shiny/driver/internal/win32/win32.go - exp - Git at Google

 // Copyright 2015 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // +build windows

 // Package win32 implements a partial shiny screen driver using the Win32 API.
 // It provides window, lifecycle, key, and mouse management, but no drawing.
 // That is left to windriver (using GDI) or gldriver (using DirectX via ANGLE).
 package win32 // import "golang.org/x/exp/shiny/driver/internal/win32"

 import (
 	"fmt"
 	"runtime"
 	"sync"
 	"syscall"
 	"unsafe"

 	"golang.org/x/exp/shiny/screen"
 	"golang.org/x/mobile/event/key"
 	"golang.org/x/mobile/event/lifecycle"
 	"golang.org/x/mobile/event/mouse"
 	"golang.org/x/mobile/event/paint"
 	"golang.org/x/mobile/event/size"
 	"golang.org/x/mobile/geom"
 )

 // screenHWND is the handle to the "Screen window".
 // The Screen window encapsulates all screen.Screen operations
 // in an actual Windows window so they all run on the main thread.
 // Since any messages sent to a window will be executed on the
 // main thread, we can safely use the messages below.
 var screenHWND syscall.Handle

 const (
 	msgCreateWindow = _WM_USER + iota
 	msgMainCallback
 	msgShow
 	msgQuit
 	msgLast
 )

 // userWM is used to generate private (WM_USER and above) window message IDs
 // for use by screenWindowWndProc and windowWndProc.
 type userWM struct {
 	sync.Mutex
 	id uint32
 }

 func (m *userWM) next() uint32 {
 	m.Lock()
 	if m.id == 0 {
 		m.id = msgLast
 	}
 	r := m.id
 	m.id++
 	m.Unlock()
 	return r
 }

 var currentUserWM userWM

 func newWindow(opts *screen.NewWindowOptions) (syscall.Handle, error) {
 	// TODO(brainman): convert windowClass to *uint16 once (in initWindowClass)
 	wcname, err := syscall.UTF16PtrFromString(windowClass)
 	if err != nil {
 		return 0, err
 	}
 	title, err := syscall.UTF16PtrFromString(opts.GetTitle())
 	if err != nil {
 		return 0, err
 	}
 	hwnd, err := _CreateWindowEx(0,
 		wcname, title,
 		_WS_OVERLAPPEDWINDOW,
 		_CW_USEDEFAULT, _CW_USEDEFAULT,
 		_CW_USEDEFAULT, _CW_USEDEFAULT,
 		0, 0, hThisInstance, 0)
 	if err != nil {
 		return 0, err
 	}
 	// TODO(andlabs): use proper nCmdShow
 	// TODO(andlabs): call UpdateWindow()

 	return hwnd, nil
 }

 // ResizeClientRect makes hwnd client rectangle opts.Width by opts.Height in size.
 func ResizeClientRect(hwnd syscall.Handle, opts *screen.NewWindowOptions) error {
 	if opts == nil || opts.Width <= 0 || opts.Height <= 0 {
 		return nil
 	}
 	var cr, wr _RECT
 	err := _GetClientRect(hwnd, &cr)
 	if err != nil {
 		return err
 	}
 	err = _GetWindowRect(hwnd, &wr)
 	if err != nil {
 		return err
 	}
 	w := (wr.Right - wr.Left) - (cr.Right - int32(opts.Width))
 	h := (wr.Bottom - wr.Top) - (cr.Bottom - int32(opts.Height))
 	return _MoveWindow(hwnd, wr.Left, wr.Top, w, h, false)
 }

 // Show shows a newly created window.
 // It sends the appropriate lifecycle events, makes the window appear
 // on the screen, and sends an initial size event.
 //
 // This is a separate step from NewWindow to give the driver a chance
 // to setup its internal state for a window before events start being
 // delivered.
 func Show(hwnd syscall.Handle) {
 	SendMessage(hwnd, msgShow, 0, 0)
 }

 func Release(hwnd syscall.Handle) {
 	SendMessage(hwnd, _WM_CLOSE, 0, 0)
 }

 func sendFocus(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr) {
 	switch uMsg {
 	case _WM_SETFOCUS:
 		LifecycleEvent(hwnd, lifecycle.StageFocused)
 	case _WM_KILLFOCUS:
 		LifecycleEvent(hwnd, lifecycle.StageVisible)
 	default:
 		panic(fmt.Sprintf("unexpected focus message: %d", uMsg))
 	}
 	return _DefWindowProc(hwnd, uMsg, wParam, lParam)
 }

