检查用户是否已存在Firebase swift 3.0

import Firebase

// ...

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
    FirebaseApp.configure()
    // ...
    return true
}

import Firebase

// ...

func checkUserExists(email: String, completion: @escaping (Bool) -> Void) {
    Auth.auth().fetchSignInMethods(forEmail: email) { (methods, error) in
        if let error = error {
            print("Error checking user existence: \(error.localizedDescription)")
            completion(false)
        } else if let methods = methods {
            completion(!methods.isEmpty)
        } else {
            completion(false)
        }
    }
}

let email = "example@example.com"

checkUserExists(email: email) { (exists) in
    if exists {
        print("User exists")
    } else {
        print("User does not exist")
    }
}