialize(); err != nil {
		panic(err)
	}
	glfw.SetMonitorCallback(func(monitor *glfw.Monitor, event glfw.PeripheralEvent) {
		cacheMonitors()
	})
	cacheMonitors()
}

func initialize() error {
	if err := glfw.Init(); err != nil {
		return err
	}
	glfw.WindowHint(glfw.Visible, glfw.False)

	// Create a window to set the initial monitor.
	w, err := glfw.CreateWindow(16, 16, "", nil, nil)
	if err != nil {
		return err
	}
	if w == nil {
		// This can happen on Windows Remote Desktop (#903).
		panic("glfw: glfw.CreateWindow must not return nil")
	}

	// Create a window and leave it as it is: this affects the result of currentMonitorFromPosition.
	theUI.window = w
	theUI.initMonitor = theUI.currentMonitor()
	v := theUI.initMonitor.GetVideoMode()
	theUI.initFullscreenWidthInDP = int(theUI.toDeviceIndependentPixel(float64(v.Width)))
	theUI.initFullscreenHeightInDP = int(theUI.toDeviceIndependentPixel(float64(v.Height)))

	return nil
}

type cachedMonitor struct {
	m  *glfw.Monitor
	vm *glfw.VidMode
	// Pos of monitor in virtual coords
	x int
	y int
}

// monitors is the monitor list cache for desktop glfw compile targets.
// populated by 'cacheMonitors' which is called on init and every
// monitor config change event.
//
// monitors must be manipulated on the main thread.
var monitors []*cachedMonitor

func cacheMonitors() {
	monitors = nil
	ms := glfw.GetMonitors()
	for _, m := range ms {
		x, y := m.GetPos()
		monitors = append(monitors, &cachedMonitor{
			m:  m,
			vm: m.GetVideoMode(),
			x:  x,
			y:  y,
		})
	}
}

// getCachedMonitor returns a monitor for the given window x/y
// returns false if monitor is not found.
//
// getCachedMonitor must be called on the main thread.
func getCachedMonitor(wx, wy int) (*cachedMonitor, bool) {
	for _, m := range monitors {
		if m.x <= wx && wx < m.x+m.vm.Width && m.y <= wy && wy < m.y+m.vm.Height {
			return m, true
		}
	}
	return nil, false
}

func (u *UserInterface) isRunning() bool {
	u.m.RLock()
	v := u.running
	u.m.RUnlock()
	return v
}

func (u *UserInterface) setRunning(running bool) {
	u.m.Lock()
	u.running = running
	u.m.Unlock()
}

func (u *UserInterface) getInitTitle() string {
	u.m.RLock()
	v := u.initTitle
	u.m.RUnlock()
	return v
}

func (u *UserInterface) setInitTitle(title string) {
	u.m.RLock()
	u.initTitle = title
	u.m.RUnlock()
}

func (u *UserInterface) isInitVsyncEnabled() bool {
	u.m.RLock()
	v := u.initVsync
	u.m.RUnlock()
	return v
}

func (u *UserInterface) isInitFullscreen() bool {
	u.m.RLock()
	v := u.initFullscreen
	u.m.RUnlock()
	return v
}

func (u *UserInterface) setInitFullscreen(initFullscreen bool) {
	u.m.Lock()
	u.initFullscreen = initFullscreen
	u.m.Unlock()
}

func (u *UserInterface) getInitCursorMode() driver.CursorMode {
	u.m.RLock()
	v := u.initCursorMode
	u.m.RUnlock()
	return v
}

func (u *UserInterface) setInitCursorMode(mode driver.CursorMode) {
	u.m.Lock()
	u.initCursorMode = mode
	u.m.Unlock()
}

func (u *UserInterface) isInitWindowDecorated() bool {
	u.m.RLock()
	v := u.initWindowDecorated
	u.m.RUnlock()
	return v
}

func (u *UserInterface) setInitWindowDecorated(decorated bool) {
	u.m.Lock()
	u.initWindowDecorated = decorated
	u.m.Unlock()
}