 func sendShow(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr) {
 	LifecycleEvent(hwnd, lifecycle.StageVisible)
 	_ShowWindow(hwnd, _SW_SHOWDEFAULT)
 	sendSize(hwnd)
 	return 0
 }

 func sendSizeEvent(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr) {
 	wp := (*_WINDOWPOS)(unsafe.Pointer(lParam))
 	if wp.Flags&_SWP_NOSIZE != 0 {
 		return 0
 	}
 	sendSize(hwnd)
 	return 0
 }

 func sendSize(hwnd syscall.Handle) {
 	var r _RECT
 	if err := _GetClientRect(hwnd, &r); err != nil {
 		panic(err) // TODO(andlabs)
 	}

 	width := int(r.Right - r.Left)
 	height := int(r.Bottom - r.Top)

 	// TODO(andlabs): don't assume that PixelsPerPt == 1
 	SizeEvent(hwnd, size.Event{
 		WidthPx:     width,
 		HeightPx:    height,
 		WidthPt:     geom.Pt(width),
 		HeightPt:    geom.Pt(height),
 		PixelsPerPt: 1,
 	})
 }

 func sendClose(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr) {
 	// TODO(ktye): DefWindowProc calls DestroyWindow by default.
 	// To intercept destruction of the window, return 0 and call
 	// DestroyWindow when appropriate.
 	LifecycleEvent(hwnd, lifecycle.StageDead)
 	return _DefWindowProc(hwnd, uMsg, wParam, lParam)
 }

 func sendMouseEvent(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr) {
 	e := mouse.Event{
 		X:         float32(_GET_X_LPARAM(lParam)),
 		Y:         float32(_GET_Y_LPARAM(lParam)),
 		Modifiers: keyModifiers(),
 	}

 	switch uMsg {
 	case _WM_MOUSEMOVE:
 		e.Direction = mouse.DirNone
 	case _WM_LBUTTONDOWN, _WM_MBUTTONDOWN, _WM_RBUTTONDOWN:
 		e.Direction = mouse.DirPress
 	case _WM_LBUTTONUP, _WM_MBUTTONUP, _WM_RBUTTONUP:
 		e.Direction = mouse.DirRelease
 	case _WM_MOUSEWHEEL:
 		// TODO: On a trackpad, a scroll can be a drawn-out affair with a
 		// distinct beginning and end. Should the intermediate events be
 		// DirNone?
 		e.Direction = mouse.DirStep

 		// Convert from screen to window coordinates.
 		p := _POINT{
 			int32(e.X),
 			int32(e.Y),
 		}
 		_ScreenToClient(hwnd, &p)
 		e.X = float32(p.X)
 		e.Y = float32(p.Y)
 	default:
 		panic("sendMouseEvent() called on non-mouse message")
 	}

 	switch uMsg {
 	case _WM_MOUSEMOVE:
 		// No-op.
 	case _WM_LBUTTONDOWN, _WM_LBUTTONUP:
 		e.Button = mouse.ButtonLeft
 	case _WM_MBUTTONDOWN, _WM_MBUTTONUP:
 		e.Button = mouse.ButtonMiddle
 	case _WM_RBUTTONDOWN, _WM_RBUTTONUP:
 		e.Button = mouse.ButtonRight
 	case _WM_MOUSEWHEEL:
 		// TODO: handle horizontal scrolling
 		delta := _GET_WHEEL_DELTA_WPARAM(wParam) / _WHEEL_DELTA
 		switch {
 		case delta > 0:
 			e.Button = mouse.ButtonWheelUp
 		case delta < 0:
 			e.Button = mouse.ButtonWheelDown
 			delta = -delta
 		default:
 			return
 		}
 		for delta > 0 {
 			MouseEvent(hwnd, e)
 			delta--
 		}
 		return
 	}

