https://medium.com/%E5%BD%BC%E5%BE%97%E6%BD%98%E7%9A%84-swift-ios-app-%E9%96%8B%E7%99%BC%E6%95%99%E5%AE%A4/swift-practice-109-firebase%E5%88%9D%E6%8E%A2-google%E5%B8%B3%E8%99%9F%E7%99%BB%E5%85%A5-3b4f4b5d0663