func (u *UserInterface) isRunnableOnUnfocused() bool {
	u.m.RLock()
	v := u.runnableOnUnfocused
	u.m.RUnlock()
	return v
}

func (u *UserInterface) setRunnableOnUnfocused(runnableOnUnfocused bool) {
	u.m.Lock()
	u.runnableOnUnfocused = runnableOnUnfocused
	u.m.Unlock()
}

func (u *UserInterface) isInitWindowResizable() bool {
	u.m.RLock()
	v := u.initWindowResizable
	u.m.RUnlock()
	return v
}

func (u *UserInterface) setInitWindowResizable(resizable bool) {
	u.m.Lock()
	u.initWindowResizable = resizable
	u.m.Unlock()
}

func (u *UserInterface) isInitScreenTransparent() bool {
	u.m.RLock()
	v := u.initScreenTransparent
	u.m.RUnlock()
	return v
}

func (u *UserInterface) setInitScreenTransparent(transparent bool) {
	u.m.RLock()
	u.initScreenTransparent = transparent
	u.m.RUnlock()
}

func (u *UserInterface) getInitIconImages() []image.Image {
	u.m.RLock()
	i := u.initIconImages
	u.m.RUnlock()
	return i
}

func (u *UserInterface) setInitIconImages(iconImages []image.Image) {
	u.m.Lock()
	u.initIconImages = iconImages
	u.m.Unlock()
}

func (u *UserInterface) getInitWindowPosition() (int, int) {
	u.m.RLock()
	defer u.m.RUnlock()
	if u.initWindowPositionXInDP != invalidPos && u.initWindowPositionYInDP != invalidPos {
		return u.initWindowPositionXInDP, u.initWindowPositionYInDP
	}
	return invalidPos, invalidPos
}

func (u *UserInterface) setInitWindowPosition(x, y int) {
	u.m.Lock()
	defer u.m.Unlock()

	u.initWindowPositionXInDP = x
	u.initWindowPositionYInDP = y
}

func (u *UserInterface) getInitWindowSize() (int, int) {
	u.m.Lock()
	w, h := u.initWindowWidthInDP, u.initWindowHeightInDP
	u.m.Unlock()
	return w, h
}

func (u *UserInterface) setInitWindowSize(width, height int) {
	u.m.Lock()
	u.initWindowWidthInDP, u.initWindowHeightInDP = width, height
	u.m.Unlock()
}

func (u *UserInterface) isInitWindowFloating() bool {
	u.m.Lock()
	f := u.initWindowFloating
	u.m.Unlock()
	return f
}

func (u *UserInterface) setInitWindowFloating(floating bool) {
	u.m.Lock()
	u.initWindowFloating = floating
	u.m.Unlock()
}

func (u *UserInterface) isInitWindowMaximized() bool {
	u.m.Lock()
	f := u.initWindowMaximized
	u.m.Unlock()
	return f
}

func (u *UserInterface) setInitWindowMaximized(floating bool) {
	u.m.Lock()
	u.initWindowMaximized = floating
	u.m.Unlock()
}

// toDeviceIndependentPixel must be called from the main thread.
func (u *UserInterface) toDeviceIndependentPixel(x float64) float64 {
	return x / u.glfwScale()
}

// toDeviceDependentPixel must be called from the main thread.
func (u *UserInterface) toDeviceDependentPixel(x float64) float64 {
	return x * u.glfwScale()
}

func (u *UserInterface) ScreenSizeInFullscreen() (int, int) {
	if !u.isRunning() {
		return u.initFullscreenWidthInDP, u.initFullscreenHeightInDP
	}

	var w, h int
	_ = u.t.Call(func() error {
		v := u.currentMonitor().GetVideoMode()
		w = int(u.toDeviceIndependentPixel(float64(v.Width)))
		h = int(u.toDeviceIndependentPixel(float64(v.Height)))
		return nil
	})
	return w, h
}