 	MouseEvent(hwnd, e)

 	return 0
 }

 // Precondition: this is called in immediate response to the message that triggered the event (so not after w.Send).
 func keyModifiers() (m key.Modifiers) {
 	down := func(x int32) bool {
 		// GetKeyState gets the key state at the time of the message, so this is what we want.
 		return _GetKeyState(x)&0x80 != 0
 	}

 	if down(_VK_CONTROL) {
 		m |= key.ModControl
 	}
 	if down(_VK_MENU) {
 		m |= key.ModAlt
 	}
 	if down(_VK_SHIFT) {
 		m |= key.ModShift
 	}
 	if down(_VK_LWIN) || down(_VK_RWIN) {
 		m |= key.ModMeta
 	}
 	return m
 }

 var (
 	MouseEvent     func(hwnd syscall.Handle, e mouse.Event)
 	PaintEvent     func(hwnd syscall.Handle, e paint.Event)
 	SizeEvent      func(hwnd syscall.Handle, e size.Event)
 	KeyEvent       func(hwnd syscall.Handle, e key.Event)
 	LifecycleEvent func(hwnd syscall.Handle, e lifecycle.Stage)

 	// TODO: use the golang.org/x/exp/shiny/driver/internal/lifecycler package
 	// instead of or together with the LifecycleEvent callback?
 )

 func sendPaint(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr) {
 	PaintEvent(hwnd, paint.Event{})
 	return _DefWindowProc(hwnd, uMsg, wParam, lParam)
 }

 var screenMsgs = map[uint32]func(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr){}

 func AddScreenMsg(fn func(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr)) uint32 {
 	uMsg := currentUserWM.next()
 	screenMsgs[uMsg] = func(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) uintptr {
 		fn(hwnd, uMsg, wParam, lParam)
 		return 0
 	}
 	return uMsg
 }

 func screenWindowWndProc(hwnd syscall.Handle, uMsg uint32, wParam uintptr, lParam uintptr) (lResult uintptr) {
 	switch uMsg {
 	case msgCreateWindow:
 		p := (*newWindowParams)(unsafe.Pointer(lParam))
 		p.w, p.err = newWindow(p.opts)
 	case msgMainCallback:
 		go func() {
 			mainCallback()
 			SendScreenMessage(msgQuit, 0, 0)
 		}()
 	case msgQuit:
 		_PostQuitMessage(0)
 	}
 	fn := screenMsgs[uMsg]
 	if fn != nil {
 		return fn(hwnd, uMsg, wParam, lParam)
 	}
 	return _DefWindowProc(hwnd, uMsg, wParam, lParam)
 }

 //go:uintptrescapes

 func SendScreenMessage(uMsg uint32, wParam uintptr, lParam uintptr) (lResult uintptr) {
 	return SendMessage(screenHWND, uMsg, wParam, lParam)
 }

 var windowMsgs = map[uint32]func(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr){
 	_WM_SETFOCUS:         sendFocus,
 	_WM_KILLFOCUS:        sendFocus,
 	_WM_PAINT:            sendPaint,
 	msgShow:              sendShow,
 	_WM_WINDOWPOSCHANGED: sendSizeEvent,
 	_WM_CLOSE:            sendClose,

 	_WM_LBUTTONDOWN: sendMouseEvent,
 	_WM_LBUTTONUP:   sendMouseEvent,
 	_WM_MBUTTONDOWN: sendMouseEvent,
 	_WM_MBUTTONUP:   sendMouseEvent,
 	_WM_RBUTTONDOWN: sendMouseEvent,
 	_WM_RBUTTONUP:   sendMouseEvent,
 	_WM_MOUSEMOVE:   sendMouseEvent,
 	_WM_MOUSEWHEEL:  sendMouseEvent,

 	_WM_KEYDOWN: sendKeyEvent,
 	_WM_KEYUP:   sendKeyEvent,
 	// TODO case _WM_SYSKEYDOWN, _WM_SYSKEYUP:
 }

 func AddWindowMsg(fn func(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr)) uint32 {
 	uMsg := currentUserWM.next()
 	windowMsgs[uMsg] = func(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) uintptr {
 		fn(hwnd, uMsg, wParam, lParam)
 		return 0
 	}
 	return uMsg
 }

 func windowWndProc(hwnd syscall.Handle, uMsg uint32, wParam uintptr, lParam uintptr) (lResult uintptr) {
 	fn := windowMsgs[uMsg]
 	if fn != nil {
 		return fn(hwnd, uMsg, wParam, lParam)
 	}
 	return _DefWindowProc(hwnd, uMsg, wParam, lParam)
 }

 type newWindowParams struct {
 	opts *screen.NewWindowOptions
 	w    syscall.Handle
 	err  error
 }

