Swift Function从异步Firebase调用返回值 [英] Swift Function returning a value from asynchronous firebase call

问题描述

我正在编写一个函数，该函数接受一个groupchatID(字符串)，并为该群聊返回一个收件人列表([String]).我正在为函数的异步部分而苦苦挣扎.当我运行该函数时，它将正确地将我要查找的用户名数组打印到控制台.虽然，当我调用该函数并尝试打印返回的值时，它始终是一个空数组，因为该函数会在Firebase调用完成之前返回该数组.我正在尝试使用回调，但是我不太了解所有语法.请看一下，让我知道需要更改的内容.

I am writing a function that takes a groupchatID (String) and returns a list of Recipients ([String]) for that group chat. I am struggling with the asynchronous part of the function however. When I run the function, it correctly prints to the console the array of usernames I was looking for. Although, when I call the function and try to print the returned value, it is always an empty array because the function returns the array before the firebase call has finished. I am trying to use a callback, but I do not quite understand the syntax of it all. Please take a look and let me know what needs to be changed.

功能:

func GetRecipientsFor(GroupChatID :String , completion: @escaping ([String]) -> ()) {
var returnArray: [String] = [""]
rootRef.child("chatMembers").child(GroupChatID).observeSingleEvent(of: .value, with: { (snapshot) in
    for child in snapshot.children.allObjects {
        var append = child as! FIRDataSnapshot
        returnArray.append((append.key as String))
        print("Return Array Currently Contains: \(returnArray)")
        //The above printout works properly and when the for loop finishes, the array is exactly as I want it
    }
    completion(returnArray)
    //BUT, this portion returns an empty array
})
}

我如何调用该函数:

GetRecipientsFor(GroupChatID: gchatID) { (result) -> () in
        print(result)
    }

新功能调用

var recipients : [String] = [""]
DispatchQueue.main.async {
    GetRecipientsFor(GroupChatID: gchatID) { result in
    print(result) //PRINTS CORRECTLY!!!
    recipients = result
    }
}
print(recipients) //PRINTS A BLANK ARRAY

推荐答案

var recipients : [String] = [""]
DispatchQueue.main.async {
    GetRecipientsFor(GroupChatID: gchatID) { result in
    print(result)
    recipients = result
    }
}
print(recipients) // Completes before recipients = result

是最后一行发生在异步调用之前.

is that the last line is happening before the async call.

为进一步解释print(recipients)发生在recipients = result之前.使用接收者的所有逻辑都需要在该完成块内发生.您所需要做的就是

To explain futher print(recipients) happens before recipients = result. All logic using recipients needs to happen within that completion block. All you need to do is

func getRecipients(completion: @escaping ([String]) -> ()) {
    var recipients : [String] = [""]
    DispatchQueue.main.async {
        GetRecipientsFor(GroupChatID: gchatID) { result in
        print(result)
        completion(result)
        }
    }
}

如果要包含其他逻辑，可以在补全内调用一个函数，即handleResults(result).我认为阅读有关关闭/completion阻止/和异步调用.

if you want to have further logic included you can call a function within the completion i.e. handleResults(result). I think it would be very beneficial to read more about closures/completion blocks/and async calls.

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