// isFullscreen must be called from the main thread.
func (u *UserInterface) isFullscreen() bool {
	if !u.isRunning() {
		panic("glfw: isFullscreen can't be called before the main loop starts")
	}
	return u.window.GetMonitor() != nil
}

func (u *UserInterface) IsFullscreen() bool {
	if !u.isRunning() {
		return u.isInitFullscreen()
	}
	b := false
	_ = u.t.Call(func() error {
		b = u.isFullscreen()
		return nil
	})
	return b
}

func (u *UserInterface) SetFullscreen(fullscreen bool) {
	if !u.isRunning() {
		u.setInitFullscreen(fullscreen)
		return
	}

	var update bool
	_ = u.t.Call(func() error {
		update = u.isFullscreen() != fullscreen
		return nil
	})
	if !update {
		return
	}

	var w, h int
	_ = u.t.Call(func() error {
		w, h = u.windowWidth, u.windowHeight
		return nil
	})
	u.setWindowSize(w, h, fullscreen)
}

func (u *UserInterface) IsFocused() bool {
	if !u.isRunning() {
		return false
	}

	var focused bool
	_ = u.t.Call(func() error {
		focused = u.window.GetAttrib(glfw.Focused) == glfw.True
		return nil
	})
	return focused
}

func (u *UserInterface) SetRunnableOnUnfocused(runnableOnUnfocused bool) {
	u.setRunnableOnUnfocused(runnableOnUnfocused)
}

func (u *UserInterface) IsRunnableOnUnfocused() bool {
	return u.isRunnableOnUnfocused()
}

func (u *UserInterface) SetVsyncEnabled(enabled bool) {
	if !u.isRunning() {
		// In general, m is used for locking init* values.
		// m is not used for updating vsync in setWindowSize so far, but
		// it should be OK since any goroutines can't reach here when
		// the game already starts and setWindowSize can be called.
		u.m.Lock()
		u.initVsync = enabled
		u.m.Unlock()
		return
	}
	_ = u.t.Call(func() error {
		if !u.vsyncInited {
			u.m.Lock()
			u.initVsync = enabled
			u.m.Unlock()
			return nil
		}
		u.vsync = enabled
		u.updateVsync()
		return nil
	})
}

func (u *UserInterface) IsVsyncEnabled() bool {
	if !u.isRunning() {
		return u.isInitVsyncEnabled()
	}
	var v bool
	_ = u.t.Call(func() error {
		if !u.vsyncInited {
			v = u.isInitVsyncEnabled()
			return nil
		}
		v = u.vsync
		return nil
	})
	return v
}

func (u *UserInterface) CursorMode() driver.CursorMode {
	if !u.isRunning() {
		return u.getInitCursorMode()
	}
	var v driver.CursorMode
	_ = u.t.Call(func() error {
		mode := u.window.GetInputMode(glfw.CursorMode)
		switch mode {
		case glfw.CursorNormal:
			v = driver.CursorModeVisible
		case glfw.CursorHidden:
			v = driver.CursorModeHidden
		case glfw.CursorDisabled:
			v = driver.CursorModeCaptured
		default:
			panic(fmt.Sprintf("invalid cursor mode: %d", mode))
		}
		return nil
	})
	return v
}

func (u *UserInterface) SetCursorMode(mode driver.CursorMode) {
	if !u.isRunning() {
		u.setInitCursorMode(mode)
		return
	}
	_ = u.t.Call(func() error {
		var c int
		switch mode {
		case driver.CursorModeVisible:
			c = glfw.CursorNormal
		case driver.CursorModeHidden:
			c = glfw.CursorHidden
		case driver.CursorModeCaptured:
			c = glfw.CursorDisabled
		default:
			panic(fmt.Sprintf("invalid cursor mode: %d", mode))
		}
		u.window.SetInputMode(glfw.CursorMode, c)
		return nil
	})
}

func (u *UserInterface) DeviceScaleFactor() float64 {
	if !u.isRunning() {
		return devicescale.GetAt(u.initMonitor.GetPos())
	}

	f := 0.0
	_ = u.t.Call(func() error {
		f = u.deviceScaleFactor()
		return nil
	})
	return f
}