 func NewWindow(opts *screen.NewWindowOptions) (syscall.Handle, error) {
 	var p newWindowParams
 	p.opts = opts
 	SendScreenMessage(msgCreateWindow, 0, uintptr(unsafe.Pointer(&p)))
 	return p.w, p.err
 }

 const windowClass = "shiny_Window"
 const screenWindowClass = "shiny_ScreenWindow"

 func initWindowClass() (err error) {
 	wcname, err := syscall.UTF16PtrFromString(windowClass)
 	if err != nil {
 		return err
 	}
 	_, err = _RegisterClass(&_WNDCLASS{
 		LpszClassName: wcname,
 		LpfnWndProc:   syscall.NewCallback(windowWndProc),
 		HIcon:         hDefaultIcon,
 		HCursor:       hDefaultCursor,
 		HInstance:     hThisInstance,
 		// TODO(andlabs): change this to something else? NULL? the hollow brush?
 		HbrBackground: syscall.Handle(_COLOR_BTNFACE + 1),
 	})
 	return err
 }

 func closeWindowClass() (err error) {
 	wcname, err := syscall.UTF16PtrFromString(windowClass)
 	if err != nil {
 		return err
 	}
 	_UnregisterClass(wcname, hThisInstance)

 	return nil
 }

 func initScreenWindow() (err error) {
 	swc, err := syscall.UTF16PtrFromString(screenWindowClass)
 	if err != nil {
 		return err
 	}
 	emptyString, err := syscall.UTF16PtrFromString("")
 	if err != nil {
 		return err
 	}
 	wc := _WNDCLASS{
 		LpszClassName: swc,
 		LpfnWndProc:   syscall.NewCallback(screenWindowWndProc),
 		HIcon:         hDefaultIcon,
 		HCursor:       hDefaultCursor,
 		HInstance:     hThisInstance,
 		HbrBackground: syscall.Handle(_COLOR_BTNFACE + 1),
 	}
 	_, err = _RegisterClass(&wc)
 	if err != nil {
 		return err
 	}
 	screenHWND, err = _CreateWindowEx(0,
 		swc, emptyString,
 		_WS_OVERLAPPEDWINDOW,
 		_CW_USEDEFAULT, _CW_USEDEFAULT,
 		_CW_USEDEFAULT, _CW_USEDEFAULT,
 		_HWND_MESSAGE, 0, hThisInstance, 0)
 	if err != nil {
 		return err
 	}
 	return nil
 }

 func closeScreenWindow() (err error) {
 	// first destroy window
 	_DestroyWindow(screenHWND)

 	// then unregister class
 	swc, err := syscall.UTF16PtrFromString(screenWindowClass)
 	if err != nil {
 		return err
 	}
 	_UnregisterClass(swc, hThisInstance)

 	return nil
 }

 var (
 	hDefaultIcon   syscall.Handle
 	hDefaultCursor syscall.Handle
 	hThisInstance  syscall.Handle
 )

 func initCommon() (err error) {
 	hDefaultIcon, err = _LoadIcon(0, _IDI_APPLICATION)
 	if err != nil {
 		return err
 	}
 	hDefaultCursor, err = _LoadCursor(0, _IDC_ARROW)
 	if err != nil {
 		return err
 	}
 	// TODO(andlabs) hThisInstance
 	return nil
 }

 //go:uintptrescapes

 func SendMessage(hwnd syscall.Handle, uMsg uint32, wParam uintptr, lParam uintptr) (lResult uintptr) {
 	return sendMessage(hwnd, uMsg, wParam, lParam)
 }

 var mainCallback func()

 func Main(f func()) (retErr error) {
 	// It does not matter which OS thread we are on.
 	// All that matters is that we confine all UI operations
 	// to the thread that created the respective window.
 	runtime.LockOSThread()

 	if err := initCommon(); err != nil {
 		return err
 	}

 	if err := initScreenWindow(); err != nil {
 		return err
 	}
 	defer func() {
 		// TODO(andlabs): log an error if this fails?
 		closeScreenWindow()
 	}()

 	if err := initWindowClass(); err != nil {
 		return err
 	}
 	defer func() {
 		// TODO(andlabs): log an error if this fails?
 		closeWindowClass()
 	}()

 	// Prime the pump.
 	mainCallback = f
 	_PostMessage(screenHWND, msgMainCallback, 0, 0)

