winauth-go/internal/win32/window_subclass_windows.go

//go:build windows

package win32

import (
	"fmt"
	"sync"
	"syscall"
	"unsafe"
)

// WindowSubclass intercepts the WM_CLOSE message on an external window
// (in our case Gio's app window) and redirects it to a Go callback. The
// caller decides whether to actually close, hide, or ignore.
//
// Implementation notes
//
//   - We swap the window procedure via SetWindowLongPtrW(GWLP_WNDPROC),
//     keeping a reference to the original so non-WM_CLOSE messages pass
//     through unchanged.
//   - The Go callback runs on the UI thread of the subclassed window
//     (because that's who DispatchMessage'd into our hook). Returning
//     false from the callback drops the message; true forwards it on.
//   - Only one subclass per HWND is supported. Calling Install a second
//     time on the same HWND returns an error.
type WindowSubclass struct {
	hwnd     uintptr
	origProc uintptr
	cb       func() bool

	once sync.Once
	gone bool
}

var (
	subclassMu sync.Mutex
	subclasses = map[uintptr]*WindowSubclass{}
)

// InstallCloseHook subclasses hwnd and routes WM_CLOSE through onClose.
// If onClose returns false the close is swallowed; if true the original
// window procedure handles it (the default behaviour, which destroys
// the window).
func InstallCloseHook(hwnd uintptr, onClose func() bool) (*WindowSubclass, error) {
	if hwnd == 0 {
		return nil, fmt.Errorf("win32: InstallCloseHook: nil hwnd")
	}
	subclassMu.Lock()
	defer subclassMu.Unlock()
	if _, exists := subclasses[hwnd]; exists {
		return nil, fmt.Errorf("win32: hwnd %#x already subclassed", hwnd)
	}

	s := &WindowSubclass{hwnd: hwnd, cb: onClose}
	proc := syscall.NewCallback(s.wndProc)
	orig, _, e := procSetWindowLongPtrW.Call(hwnd, gwlpWndProc, proc)
	if orig == 0 {
		if errno, ok := e.(syscall.Errno); ok && errno != 0 {
			return nil, fmt.Errorf("win32: SetWindowLongPtrW: %w", errno)
		}
		return nil, fmt.Errorf("win32: SetWindowLongPtrW returned 0")
	}
	s.origProc = orig
	subclasses[hwnd] = s
	return s, nil
}

// Remove restores the original window procedure. Safe to call more than
// once; subsequent calls are no-ops.
func (s *WindowSubclass) Remove() {
	s.once.Do(func() {
		s.gone = true
		subclassMu.Lock()
		delete(subclasses, s.hwnd)
		subclassMu.Unlock()
		procSetWindowLongPtrW.Call(s.hwnd, gwlpWndProc, s.origProc)
	})
}

func (s *WindowSubclass) wndProc(hwnd uintptr, msg uint32, wParam, lParam uintptr) uintptr {
	if msg == wmClose && s.cb != nil && !s.gone {
		if !s.cb() {
			return 0
		}
	}
	r, _, _ := procCallWindowProcW.Call(s.origProc, hwnd, uintptr(msg), wParam, lParam)
	return r
}

// ShowWindow toggles a window's visibility. cmdShow uses the standard
// SW_* constants (SW_HIDE = 0, SW_SHOW = 5, SW_RESTORE = 9).
func ShowWindow(hwnd uintptr, cmdShow int32) {
	procShowWindowProc.Call(hwnd, uintptr(cmdShow))
}

// SW_* constants exposed so callers do not need to redefine them.
const (
	SW_HIDE    int32 = 0
	SW_SHOW    int32 = 5
	SW_RESTORE int32 = 9
)

// FindWindowByTitle searches top-level windows for one whose title
// exactly matches. Returns 0 when nothing matches.
func FindWindowByTitle(title string) uintptr {
	wtitle, err := syscall.UTF16PtrFromString(title)
	if err != nil {
		return 0
	}
	r, _, _ := procFindWindowW.Call(0, uintptr(unsafe.Pointer(wtitle)))
	return r
}

// PostCloseMessage asks the window to close the same way the user
// clicking the X would. Combined with InstallCloseHook this lets the
// tray "Exit" item route through the same path as a real close, so the
// hook can let it through after setting an "exiting" flag.
func PostCloseMessage(hwnd uintptr) {
	procPostMessageW.Call(hwnd, uintptr(wmClose), 0, 0)
}

// -----------------------------------------------------------------------------
// raw syscalls

var (
	procSetWindowLongPtrW = user32.NewProc(setWindowLongPtrName)
	procCallWindowProcW   = user32.NewProc("CallWindowProcW")
)

const (
	wmClose uint32 = 0x0010
)

// gwlpWndProc is the GWLP_WNDPROC index. Declared as a var (not a const)
// because Go forbids constant negative-to-uintptr conversion.
var gwlpWndProc = func() uintptr {
	x := int32(-4)
	return uintptr(x)
}()