ios - 调度组错误

Question

因此，我正在尝试重组我的 API 调用以利用 dispatchGroups，这样我就不必如此快速地重新加载 collectionViews 和其他元素。我知道对于调度组，您通常必须创建一个，进入，然后离开一定次数。完成此操作后，您通常会通知主队列执行一些操作。但是，在下面的代码片段中，它甚至在我进入调度组之前就通知了主队列。这是把事情扔掉了。如果有人可以查看我的代码并告诉我出了什么问题，我将不胜感激。

 static func showFeaturedEvent(for currentLocation: CLLocation,completion: @escaping ([Event]) -> Void) {
        //getting firebase root directory
        let dispatchGroup1 = DispatchGroup()
        var currentEvents:[Event]?
        var geoFireRef: DatabaseReference?
        var geoFire:GeoFire?
        geoFireRef = Database.database().reference().child("featuredeventsbylocation")
        geoFire = GeoFire(firebaseRef: geoFireRef!)
        let circleQuery = geoFire?.query(at: currentLocation, withRadius: 17.0)
        circleQuery?.observe(.keyEntered, with: { (key: String!, location: CLLocation!) in
            print("Key '\(key)' entered the search area and is at location '\(location)'")
            dispatchGroup1.enter()
            print("entered dispatch group")
            EventService.show(forEventKey: key, completion: { (event) in
                if let newEvent = event {
                    currentEvents?.append(newEvent)
                    dispatchGroup1.leave()
                    print("left dispatch group")
                }
            })

        })
        dispatchGroup1.notify(queue: .main, execute: {
            if let currentEventsFinal = currentEvents{
                completion(currentEventsFinal)
            }
        })
    }

我在其他地方运行调度组。我不确定这是否会影响任何事情，但我觉得在这个问题中注意这一点很重要。

Answer 1

您需要在开始异步任务之前进入组，并在异步任务完成时离开。在第一个异步任务完成之前，您不会执行第一个 enter，因此当执行命中您的 notify 时，调度组为空并立即触发。

调用 leave 的次数与调用 enter 的次数相同也很重要，否则该组将永远不会为空，因此请谨慎调用 leave 在条件语句中。

static func showFeaturedEvent(for currentLocation: CLLocation,completion: @escaping ([Event]) -> Void) {
    //getting firebase root directory
    let dispatchGroup1 = DispatchGroup()
    var currentEvents:[Event]?
    var geoFireRef: DatabaseReference?
    var geoFire:GeoFire?
    geoFireRef = Database.database().reference().child("featuredeventsbylocation")
    geoFire = GeoFire(firebaseRef: geoFireRef!)
    let circleQuery = geoFire?.query(at: currentLocation, withRadius: 17.0)

    disatchGroup1.enter()

    circleQuery?.observe(.keyEntered, with: { (key: String!, location: CLLocation!) in
        print("Key '\(key)' entered the search area and is at location '\(location)'")

        dispatchGroup1.enter()

        EventService.show(forEventKey: key, completion: { (event) in
            if let newEvent = event {
                currentEvents?.append(newEvent)
                print("left dispatch group")
            }

            dispatchGroup1.leave()

        })

        dispatchGroup1.leave()

    })

    dispatchGroup1.notify(queue: .main, execute: {
        if let currentEventsFinal = currentEvents{
            completion(currentEventsFinal)
        }
    })
}