Swift 中的相互依赖注入

Question

我按如下方式相互注入类，但是当我运行应用程序时，应用程序崩溃。 这个实现有什么问题？ 它显示 unsafeMutableAddressor。

class NetworkManager {
  
  static let shared = NetworkManager()
  let analyticsManager: ServiceManager

  init(serviceManager:ServiceManager = .shared) {
    self.serviceManager = serviceManager
  }
}

class ServiceManager {
  
  static let shared = ServiceManager()
  let networkManager: NetworkManager

  init(networkManager:NetworkManager = .shared) {
    self.networkManager = networkManager
  }
}