 	// Main message pump.
 	var m _MSG
 	for {
 		done, err := _GetMessage(&m, 0, 0, 0)
 		if err != nil {
 			return fmt.Errorf("win32 GetMessage failed: %v", err)
 		}
 		if done == 0 { // WM_QUIT
 			break
 		}
 		_TranslateMessage(&m)
 		_DispatchMessage(&m)
 	}

 	return nil
 }
	// Copyright 2015 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// +build windows

	// Package win32 implements a partial shiny screen driver using the Win32 API.
	// It provides window, lifecycle, key, and mouse management, but no drawing.
	// That is left to windriver (using GDI) or gldriver (using DirectX via ANGLE).
	package win32 // import "golang.org/x/exp/shiny/driver/internal/win32"

	import (
	"fmt"
	"runtime"
	"sync"
	"syscall"
	"unsafe"

	"golang.org/x/exp/shiny/screen"
	"golang.org/x/mobile/event/key"
	"golang.org/x/mobile/event/lifecycle"
	"golang.org/x/mobile/event/mouse"
	"golang.org/x/mobile/event/paint"
	"golang.org/x/mobile/event/size"
	"golang.org/x/mobile/geom"
	)

	// screenHWND is the handle to the "Screen window".
	// The Screen window encapsulates all screen.Screen operations
	// in an actual Windows window so they all run on the main thread.
	// Since any messages sent to a window will be executed on the
	// main thread, we can safely use the messages below.
	var screenHWND syscall.Handle

	const (
	msgCreateWindow = _WM_USER + iota
	msgMainCallback
	msgShow
	msgQuit
	msgLast
	)

	// userWM is used to generate private (WM_USER and above) window message IDs
	// for use by screenWindowWndProc and windowWndProc.
	type userWM struct {
	sync.Mutex
	id uint32
	}

	func (m *userWM) next() uint32 {
	m.Lock()
	if m.id == 0 {
	m.id = msgLast
	}
	r := m.id
	m.id++
	m.Unlock()
	return r
	}

	var currentUserWM userWM

	func newWindow(opts *screen.NewWindowOptions) (syscall.Handle, error) {
	// TODO(brainman): convert windowClass to *uint16 once (in initWindowClass)
	wcname, err := syscall.UTF16PtrFromString(windowClass)
	if err != nil {
	return 0, err
	}
	title, err := syscall.UTF16PtrFromString(opts.GetTitle())
	if err != nil {
	return 0, err
	}
	hwnd, err := _CreateWindowEx(0,
	wcname, title,
	_WS_OVERLAPPEDWINDOW,
	_CW_USEDEFAULT, _CW_USEDEFAULT,
	_CW_USEDEFAULT, _CW_USEDEFAULT,
	0, 0, hThisInstance, 0)
	if err != nil {
	return 0, err
	}
	// TODO(andlabs): use proper nCmdShow
	// TODO(andlabs): call UpdateWindow()

	return hwnd, nil
	}

	// ResizeClientRect makes hwnd client rectangle opts.Width by opts.Height in size.
	func ResizeClientRect(hwnd syscall.Handle, opts *screen.NewWindowOptions) error {
	if opts == nil \|\| opts.Width <= 0 \|\| opts.Height <= 0 {
	return nil
	}
	var cr, wr _RECT
	err := _GetClientRect(hwnd, &cr)
	if err != nil {
	return err
	}
	err = _GetWindowRect(hwnd, &wr)
	if err != nil {
	return err
	}
	w := (wr.Right - wr.Left) - (cr.Right - int32(opts.Width))
	h := (wr.Bottom - wr.Top) - (cr.Bottom - int32(opts.Height))
	return _MoveWindow(hwnd, wr.Left, wr.Top, w, h, false)
	}

