How to calculate download speed?

type
  TCallbackProc = procedure (aMessage: string; aMessageInfo: Integer = 0) of object;
 
  TCallbackThread = class(TThread)
  private
    FCallBack: TCallbackProc;
    FCallbackMsg: string;
    FCallbackMsgInfo: integer;
    procedure SynchedCallback;
  protected
    procedure DoCallback(aMsg: string; aMsgInfo: integer = 0); virtual;
    property Callback: TCallbackProc read FCallback;
  public
    constructor Create(aCallback: TCallbackProc; CreateSuspended: Boolean = False); reintroduce; virtual;
  end;
 
  TDownloadThread = class(TCallbackThread)
  private
    FURL: String;
    FFileName: String;
    FProgress: Int64;
    FDuration: Integer;
    FFileSize: Int64;
    FDownloadPercent: Integer;
    fLastTime: TDateTime;
    fLastDownloaded: Int64;
  protected
    procedure Execute; override;
  public
    constructor Create(aURL,aSaveToFile: String; aCallbackProc: TCallbackProc); reintroduce;
    destructor Destroy; override;
    procedure URL_OnDownloadProgress(Sender: TDownLoadURL;
      aProgress, aProgressMax: Cardinal;
      StatusCode: TURLDownloadStatus;
      StatusText: String; var Cancel: Boolean);
  end;
 
implementation
 
{ TCallbackThread }
 
constructor TCallbackThread.Create(aCallback: TCallbackProc; CreateSuspended: Boolean = False);
begin
  inherited Create(CreateSuspended);
  FreeOnTerminate := True;
  FCallback := aCallback;
end;
 
procedure TCallbackThread.DoCallback(aMsg: string; aMsgInfo: Integer = 0);
begin
  FCallbackMsg := aMsg;
  FCallbackMsgInfo := aMsgInfo;
  Synchronize(SynchedCallback);
end;
 
procedure TCallbackThread.SynchedCallback;
begin
  if Assigned(FCallback) then
    FCallBack(FCallbackMsg, FCallbackMsgInfo);
end;
 
{ TDownloadThread }
 
constructor TDownloadThread.Create(aURL, aSaveToFile: string; aCallback: TCallbackProc);
begin
  inherited Create(aCallback, False);
  FURL := aURL;
  FFileName := aSaveToFile;
  fLastTime := Now;
  FreeOnTerminate := True;
end;
 
procedure TDownloadThread.Execute;
begin
  DoCallback('Starting download of ' + fUrl + '/' + fFileName, 0);
  with TDownloadURL.Create(nil) do
  try
    URL:= FURL;
    FileName := FFileName;
    OnDownloadProgress := URL_OnDownloadProgress;
    fLastTime := Now;
    fLastDownloaded := 0;
    ExecuteTarget(nil);
  finally
    Free;
  end;
  DoCallbakc('Download finished', 10);
end;
 
destructor TDownloadThread.Destroy;
begin
  //Downloading := False;
  inherited Destroy;
end;
 
procedure TDownloadThread.URL_OnDownloadProgress(Sender: TDownLoadURL;
  aProgress, aProgressMax: Cardinal;
  StatusCode: TURLDownloadStatus;
  StatusText: String; var Cancel: Boolean);
  deltaDownloaded: Int64;
begin
  // Downloading := True;
  fProgress := aProgress;
  fFileSize := aProgressMax;
 
  if (fProgress > 0) and (fFileSize > 0) then
    fDownloadPercent := Trunc(((fProgress / fFileSize) * 100));
  // Show percent complete  
  DoCallback(FloatToStr(fDownloadPercent), 1);
  // Show speed 
  deltaDownloaded := fProgress - fLastDownloaded;
  DoCallback(FloatToStr(deltaDownloaded / 1024 / SecondSpan(Now, fLastTime)), 2);
  fLastTime := Now;
  fLastDownloaded := fProgress;
end;

                                              

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