Nim

import wNim
import winim/lean

const
  NOTIFY_FOR_THIS_SESSION: DWORD = 0
  WM_WTSSESSION_CHANGE: DWORD = 0x2b1
  WTS_SESSION_LOCK: DWORD = 0x7
  WTS_SESSION_UNLOCK: DWORD = 0x8
  WTS_SESSION_REMOTE_CONTROL: DWORD = 0x9

{.push stdcall, dynlib: "wtsapi32".}
proc WTSRegisterSessionNotification*(hwnd: HANDLE, dwFlags: DWORD): bool
  {.importc: "WTSRegisterSessionNotification".}
proc WTSUnRegisterSessionNotification*(hwnd: HANDLE): bool
  {.importc: "WTSUnRegisterSessionNotification".}
{.pop.}

when isMainModule:
  let
    app = App()
    frm = Frame(title="Frame", size=(640, 480))
    sn = WTSRegisterSessionNotification(frm.getHandle, NOTIFY_FOR_THIS_SESSION)

  frm.WM_WTSSESSION_CHANGE do (ev: wEvent):
    echo ev.getWindow.handle
    case ev.getwParam
    of WTS_SESSION_LOCK: echo "lock"
    of WTS_SESSION_UNLOCK: echo "unlock"
    of WTS_SESSION_REMOTE_CONTROL: echo "remote control"
    else: echo "unknown"

  frm.center
  frm.show

  if not sn:
    echo "Error: can not register WTS Session Notification"
  else:
    defer:
      discard WTSUnRegisterSessionNotification(frm.getHandle)
    app.mainLoop