Call CopyFileEx and let the callback update a progress bar

Problem/Question/Abstract:

Does anyone have an example of using CopyFileEx with a CopyProgressRoutine? I have created a function that takes the same parameters as the CopyProgressRoutine, but when I pass it using @ or Addr() I get a Variable Required error message.

Answer:

Let's assume you call CopyFileEx and want the callback to update a progress bar. The callback cannot be an objects method but you can use the lpData parameter of CopyFileEx to pass any kind of data to the callback, e.g. a form reference. So, if you want to serve a progress form in the callback that would look like this (untested !):

type
  TProgressForm = class(TForm)
    AbortButton: TButton;
    ProgressBar: TProgressBar;
    procedure AbortButtonClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
    FCancel: BOOL;
  end;
  {form has fsStayOnTop formstyle!}

implementation

{$R *.DFM}

procedure TProgressForm.AbortButtonClick(Sender: TObject);
begin
  FCancel := True;
end;

{Note: could use int64 instead of COMP, but that would make this D4 specific}

function CopyCallback(TotalFileSize, TotalBytesTransferred, StreamSize,
  StreamBytesTransferred: COMP; dwStreamNumber, dwCallbackReason: DWORD;
  hSourceFile, hDestinationFile: THandle; progressform: TProgressForm): DWORD; stdcall;
var
  newpos: Integer;
begin
  Result := PROCESS_CONTINUE;
  if dwCallbackReason = CALLBACK_CHUNK_FINISHED then
  begin
    newpos := Round(TotalBytesTransferred / TotalFileSize * 100);
    with progressform.Progressbar do
      if newpos <> Position then
        Position := newpos;
    Application.ProcessMessages;
  end;
end;

function DoFilecopy(const source, target: string): Boolean;
var
  progressform: TProgressForm;
begin
  progressform := TProgressform.Create;
  try
    progressform.Show;
    Application.ProcessMessages;
    Result := CopyFileEx(PChar(source), PChar(target), @CopyCallback,
                 Pointer(progressform), @progressform.FCancel, 0);
  finally
    progressform.Hide;
    progressform.free;
  end;
end;