ios - swift 警告 : Application delegate received call to -application:performFetchWithCompletionHandler: but the completion handler was never called

Question

我在下面的模拟器中调用这个函数来模拟后台获取。

然后我在日志中收到这个警告:

Swift Warning: Application delegate received call to -application:performFetchWithCompletionHandler: but the completion handler was never called.

我看到其他 Stack Iverflow 回答说我只需要添加 completionhandler()。我试过了，它说我需要添加一个参数，这就是我迷路的地方。

func application(_ application: UIApplication, performFetchWithCompletionHandler completionHandler:@escaping (UIBackgroundFetchResult) -> Void) {
    let db = Firestore.firestore()
    guard let uid = Auth.auth().currentUser?.uid else { return }

    //check if user online
    let docRef = db.collection("Users").document(uid)
    docRef.getDocument { (document, error) in
        if let document = document {
            if document.exists {
                guard let dictionary = document.data() else { return }
                guard let onlineOfflineStatus = dictionary["Online Offline Status"] as? String else { return }

                // if online create value to set offline an alert
                if onlineOfflineStatus == "Online" {
                    print("user is Online and inactive, will set value to trigger notification asking if they would like to go offline")
                    db.collection("sendGoOffline").document(uid).setData(["OfflineAlert" : 1200], completion: { (error) in
                        if let error = error {
                            print("there was an error", error)
                        }
                    })
                }
            }
        }

        if let error = error {
            print("failed to fetch user", error)
        }
    }
}

Answer 1

警告是告诉你添加这个方法:

completionHandler(argument)

参数是以下之一:

UIBackgroundFetchResult.noData
UIBackgroundFetchResult.newData
UIBackgroundFetchResult.failed

目的是告诉系统你已经完成了。

func application(_ application: UIApplication, performFetchWithCompletionHandler completionHandler:@escaping (UIBackgroundFetchResult) -> Void) {

    // do backgound data fetch

    // process it

    // finished
    completionHandler(UIBackgroundFetchResult.newData)
}

在这里阅读更多:

• documentation
• old but good article
• UIBackgroundFetchResult

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