type

TPrimeInfoEvent = procedure(Sender: TPrimeFinderThread; number: Integer) of Object;

TPrimeFinderThread = class(TThread)
  private
    testNumber: Integer;
    isPrime: Boolean;
  protected
    procedure Execute; override;
    procedure ShowProgress;
    function IsNumberPrime(number: Integer);
    fProgressCallback: TPrimeInfoEvent;
  public
    { Public-Deklarationen }
    constructor Create(Form1: TForm1);
end;

implementation

constructor PrimeFinderThread.Create(callback: TPrimeInfoEvent);
begin
  inherited Create(False);
  Self.fProgressCallback = callback;
  FreeOnTerminate := True;
end;

procedure PrimeFinderThread.Execute;
begin
  testNumber = 10000;
  while not Terminated and testNumber < 1e7 do
  begin
    if IsNumberPrime(testNumber) then
      Synchronize(ShowProgress);

    testNumber := testNumber+1;
  end;
end;

procedure PrimeFinderThread.ShowProgress;
begin
  fProgressCallback(Self, testNumber);
end;

function PrimeFinderThread.IsNumberPrime(number: Integer): Boolean;
var
  i: Integer;
  divisorCount: Integer;
  remainder: Integer;
begin
  divisorCount := 0;
  for i := 1 to number do
  begin
    remainder := number;
    while remainder >= i do { remainder := number mod i }
    begin
      remainder := remainder - i;
    end
    if remainder = 0 then
      inc(divisorCount);
  end
  Result := divisorCount = 2;
end;

end.