Guzzle-> getAsync（）必须wait（）才能下载文件

I don't understand why but Guzzle doesn't download my file with getAsync if I don't wait the promise. But the reason I'm using getAsync is to continue my script while downloading the image.

Here is how I'm using it

protected function downloadImage ($url, $subDir, $filename)
{
  $dir = '../public/files/' . $this->getAccountId() . '/';
  if (!is_dir($dir)) {
    mkdir($dir);
  }
  if (!is_dir($dir . $subDir . '/')) {
    mkdir($dir . $subDir . '/');
  }
  $file = $dir . $subDir . '/' . $filename . '.jpg';

  $client = new Client();
  $promise = $client->getAsync($url, [ 'sink' => $file ]);
  //$res = $promise->wait();

  return $file;
}

public function myFunction ()
{
  foreach ($aUrl as $url) {
    $this->downloadImage($url, 'subFolder', 'myfilename')
  }
}

Of course the final filename is different for each files but that's not the problem here