How to copy a file and display it's progress

This is just a demo, it can be modified to copy files using thread(s) and display the progress in a separate window(like Total Commander does).

procedure TurboCopyFile(
  const
    SourceFile, (* the source file *)
    DestinationFile: String; (* destination file *)
  Display: PProgressbar (* this is a pointer to progress bar
    which we update if assigned,
    this procedure can be called without a
    progress bar like so:
    TurboCopyFile(Source, Destination, NIL); *)
  );
type
  (* define an array of 4096 bytes which holds the bytes *)
  TurboBuffer = array[1..4096] of Byte;
const
  (* we need to set the size of the buffer in a constant *)
  szBuffer = sizeof(TurboBuffer);
var
  (* File streams with which we work *)
  InStream,
  OutStream: TFileStream;
  (* we need this to store a logical operation's result *)
  CanCopy: Boolean;
  (* holds the number of bytes left when
     InStream.Size -InStream.Position < 4096 bytes *)
  BytesLeft: Integer;
  (* the Buffer almighty *)
  Buffer: TurboBuffer;
begin
  (* Open the source file so we can read it's bytes *)
  InStream := TFileStream.Create(SourceFile, fmOpenRead);
  (* create a new file to the desired destination *)
  OutStream := TFileStream.Create(DestinationFile, fmCreate);

  (* check if a progress bar was passed *)
  if Assigned(Display) then
    Display^.Max := InStream.Size;

  (* this is the operation I was talking about in the
     variable section, this checks if we can read a
     full buffer(4096 bytes) *)
  CanCopy :=
    (InStream.Size > InStream.Position) and
    ((InStream.Size -InStream.Position) >= szBuffer);

  (* this ensures that the progress bar is being painted *)
  Application.ProcessMessages;

  (* loop while we CanCopy *)
  while CanCopy do begin
    (* read 4096 bytes from source file *)
    InStream.ReadBuffer(Buffer, szBuffer);
    (* then write it to destination file *)
    OutStream.WriteBuffer(Buffer, szBuffer);
    (* check if display is assigned *)
    if Assigned(Display) then
      (* update it's position *)
      Display^.Position := InStream.Position;
    CanCopy :=
      (InStream.Size > InStream.Position) and
      ((InStream.Size -InStream.Position) >= szBuffer);
  end;

  (* store the number of bytes that is less than 4096 into
     a local variable *)
  BytesLeft := InStream.Size -InStream.Position;

  (* do we have some bytes left ? we don't care how many,
     we just know it's less than 4096 bytes *)
  if BytesLeft > 0 then begin
    (* surprise, or not we have some bytes left,
       read them all into the buffer *)
    InStream.ReadBuffer(Buffer, BytesLeft);
    (* write them to destination file *)
    OutStream.WriteBuffer(Buffer, BytesLeft);
  end;

  (* do we have a progress bar ? *)
  if Assigned(Display) then
    (* we do? then update it's progress *)
    Display^.Position := Display^.Max;

  (* free the memory *)
  FreeAndNil(InStream);
  FreeAndNil(OutStream);
end; (* end of TurboCopyFile *)

A demo application can be downloaded as
binary
or
sourcecode.