为此使用NSMutableAttributedString

private let signUpButton = UIButton().then {
  // Creating gray text part
  let grayButtonText = NSAttributedString(string: "FirstPart", attributes: [.font: UIFont.systemFont(ofSize: 12), .foregroundColor: UIColor.gray ])
  // Creating black text part
  let blackButtonText = NSAttributedString(string: "SecondPart", attributes: [.font: UIFont.systemFont(ofSize: 12), .foregroundColor: UIColor.black ])

  // Merging two parts together
  let buttonTitle = NSMutableAttributedString(attributedString: grayButtonText)
  buttonTitle.append(blackButtonText)
  
  // Set it as a title for ".normal" state
  $0.setAttributedTitle(buttonTitle, for: .normal)
}

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