void 函数中出现意外的非空返回值 - Swift 4(Firebase、Firestore) [英] Unexpected non-void return value in void function - Swift 4 (Firebase, Firestore)

问题描述

所以我想要一个布尔函数，它根据 users 集合中的用户是否存在给定的电子邮件来返回 true 或 false.

So I want a Boolean function that returns true or false depending on whether the given email exists for a user in the users collection.

但是，如果我尝试在 getDocuments 调用中返回 True 或 False，我会收到错误:non-void return value在 void 函数中

However if I try and return True or False within the getDocuments call I get the error: non-void return value in void function

func checkUserWith(email: String) -> Bool
{
    let usersDB = database.collection("users")
    usersDB.whereField("email", isEqualTo: email).getDocuments { (snapshot, error) in

        if error != nil
        {
            print("Error: (error?.localizedDescription ?? "")")
            return false
        }

        for document in (snapshot?.documents)! {
            if document.data()["email"]! as! String == email {
                return true
            }
        }

        return false
    }
}

我有一种感觉，这是因为我试图在 Firestore 调用中返回一个布尔值，该布尔值需要不同的变量类型?

I have a feeling it is because I am trying to return a boolean within the Firestore call which is expecting a different variable type?

推荐答案

由于 firebase 操作给你一个回调闭包，而且调用是异步的，我相信你不可能直接从闭包返回.但是，您可以按如下方式返回一个指示 true 或 false 的转义闭包...

Since the firebase operation gives you a callback closure, and the calls made asynchronously, I believe it wont be possible for you to directly return from closures. However, you can return an escaping closure indicating true or false as follows...

func checkUserWith(email: String, completion: @escaping (Bool) -> Void)
{
    let usersDB = database.collection("users")
    usersDB.whereField("email", isEqualTo: email).getDocuments { (snapshot, error) in

        if error != nil
        {
            print("Error: (error?.localizedDescription ?? "")")
            completion(false)
        }

        for document in (snapshot?.documents)! {
            if document.data()["email"]! as! String == email {
                completion(true)
                return
            }
        }

        completion(false)
    }
}

然后当你调用这个方法时:

Then when you call this method:

checkUserWith(email: emailHere) { (isSucceeded) in
    if isSucceeded {
        //it exists, do something
    } else {
        //user does not exist, do something else
    }
}

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