	// Show shows a newly created window.
	// It sends the appropriate lifecycle events, makes the window appear
	// on the screen, and sends an initial size event.
	//
	// This is a separate step from NewWindow to give the driver a chance
	// to setup its internal state for a window before events start being
	// delivered.
	func Show(hwnd syscall.Handle) {
	SendMessage(hwnd, msgShow, 0, 0)
	}

	func Release(hwnd syscall.Handle) {
	SendMessage(hwnd, _WM_CLOSE, 0, 0)
	}

	func sendFocus(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr) {
	switch uMsg {
	case _WM_SETFOCUS:
	LifecycleEvent(hwnd, lifecycle.StageFocused)
	case _WM_KILLFOCUS:
	LifecycleEvent(hwnd, lifecycle.StageVisible)
	default:
	panic(fmt.Sprintf("unexpected focus message: %d", uMsg))
	}
	return _DefWindowProc(hwnd, uMsg, wParam, lParam)
	}

	func sendShow(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr) {
	LifecycleEvent(hwnd, lifecycle.StageVisible)
	_ShowWindow(hwnd, _SW_SHOWDEFAULT)
	sendSize(hwnd)
	return 0
	}

	func sendSizeEvent(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr) {
	wp := (*_WINDOWPOS)(unsafe.Pointer(lParam))
	if wp.Flags&_SWP_NOSIZE != 0 {
	return 0
	}
	sendSize(hwnd)
	return 0
	}

	func sendSize(hwnd syscall.Handle) {
	var r _RECT
	if err := _GetClientRect(hwnd, &r); err != nil {
	panic(err) // TODO(andlabs)
	}

	width := int(r.Right - r.Left)
	height := int(r.Bottom - r.Top)

	// TODO(andlabs): don't assume that PixelsPerPt == 1
	SizeEvent(hwnd, size.Event{
	WidthPx: width,
	HeightPx: height,
	WidthPt: geom.Pt(width),
	HeightPt: geom.Pt(height),
	PixelsPerPt: 1,
	})
	}

	func sendClose(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr) {
	// TODO(ktye): DefWindowProc calls DestroyWindow by default.
	// To intercept destruction of the window, return 0 and call
	// DestroyWindow when appropriate.
	LifecycleEvent(hwnd, lifecycle.StageDead)
	return _DefWindowProc(hwnd, uMsg, wParam, lParam)
	}

	func sendMouseEvent(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr) {
	e := mouse.Event{
	X: float32(_GET_X_LPARAM(lParam)),
	Y: float32(_GET_Y_LPARAM(lParam)),
	Modifiers: keyModifiers(),
	}

	switch uMsg {
	case _WM_MOUSEMOVE:
	e.Direction = mouse.DirNone
	case _WM_LBUTTONDOWN, _WM_MBUTTONDOWN, _WM_RBUTTONDOWN:
	e.Direction = mouse.DirPress
	case _WM_LBUTTONUP, _WM_MBUTTONUP, _WM_RBUTTONUP:
	e.Direction = mouse.DirRelease
	case _WM_MOUSEWHEEL:
	// TODO: On a trackpad, a scroll can be a drawn-out affair with a
	// distinct beginning and end. Should the intermediate events be
	// DirNone?
	e.Direction = mouse.DirStep

	// Convert from screen to window coordinates.
	p := _POINT{
	int32(e.X),
	int32(e.Y),
	}
	_ScreenToClient(hwnd, &p)
	e.X = float32(p.X)
	e.Y = float32(p.Y)
	default:
	panic("sendMouseEvent() called on non-mouse message")
	}

	switch uMsg {
	case _WM_MOUSEMOVE:
	// No-op.
	case _WM_LBUTTONDOWN, _WM_LBUTTONUP:
	e.Button = mouse.ButtonLeft
	case _WM_MBUTTONDOWN, _WM_MBUTTONUP:
	e.Button = mouse.ButtonMiddle
	case _WM_RBUTTONDOWN, _WM_RBUTTONUP:
	e.Button = mouse.ButtonRight
	case _WM_MOUSEWHEEL:
	// TODO: handle horizontal scrolling
	delta := _GET_WHEEL_DELTA_WPARAM(wParam) / _WHEEL_DELTA
	switch {
	case delta > 0:
	e.Button = mouse.ButtonWheelUp
	case delta < 0:
	e.Button = mouse.ButtonWheelDown
	delta = -delta
	default:
	return
	}
	for delta > 0 {
	MouseEvent(hwnd, e)
	delta--
	}
	return
	}

