Application.ProcessMessages

//如果有这样一个循环, 是非常可怕的; 因为它完不了, 你得等着.
procedure TForm1.Button1Click(Sender: TObject);
var
  i: Integer;
begin
  for i := 0 to MaxInt do
  begin
    Text := IntToStr(i);
  end;
end;


//即使这样也无济于事, 因为在循环期间你执行不了 Button2Click
var
  b: Boolean;

procedure TForm1.Button1Click(Sender: TObject);
var
  i: Integer;
begin
  b := True;
  for i := 0 to MaxInt do
  begin
    if b then Text := IntToStr(i) else Exit;
  end;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  b := False;
end;


//如果在循环期间...
for i := 0 to MaxInt do
  begin
    if b then Text := IntToStr(i) else Exit;
    {看看还有其他什么事情发生}
  end;


//Application.ProcessMessages 就是干这个的!
var
  b: Boolean;

procedure TForm1.Button1Click(Sender: TObject);
var
  i: Integer;
begin
  b := True;
  for i := 0 to MaxInt do
  begin
    if b then Text := IntToStr(i) else Exit;
    Application.ProcessMessages; {!}
  end;
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  b := False;
end;