// deviceScaleFactor must be called from the main thread.
func (u *UserInterface) deviceScaleFactor() float64 {
	// Before calling SetWindowPosition, the window's positin is not reliable.
	if u.iwindow.setPositionCalled {
		// Avoid calling monitor.GetPos if we have the monitor position cached already.
		if cm, ok := getCachedMonitor(u.window.GetPos()); ok {
			return devicescale.GetAt(cm.x, cm.y)
		}
	}
	return devicescale.GetAt(u.currentMonitor().GetPos())
}

func init() {
	// Lock the main thread.
	runtime.LockOSThread()
}

func (u *UserInterface) Run(uicontext driver.UIContext) error {
	u.context = uicontext

	// Initialize the main thread first so the thread is available at u.run (#809).
	u.t = thread.New()
	u.Graphics().SetThread(u.t)

	ctx, cancel := context.WithCancel(context.Background())

	ch := make(chan error, 1)
	go func() {
		defer cancel()
		defer close(ch)
		if err := u.run(); err != nil {
			ch <- err
		}
	}()

	u.setRunning(true)
	u.t.Loop(ctx)
	u.setRunning(false)
	return <-ch
}

func (u *UserInterface) RunWithoutMainLoop(context driver.UIContext) {
	panic("glfw: RunWithoutMainLoop is not implemented")
}

// createWindow creates a GLFW window.
//
// createWindow must be called from the main thread.
//
// createWindow does not set the position or size so far.
func (u *UserInterface) createWindow() error {
	if u.window != nil {
		panic("glfw: u.window must not exist at createWindow")
	}

	// As a start, create a window with temporary size to create OpenGL context thread.
	window, err := glfw.CreateWindow(16, 16, "", nil, nil)
	if err != nil {
		return err
	}
	u.window = window

	if u.Graphics().IsGL() {
		u.window.MakeContextCurrent()
	}

	u.window.SetInputMode(glfw.StickyMouseButtonsMode, glfw.True)
	u.window.SetInputMode(glfw.StickyKeysMode, glfw.True)

	mode := glfw.CursorNormal
	switch u.getInitCursorMode() {
	case driver.CursorModeHidden:
		mode = glfw.CursorHidden
	case driver.CursorModeCaptured:
		mode = glfw.CursorDisabled
	}
	u.window.SetInputMode(glfw.CursorMode, mode)
	u.window.SetTitle(u.title)
	// TODO: Set icons

	u.window.SetSizeCallback(func(_ *glfw.Window, width, height int) {
		if u.window.GetAttrib(glfw.Resizable) == glfw.False {
			return
		}
		if u.isFullscreen() {
			return
		}
		u.reqWidth = width
		u.reqHeight = height
	})

	return nil
}

func (u *UserInterface) run() error {
	if err := u.t.Call(func() error {
		// The window is created at initialize().
		u.window.Destroy()
		u.window = nil

		if u.Graphics().IsGL() {
			glfw.WindowHint(glfw.ContextVersionMajor, 2)
			glfw.WindowHint(glfw.ContextVersionMinor, 1)
		} else {
			glfw.WindowHint(glfw.ClientAPI, glfw.NoAPI)
		}

		decorated := glfw.False
		if u.isInitWindowDecorated() {
			decorated = glfw.True
		}
		glfw.WindowHint(glfw.Decorated, decorated)

		transparent := glfw.False
		if u.isInitScreenTransparent() {
			transparent = glfw.True
		}
		glfw.WindowHint(glfw.TransparentFramebuffer, transparent)
		u.Graphics().SetTransparent(u.isInitScreenTransparent())

		resizable := glfw.False
		if u.isInitWindowResizable() {
			resizable = glfw.True
		}
		glfw.WindowHint(glfw.Resizable, resizable)

		floating := glfw.False
		if u.isInitWindowFloating() {
			floating = glfw.True
		}
		glfw.WindowHint(glfw.Floating, floating)

