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How to call CopyFileEx and let the callback update a progress bar

Question
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.

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);
  public
    FCancel: BOOL;
  end;
  {form has fsStayOnTop formstyle!}

implementation

{$R *.DFM}

procedure TProgressForm.AbortButtonClick(Sender: TObject);
begin
  FCancel := True;
end;
   
function CopyCallback(TotalFileSize, TotalBytesTransferred, StreamSize,
  StreamBytesTransferred: Int64; 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;

Slightly modified by Peter Johnson

Original resource: The Delphi Pool
Author: Peter Below
Added: 2010-06-02
Last updated: 2010-06-02

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