将 firebase 数据库值转换为 Swift 变量

Question

我声明了一个 var，我可以从 Firebase 数据库中检索该值，但是当我在 ViewDidLoad 中打印该 var 时，它是空的，我不明白出了什么问题。感谢大家 这是我打印 var 时得到的答案：这是语言

//
var language: String = ""

//
func getUserLanguage(completion:((String) -> Void)?) {
    let ref = Database.database().reference()
    let uid = Auth.auth().currentUser!.uid
    ref.child("users").child(uid).observe(.value, with: { (snapshot) in
       let value = snapshot.value as? NSDictionary
       let languageResult = value?["language"] as? String ?? ""
       completion?(languageResult)
    }
)}

//
getUserLanguage { (languageResult) in
   self.language = languageResult
}

// I print the value in ViewDidload
print("this is the language\(self.language)")

Answer 1

1. 尝试在函数中打印languageResult - 也许你没有在函数内部得到它并且变量没有被赋值
2. 公开language变量

Answer 2

我认为您在这里缺少代码的异步特性。 getUserLanguage 函数只有在从 firebase 的 observe(.value, with: { (snapshot) 方法获得回调时才会调用完成。它是异步的。在viewDidLoad 中调用getUserLanguage 后，您不会立即获得language 的值。

override func viewDidLoad() {
    super.viewDidLoad()
    
    getUserLanguage { (language) in
        print(language) // --> prints the expected value
        self.language = language
    }
    print(language) // --> prints ""
}

func getUserLanguage(completion: @escaping (String) -> Void) {
    let ref = Database.database().reference()
    let uid = Auth.auth().currentUser!.uid
    ref.child("users").child(uid).observe(.value, with: { (snapshot) in
        let value = snapshot.value as? [String: Any]

        let languageResult = value?["language"] as? String ?? ""
        print("language: ", languageResult) // --> prints the expected value

        completion(languageResult)
    })
}