	MouseEvent(hwnd, e)

	return 0
	}

	// Precondition: this is called in immediate response to the message that triggered the event (so not after w.Send).
	func keyModifiers() (m key.Modifiers) {
	down := func(x int32) bool {
	// GetKeyState gets the key state at the time of the message, so this is what we want.
	return _GetKeyState(x)&0x80 != 0
	}

	if down(_VK_CONTROL) {
	m \|= key.ModControl
	}
	if down(_VK_MENU) {
	m \|= key.ModAlt
	}
	if down(_VK_SHIFT) {
	m \|= key.ModShift
	}
	if down(_VK_LWIN) \|\| down(_VK_RWIN) {
	m \|= key.ModMeta
	}
	return m
	}

	var (
	MouseEvent func(hwnd syscall.Handle, e mouse.Event)
	PaintEvent func(hwnd syscall.Handle, e paint.Event)
	SizeEvent func(hwnd syscall.Handle, e size.Event)
	KeyEvent func(hwnd syscall.Handle, e key.Event)
	LifecycleEvent func(hwnd syscall.Handle, e lifecycle.Stage)

	// TODO: use the golang.org/x/exp/shiny/driver/internal/lifecycler package
	// instead of or together with the LifecycleEvent callback?
	)

	func sendPaint(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr) {
	PaintEvent(hwnd, paint.Event{})
	return _DefWindowProc(hwnd, uMsg, wParam, lParam)
	}

	var screenMsgs = map[uint32]func(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr){}

	func AddScreenMsg(fn func(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr)) uint32 {
	uMsg := currentUserWM.next()
	screenMsgs[uMsg] = func(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) uintptr {
	fn(hwnd, uMsg, wParam, lParam)
	return 0
	}
	return uMsg
	}

	func screenWindowWndProc(hwnd syscall.Handle, uMsg uint32, wParam uintptr, lParam uintptr) (lResult uintptr) {
	switch uMsg {
	case msgCreateWindow:
	p := (*newWindowParams)(unsafe.Pointer(lParam))
	p.w, p.err = newWindow(p.opts)
	case msgMainCallback:
	go func() {
	mainCallback()
	SendScreenMessage(msgQuit, 0, 0)
	}()
	case msgQuit:
	_PostQuitMessage(0)
	}
	fn := screenMsgs[uMsg]
	if fn != nil {
	return fn(hwnd, uMsg, wParam, lParam)
	}
	return _DefWindowProc(hwnd, uMsg, wParam, lParam)
	}

	//go:uintptrescapes

	func SendScreenMessage(uMsg uint32, wParam uintptr, lParam uintptr) (lResult uintptr) {
	return SendMessage(screenHWND, uMsg, wParam, lParam)
	}

	var windowMsgs = map[uint32]func(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) (lResult uintptr){
	_WM_SETFOCUS: sendFocus,
	_WM_KILLFOCUS: sendFocus,
	_WM_PAINT: sendPaint,
	msgShow: sendShow,
	_WM_WINDOWPOSCHANGED: sendSizeEvent,
	_WM_CLOSE: sendClose,

	_WM_LBUTTONDOWN: sendMouseEvent,
	_WM_LBUTTONUP: sendMouseEvent,
	_WM_MBUTTONDOWN: sendMouseEvent,
	_WM_MBUTTONUP: sendMouseEvent,
	_WM_RBUTTONDOWN: sendMouseEvent,
	_WM_RBUTTONUP: sendMouseEvent,
	_WM_MOUSEMOVE: sendMouseEvent,
	_WM_MOUSEWHEEL: sendMouseEvent,

	_WM_KEYDOWN: sendKeyEvent,
	_WM_KEYUP: sendKeyEvent,
	// TODO case _WM_SYSKEYDOWN, _WM_SYSKEYUP:
	}

	func AddWindowMsg(fn func(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr)) uint32 {
	uMsg := currentUserWM.next()
	windowMsgs[uMsg] = func(hwnd syscall.Handle, uMsg uint32, wParam, lParam uintptr) uintptr {
	fn(hwnd, uMsg, wParam, lParam)
	return 0
	}
	return uMsg
	}

	func windowWndProc(hwnd syscall.Handle, uMsg uint32, wParam uintptr, lParam uintptr) (lResult uintptr) {
	fn := windowMsgs[uMsg]
	if fn != nil {
	return fn(hwnd, uMsg, wParam, lParam)
	}
	return _DefWindowProc(hwnd, uMsg, wParam, lParam)
	}

	type newWindowParams struct {
	opts *screen.NewWindowOptions
	w syscall.Handle
	err error
	}