		// Set the window visible explicitly or the application freezes on Wayland (#974).
		if os.Getenv("WAYLAND_DISPLAY") != "" {
			glfw.WindowHint(glfw.Visible, glfw.True)
		}

		if err := u.createWindow(); err != nil {
			return err
		}

		if i := u.getInitIconImages(); i != nil {
			u.window.SetIcon(i)
		}
		return nil
	}); err != nil {
		return err
	}

	setPosition := func() {
		u.iwindow.SetPosition(u.getInitWindowPosition())
	}
	setSize := func() {
		ww, wh := u.getInitWindowSize()
		ww = int(u.toDeviceDependentPixel(float64(ww)))
		wh = int(u.toDeviceDependentPixel(float64(wh)))
		u.setWindowSize(ww, wh, u.isFullscreen())
	}

	// Set the window size and the window position in this order on Linux or other UNIX using X (#1118),
	// but this should be inverted on Windows. This is very tricky, but there is no obvious way to solve this.
	// This doesn't matter on macOS.
	if runtime.GOOS == "windows" {
		setPosition()
		setSize()
	} else {
		setSize()
		setPosition()
	}

	// Maximizing a window requires a proper size and position. Call Maximize here (#1117).
	if u.isInitWindowMaximized() {
		_ = u.t.Call(func() error {
			u.window.Maximize()
			return nil
		})
	}

	_ = u.t.Call(func() error {
		u.title = u.getInitTitle()
		u.window.SetTitle(u.title)
		u.window.Show()
		return nil
	})

	var w unsafe.Pointer
	_ = u.t.Call(func() error {
		w = u.nativeWindow()
		return nil
	})
	if g, ok := u.Graphics().(interface{ SetWindow(unsafe.Pointer) }); ok {
		g.SetWindow(w)
	}
	return u.loop()
}

func (u *UserInterface) updateSize() {
	var w, h int
	_ = u.t.Call(func() error {
		w, h = u.windowWidth, u.windowHeight
		return nil
	})
	u.setWindowSize(w, h, u.isFullscreen())

	sizeChanged := false
	_ = u.t.Call(func() error {
		if !u.toChangeSize {
			return nil
		}

		u.toChangeSize = false
		sizeChanged = true
		return nil
	})
	if sizeChanged {
		var w, h float64
		_ = u.t.Call(func() error {
			var ww, wh int
			if u.isFullscreen() {
				v := u.currentMonitor().GetVideoMode()
				ww = v.Width
				wh = v.Height
			} else {
				// Instead of u.windowWidth and u.windowHeight, use the actual window size here.
				// On Windows, the specified size at SetSize and the actual window size might not
				// match (#1163).
				ww, wh = u.window.GetSize()
			}
			w = u.toDeviceIndependentPixel(float64(ww))
			h = u.toDeviceIndependentPixel(float64(wh))
			return nil
		})
		u.context.Layout(w, h)
	}
}

func (u *UserInterface) update() error {
	shouldClose := false
	_ = u.t.Call(func() error {
		shouldClose = u.window.ShouldClose()
		return nil
	})
	if shouldClose {
		return driver.RegularTermination
	}

	if u.isInitFullscreen() {
		var w, h int
		_ = u.t.Call(func() error {
			w, h = u.window.GetSize()
			return nil
		})
		u.setWindowSize(w, h, true)
		u.setInitFullscreen(false)
	}

	// This call is needed for initialization.
	u.updateSize()

	_ = u.t.Call(func() error {
		glfw.PollEvents()
		return nil
	})
	u.input.update(u.window, u.context)
	_ = u.t.Call(func() error {
		defer hooks.ResumeAudio()

		for !u.isRunnableOnUnfocused() && u.window.GetAttrib(glfw.Focused) == 0 {
			hooks.SuspendAudio()
			// Wait for an arbitrary period to avoid busy loop.
			time.Sleep(time.Second / 60)
			glfw.PollEvents()
			if u.window.ShouldClose() {
				return nil
			}
		}
		return nil
	})
	if err := u.context.Update(); err != nil {
		return err
	}
	if err := u.context.Draw(); err != nil {
		return err
	}

