delphi – 为什么截图不起作用(黑屏)？

unit Unit1;

interface

uses
Windows,Messages,SysUtils,Classes,Graphics,Controls,SvcMgr,Dialogs;

type
  TCopyDesk = class(TService)
  procedure ServiceContinue(Sender: TService; var Continued: Boolean);
  procedure ServiceExecute(Sender: TService);
  procedure ServicePause(Sender: TService; var Paused: Boolean);
  procedure ServiceShutdown(Sender: TService);
  procedure ServiceStart(Sender: TService; var Started: Boolean);
  procedure ServiceStop(Sender: TService; var Stopped: Boolean);
  private
    procedure CopyScreen(const Index: Integer);
  public
    function GetServiceController: TServiceController; override;
  end;

var
  CopyDesk: TCopyDesk;

implementation

{$R *.DFM}

procedure ServiceController(CtrlCode: DWord); stdcall;
begin
  CopyDesk.Controller(CtrlCode);
end;

procedure TCopyDesk.CopyScreen(const Index: Integer);
const
  DefaultWindowStation = 'WinSta0';
  DefaultDesktop = 'Default';
  CAPTUREBLT = $40000000;
  WINSTA_ALL_ACCESS = $0000037f;
var
  Bmp: TBitmap;
  hwinstaSave: HWINSTA;
  hdeskSave: HDESK;
  hwinstaUser: HWINSTA;
  hdeskUser: HDESK;
  dwThreadId: DWORD;
  hdcScreen : HDC;
  hdcCompatible : HDC;
  hbmScreen : HBITMAP;
begin
  hwinstaUser:= OpenWindowStation(DefaultWindowStation,FALSE,WINSTA_ALL_ACCESS);

  hwinstaSave:= GetProcessWindowStation;
  if hwinstaUser = 0 then
  begin
    OutputDebugString(PChar('OpenWindowStation failed' + SysErrorMessage       (GetLastError)));
    exit;
  end;

  if not SetProcessWindowStation(hwinstaUser) then
  begin
    OutputDebugString('SetProcessWindowStation failed');
    exit;
  end;

//  hdeskUser:= OpenDesktop(DefaultDesktop,MAXIMUM_ALLOWED);
  hdeskUser:= OpenInputDesktop(0,False,MAXIMUM_ALLOWED);
  if hdeskUser = 0 then
  begin
    OutputDebugString('OpenDesktop failed');
    SetProcessWindowStation (hwinstaSave);
    CloseWindowStation (hwinstaUser);
    exit;
  end;
  dwThreadId:= GetCurrentThreadID;

  hdeskSave:= GetThreadDesktop(dwThreadId);

  if not SetThreadDesktop(hdeskUser) then
  begin
    OutputDebugString(PChar('SetThreadDesktop' + SysErrorMessage(GetLastError)));
    Exit;
  end;

  try
    hdcScreen := GetDC(0);//GetDC(GetDesktopWindow);//CreateDC('DISPLAY',nil,nil);
    hdcCompatible := CreateCompatibleDC(hdcScreen);
    hbmScreen := CreateCompatibleBitmap(hdcScreen,GetDeviceCaps(hdcScreen,HORZRES),VERTRES));
    SelectObject(hdcCompatible,hbmScreen);
    bmp:= TBitmap.Create;
    bmp.Handle:= hbmScreen;
    BitBlt(hdcCompatible,bmp.Width,bmp.Height,hdcScreen,SRCCOPY OR CAPTUREBLT);
    Bmp.SaveToFile('C:UsersPublicScreenShot' + IntToStr(Index) + '.bmp');
  finally
    DeleteDC(hdcScreen);
    DeleteDC(hdcCompatible);
    Bmp.Free;
    Bmp:= nil;
  end;
  SetThreadDesktop(hdeskSave);
  SetProcessWindowStation(hwinstaSave);
  if hwinstaUser <> 0 then
    CloseWindowStation(hwinstaUser);
  if hdeskUser <> 0 then
    CloseDesktop(hdeskUser);
end;

function TCopyDesk.GetServiceController: TServiceController;
begin
  Result := ServiceController;
end;

procedure TCopyDesk.ServiceContinue(Sender: TService;
  var Continued: Boolean);
begin
  while not Terminated do
  begin
    Sleep(10);
    ServiceThread.ProcessRequests(False);
  end;
end;

procedure TCopyDesk.ServiceExecute(Sender: TService);
var
  Index: Integer;
begin
  Index:= 0;
  while not Terminated do
  begin
    CopyScreen(Index);
    Inc(Index);
    ServiceThread.ProcessRequests(False);
//    Sleep(1000);
//    if Index = 4 then
      DoStop;
  end;
end;

procedure TCopyDesk.ServicePause(Sender: TService; var Paused: Boolean);
begin
  Paused:= True;
end;

procedure TCopyDesk.ServiceShutdown(Sender: TService);
begin
  Status:= csStopped;
  ReportStatus();
end;

procedure TCopyDesk.ServiceStart(Sender: TService; var Started: Boolean);
begin
  Started:= True;
end;

procedure TCopyDesk.ServiceStop(Sender: TService; var Stopped: Boolean);
begin
  Stopped:= True;
end;
end.