	func NewWindow(opts *screen.NewWindowOptions) (syscall.Handle, error) {
	var p newWindowParams
	p.opts = opts
	SendScreenMessage(msgCreateWindow, 0, uintptr(unsafe.Pointer(&p)))
	return p.w, p.err
	}

	const windowClass = "shiny_Window"
	const screenWindowClass = "shiny_ScreenWindow"

	func initWindowClass() (err error) {
	wcname, err := syscall.UTF16PtrFromString(windowClass)
	if err != nil {
	return err
	}
	_, err = _RegisterClass(&_WNDCLASS{
	LpszClassName: wcname,
	LpfnWndProc: syscall.NewCallback(windowWndProc),
	HIcon: hDefaultIcon,
	HCursor: hDefaultCursor,
	HInstance: hThisInstance,
	// TODO(andlabs): change this to something else? NULL? the hollow brush?
	HbrBackground: syscall.Handle(_COLOR_BTNFACE + 1),
	})
	return err
	}

	func closeWindowClass() (err error) {
	wcname, err := syscall.UTF16PtrFromString(windowClass)
	if err != nil {
	return err
	}
	_UnregisterClass(wcname, hThisInstance)

	return nil
	}

	func initScreenWindow() (err error) {
	swc, err := syscall.UTF16PtrFromString(screenWindowClass)
	if err != nil {
	return err
	}
	emptyString, err := syscall.UTF16PtrFromString("")
	if err != nil {
	return err
	}
	wc := _WNDCLASS{
	LpszClassName: swc,
	LpfnWndProc: syscall.NewCallback(screenWindowWndProc),
	HIcon: hDefaultIcon,
	HCursor: hDefaultCursor,
	HInstance: hThisInstance,
	HbrBackground: syscall.Handle(_COLOR_BTNFACE + 1),
	}
	_, err = _RegisterClass(&wc)
	if err != nil {
	return err
	}
	screenHWND, err = _CreateWindowEx(0,
	swc, emptyString,
	_WS_OVERLAPPEDWINDOW,
	_CW_USEDEFAULT, _CW_USEDEFAULT,
	_CW_USEDEFAULT, _CW_USEDEFAULT,
	_HWND_MESSAGE, 0, hThisInstance, 0)
	if err != nil {
	return err
	}
	return nil
	}

	func closeScreenWindow() (err error) {
	// first destroy window
	_DestroyWindow(screenHWND)

	// then unregister class
	swc, err := syscall.UTF16PtrFromString(screenWindowClass)
	if err != nil {
	return err
	}
	_UnregisterClass(swc, hThisInstance)

	return nil
	}

	var (
	hDefaultIcon syscall.Handle
	hDefaultCursor syscall.Handle
	hThisInstance syscall.Handle
	)

	func initCommon() (err error) {
	hDefaultIcon, err = _LoadIcon(0, _IDI_APPLICATION)
	if err != nil {
	return err
	}
	hDefaultCursor, err = _LoadCursor(0, _IDC_ARROW)
	if err != nil {
	return err
	}
	// TODO(andlabs) hThisInstance
	return nil
	}

	//go:uintptrescapes

	func SendMessage(hwnd syscall.Handle, uMsg uint32, wParam uintptr, lParam uintptr) (lResult uintptr) {
	return sendMessage(hwnd, uMsg, wParam, lParam)
	}

	var mainCallback func()

	func Main(f func()) (retErr error) {
	// It does not matter which OS thread we are on.
	// All that matters is that we confine all UI operations
	// to the thread that created the respective window.
	runtime.LockOSThread()

	if err := initCommon(); err != nil {
	return err
	}

	if err := initScreenWindow(); err != nil {
	return err
	}
	defer func() {
	// TODO(andlabs): log an error if this fails?
	closeScreenWindow()
	}()

	if err := initWindowClass(); err != nil {
	return err
	}
	defer func() {
	// TODO(andlabs): log an error if this fails?
	closeWindowClass()
	}()

	// Prime the pump.
	mainCallback = f
	_PostMessage(screenHWND, msgMainCallback, 0, 0)

	// Main message pump.
	var m _MSG
	for {
	done, err := _GetMessage(&m, 0, 0, 0)
	if err != nil {
	return fmt.Errorf("win32 GetMessage failed: %v", err)
	}
	if done == 0 { // WM_QUIT
	break
	}
	_TranslateMessage(&m)
	_DispatchMessage(&m)
	}

	return nil
	}