	// Update the screen size when the window is resizable.
	var w, h int
	_ = u.t.Call(func() error {
		w, h = u.reqWidth, u.reqHeight
		return nil
	})
	if w != 0 || h != 0 {
		u.setWindowSize(w, h, u.isFullscreen())
	}
	_ = u.t.Call(func() error {
		u.reqWidth = 0
		u.reqHeight = 0
		return nil
	})
	return nil
}

func (u *UserInterface) loop() error {
	defer func() {
		_ = u.t.Call(func() error {
			glfw.Terminate()
			return nil
		})
	}()
	for {
		var unfocused bool

		// On Windows, the focusing state might be always false (#987).
		// On Windows, even if a window is in another workspace, vsync seems to work.
		// Then let's assume the window is always 'focused' as a workaround.
		if runtime.GOOS != "windows" {
			unfocused = u.window.GetAttrib(glfw.Focused) == glfw.False
		}

		var t1, t2 time.Time

		if unfocused {
			t1 = time.Now()
		}
		if err := u.update(); err != nil {
			return err
		}

		_ = u.t.Call(func() error {
			u.swapBuffers()
			return nil
		})
		if unfocused {
			t2 = time.Now()
		}

		// When a window is not focused, SwapBuffers might return immediately and CPU might be busy.
		// Mitigate this by sleeping (#982).
		if unfocused {
			d := t2.Sub(t1)
			const wait = time.Second / 60
			if d < wait {
				time.Sleep(wait - d)
			}
		}
	}
}

// swapBuffers must be called from the main thread.
func (u *UserInterface) swapBuffers() {
	if u.Graphics().IsGL() {
		u.window.SwapBuffers()
	}
}

func (u *UserInterface) setWindowSize(width, height int, fullscreen bool) {
	windowRecreated := false

	_ = u.t.Call(func() error {
		if u.windowWidth == width && u.windowHeight == height && u.isFullscreen() == fullscreen && u.lastDeviceScaleFactor == u.deviceScaleFactor() {
			return nil
		}

		if width < 1 {
			width = 1
		}
		if height < 1 {
			height = 1
		}

		u.lastDeviceScaleFactor = u.deviceScaleFactor()

		// To make sure the current existing framebuffers are rendered,
		// swap buffers here before SetSize is called.
		u.swapBuffers()

		if fullscreen {
			if u.origPosX == invalidPos || u.origPosY == invalidPos {
				u.origPosX, u.origPosY = u.window.GetPos()
			}
			m := u.currentMonitor()
			v := m.GetVideoMode()
			u.window.SetMonitor(m, 0, 0, v.Width, v.Height, v.RefreshRate)

			// Swapping buffer is necesary to prevent the image lag (#1004).
			// TODO: This might not work when vsync is disabled.
			if u.Graphics().IsGL() {
				glfw.PollEvents()
				u.swapBuffers()
			}
		} else {
			if u.window.GetMonitor() != nil {
				if u.Graphics().IsGL() {
					// When OpenGL is used, swapping buffer is enough to solve the image-lag
					// issue (#1004). Rather, recreating window destroys GPU resources.
					// TODO: This might not work when vsync is disabled.
					u.window.SetMonitor(nil, 0, 0, 16, 16, 0)
					glfw.PollEvents()
					u.swapBuffers()
				} else {
					// Recreate the window since an image lag remains after coming back from
					// fullscreen (#1004).
					if u.window != nil {
						u.window.Destroy()
						u.window = nil
					}
					if err := u.createWindow(); err != nil {
						// TODO: This should return an error.
						panic(fmt.Sprintf("glfw: failed to recreate window: %v", err))
					}
					u.window.Show()
					windowRecreated = true
				}
			}

