如何通过 Firebase 返回我在数据服务类中创建的对象? [英] How can I return an object that I create in my data service class through Firebase?

问题描述

我正在尝试遵守 MVC 实践，并将所有网络代码保存在我在应用程序中使用的数据服务类中.在一个屏幕上，我有需要显示的用户名和用户名.更新此内容时，我正在调用此函数:

I am trying to adhere to MVC practices and keep all the network code inside the data service class that I am using in my app. On one screen I have the user's name and username that needs to be displayed. When updating this, I am calling this function:

func grabUserData() -> User {
    REF_USERS.child(getCurrentUID()).observeSingleEvent(of: .value) { 
        (snapshot) in
        if let userDict = snapshot.value as? Dictionary<String, String> {
            let user = User(
                first:    userDict["firstName"]!, 
                last:     userDict["lastName"]!, 
                username: userDict["username"]!
            )
            return user
        } 
    }
}

但是我在尝试返回用户时遇到错误！它说:

But I am getting an error trying to return the user! It says:

void 函数中出现意外的非 void 返回值.

Unexpected non-void return value in void function.

但是这个函数显然不是空的.那我该怎么办?

But the function clearly isn't void. So what do I do?

推荐答案

你混淆了 grabUserData function 返回值和 Firebase closure返回值 — 前者是 User 而后者是 Void ;)

You are confusing your grabUserData function return value with the Firebase closure return value — the former is User but the latter is Void ;)

你实际上是从这个关闭返回— 我现在使用 explicit 返回类型来明确:

You are actually returning from this closure — I'm now using a explicit return type to be clear:

{ 
    (snapshot) -> Void in
    if let userDict = snapshot.value as? Dictionary<String,String> {
        let user = User(
            first:    userDict["firstName"]!, 
            last:     userDict["lastName"]!, 
            username: userDict["username"]!
        )
        return user
    }
}

作为最后一个参数传递给 observeSingleEvent Firebase 函数.这是一个非常的错误;)

which is passed as the last argument to the observeSingleEvent Firebase function. This is a very common mistake ;)

完成处理程序.这里的标准模式是通过完成处理程序返回所需的User.该解决方案很好地模拟了网络请求(例如 Firebase 数据库调用)的异步性质.例如:

Completion handler. A standard pattern here is to return the desired User via a completion handler instead. This solution nicely models the asynchronous nature of network requests such as Firebase database calls. For instance:

func grabUserData(completion: @escaping (User?) -> Void) {
    REF_USERS.child(getCurrentUID()).observeSingleEvent(of: .value) { 
        (snapshot) in
        if let userDict = snapshot.value as? Dictionary<String, String> {
            let user = User(
                first:    userDict["firstName"]!, 
                last:     userDict["lastName"]!, 
                username: userDict["username"]!
            )
            completion(user) // Returns user!
        } else {
            completion(nil) // User not found!
        }
    }
}

最后，在您的数据服务客户端代码中，像这样调用它:

Finally, in your data service client code, call it like this:

grabUserData() { 
    (user) in
    if let user = user {
        print("Grabbed user: (user)")
    } else {
        print("User not found!")
    }        
}

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