无法为闭包内部的变量赋值 [英] Can't assign a value to variable inside of closure Swift

问题描述

我正在尝试从Firebase获取属于特殊用户的当前文档数.每当我尝试将计数值分配给闭包中的变量时，值始终显示为nill.因此，我进行了一些研究，结果我发现联网有时会花费很长时间，并且异步发生.因此，如果我没错，因为异步返回值时，可能会在赋值之前在函数内部返回一个值.

I'm trying to get current document count belongs to spesific user from Firebase. Whenever i try to assign count value to my variable in closure, value is always shown as nill. So that i did couple of research, as a result i figured out networking sometimes takes so long and it happens asynchronously. So if i'm not wrong due to asynchronous returning a value inside a fucntion might happen before assign value .

我试图添加dispatchqueue.main.async，但是对我来说不起作用...

I tried to add dispatchqueue.main.async but it didn't work for me...

这是我的代码

 func getEventCount () -> Int? {
    var count: Int?
    db.collection("Events").whereField("owner", isEqualTo: currentUser.email).getDocuments { (snapshot, error) in
        if error != nil {
            print(error)
        }else {
            DispatchQueue.main.async {

                if let snapshot = snapshot {
                    count = snapshot.count
                }

            }
        }
    }
    return count
}

我的主要目标是从数据库获取计数数据并将其分配给"count"变量.那么，为什么我需要该计数变量?因为我要将该计数值传递给tableView数据源方法numberOfRowsInSection，该方法需要一个int值.使用该值，我将在表视图中表示来自Firestore的事件"文档中的一些数据.

注意:当我尝试在闭包中打印计数值时，它会显示所需的值，但是当函数返回值时，它会显示nill ...

note: When i try to print count value in closure it shows desired value, but when function return value it's shown nill...

推荐答案

一旦它是异步调用，您就无法从函数中同步返回值.您应该接受将接受计数的函数的回调.该回调函数或闭包将异步传递该值.

Once it is a Async call - you cannot synchronously return the value from the function. You should accept a callback to the function that will accept the count. That callback function or closure will be passed the value asynchronously.

func getEventCount (callback: @escaping(Result<Int, Error>) -> Void) {
    db.collection("Events").whereField("owner", isEqualTo: currentUser.email).getDocuments { (snapshot, error) in
        if error != nil {
            let result = Result.failure(error)
            callback(result)
        }else if let snapshot = snapshot {
               let result = Result.success(snapshot.count)
               callback(result)
        } else {
            let result = Result.failure(SomeCustomAppError)
            callback(result)
        }
    }
}

然后您可以通过回调函数调用此函数

Then you can call this function passing in a callback

self.getCount() { result in
  switch result {
   case .success(let count): /// use count
   print(count)
   // only here u can assign the count value to ur variable
   case .error(let error): /// handle error
   print(error.localizedDescription)
  }
}

注意:在上面，我使用了Swift标准库中的Result数据类型- https://developer.apple.com/documentation/swift/result ，以便两者均出错或结果可以传回

Note: In the above I've used the Result datatype from Swift standard library - https://developer.apple.com/documentation/swift/result so that both error or result can be passed back

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