协议:

@objc protocol CaptureDelegate: NSObjectProtocol{
    @objc optional func settingView(_ settingView: LQCaptureView , preview previewSwitch: Bool)
}

class实现:

class  LQCaptureView{
   var tapCallback: CaptureDelegate?

   func ifPerform() {
      self.delegate?.settingView?(self, preview: isOn)
    }
}

重点:

1 协议使用@objc修饰符
2 协议方法使用@objc optional修饰
3 调用方法时方法名后边添加 ?, 如下图

self.delegate?.settingView?(self, preview: isOn)