			// On Windows, giving a too small width doesn't call a callback (#165).
			// To prevent hanging up, return asap if the width is too small.
			// 126 is an arbitrary number and I guess this is small enough.
			minWindowWidth := int(u.toDeviceDependentPixel(126))
			if u.window.GetAttrib(glfw.Decorated) == glfw.False {
				minWindowWidth = 1
			}
			if width < minWindowWidth {
				width = minWindowWidth
			}

			if u.origPosX != invalidPos && u.origPosY != invalidPos {
				x := u.origPosX
				y := u.origPosY
				u.window.SetPos(x, y)
				// Dirty hack for macOS (#703). Rendering doesn't work correctly with one SetPos, but
				// work with two or more SetPos.
				if runtime.GOOS == "darwin" {
					u.window.SetPos(x+1, y)
					u.window.SetPos(x, y)
				}
				u.origPosX = invalidPos
				u.origPosY = invalidPos
			}

			// Set the window size after the position. The order matters.
			// In the opposite order, the window size might not be correct when going back from fullscreen with multi monitors.
			oldW, oldH := u.window.GetSize()
			newW := width
			newH := height
			if oldW != newW || oldH != newH {
				ch := make(chan struct{})
				u.window.SetFramebufferSizeCallback(func(_ *glfw.Window, _, _ int) {
					u.window.SetFramebufferSizeCallback(nil)
					close(ch)
				})
				u.window.SetSize(newW, newH)
			event:
				for {
					glfw.PollEvents()
					select {
					case <-ch:
						break event
					default:
					}
				}
			}

			// Window title might be lost on macOS after coming back from fullscreen.
			u.window.SetTitle(u.title)
		}

		// As width might be updated, update windowWidth/Height here.
		u.windowWidth = width
		u.windowHeight = height

		if !u.vsyncInited {
			// Initialize vsync after SetMonitor is called. See the comment in updateVsync.
			u.vsync = u.isInitVsyncEnabled()
			u.updateVsync()
			u.vsyncInited = true
		}

		u.toChangeSize = true
		return nil
	})

	if windowRecreated {
		if g, ok := u.Graphics().(interface{ SetWindow(unsafe.Pointer) }); ok {
			g.SetWindow(u.nativeWindow())
		}
	}
}

// updateVsync must be called on the main thread.
func (u *UserInterface) updateVsync() {
	if u.Graphics().IsGL() {
		// SwapInterval is affected by the current monitor of the window.
		// This needs to be called at least after SetMonitor.
		// Without SwapInterval after SetMonitor, vsynch doesn't work (#375).
		//
		// TODO: (#405) If triple buffering is needed, SwapInterval(0) should be called,
		// but is this correct? If glfw.SwapInterval(0) and the driver doesn't support triple
		// buffering, what will happen?
		if u.vsync {
			glfw.SwapInterval(1)
		} else {
			glfw.SwapInterval(0)
		}
	}
	u.Graphics().SetVsyncEnabled(u.vsync)
}

// currentMonitor returns the monitor most suitable with the current window.
//
// currentMonitor must be called on the main thread.
func (u *UserInterface) currentMonitor() *glfw.Monitor {
	if w := u.window; w != nil {
		// GetMonitor is available only on fullscreen.
		if m := w.GetMonitor(); m != nil {
			return m
		}
	}
	// Get the monitor which the current window belongs to. This requires OS API.
	return u.currentMonitorFromPosition()
}

func (u *UserInterface) SetScreenTransparent(transparent bool) {
	if !u.isRunning() {
		u.setInitScreenTransparent(transparent)
		return
	}
	panic("glfw: SetScreenTransparent can't be called after the main loop starts")
}

func (u *UserInterface) IsScreenTransparent() bool {
	if !u.isRunning() {
		return u.isInitScreenTransparent()
	}
	val := false
	_ = u.t.Call(func() error {
		val = u.window.GetAttrib(glfw.TransparentFramebuffer) == glfw.True
		return nil
	})
	return val
}

func (u *UserInterface) ResetForFrame() {
	// The offscreens must be updated every frame (#490).
	u.updateSize()
	u.input.resetForFrame()
}

func (u *UserInterface) Input() driver.Input {
	return &u.input
}

func (u *UserInterface) Window() driver.Window {
	return &u.iwindow
}