Question

我正在使用GCD通知主线程（该函数内部有2个异步调用）

我的代码：

func getWavesByMostRecent(closure: @escaping ([Wave]?) -> Void) {
    var waves = [Wave]()
    let dispatchGroup = DispatchGroup()

    self.query = DatabaseManager.waveRef.queryOrdered(byChild: Constants.reverseTimeStampKey)
    self.handle = self.query?.observe(.value, with: { (snapshot) in

        for value in snapshot.children {
            guard let wave = Wave(snapshot: value as! DataSnapshot) else { return }

            self.geoFire = GeoFire(firebaseRef: DatabaseManager.waveRef)
            let currentLocation = LocationManager.shared.getCurrentLocation()

            dispatchGroup.enter()
            self.geoFire?.getLocationForKey(wave.waveID, withCallback: { (location, error) in
                guard let location = location else { return }
                if error == nil {
                    if location.distance(from: currentLocation) < Constants.distance {
                        print("Wave", wave.waveID, "is in range")
                        waves.append(wave)
                    } else {
                        print("Wave", wave.waveID, "is out of range")
                    }
                } else {
                    print(error?.localizedDescription ?? "")
                }
                dispatchGroup.leave()
            })
        }
        dispatchGroup.notify(queue: .main) {
            print("THERE ARE SO MANY WAVES:", waves.count)
            closure(waves)
        }
    })
}

但是.notify闭包不起作用，我不能称呼我的“主要”闭包权利。我究竟做错了什么？任何建议将不胜感激。

Answer 1

尝试此更改：

self.geoFire?.getLocationForKey(wave.waveID, withCallback: { (location, error) in
    defer { dispatchGroup.leave() }
    guard let location = location else { return }
    if error == nil {
        if location.distance(from: currentLocation) < Constants.distance {
            print("Wave", wave.waveID, "is in range")
            waves.append(wave)
        } else {
            print("Wave", wave.waveID, "is out of range")
        }
    } else {
        print(error?.localizedDescription ?? "")
    }
})

如马特（Matt）的评论defer所述，它是一个总是在离开时做点什么的好工具。

这是另一个问题，但是同时从多个线程更新数组会导致一些问题。它很少发生，因此可能是一个难以修复的错误。

我不确定GeoFire是否在主线程中调用其回调，但是，如果不这样，则最好将所有回调代码包含在DispatchQueue.main.async {...}中。

Answer 2

dispatchGroup.leave（）仍在闭包中，而是应位于for循环的末尾，如下所示：

func getWavesByMostRecent(closure: @escaping ([Wave]?) -> Void) {
    var waves = [Wave]()
    let dispatchGroup = DispatchGroup()

    self.query = DatabaseManager.waveRef.queryOrdered(byChild: Constants.reverseTimeStampKey)
    self.handle = self.query?.observe(.value, with: { (snapshot) in

        for value in snapshot.children {
            guard let wave = Wave(snapshot: value as! DataSnapshot) else { return }

            self.geoFire = GeoFire(firebaseRef: DatabaseManager.waveRef)
            let currentLocation = LocationManager.shared.getCurrentLocation()

            dispatchGroup.enter()
            self.geoFire?.getLocationForKey(wave.waveID, withCallback: { (location, error) in
                guard let location = location else { return }
                if error == nil {
                    if location.distance(from: currentLocation) < Constants.distance {
                        print("Wave", wave.waveID, "is in range")
                        waves.append(wave)
                    } else {
                        print("Wave", wave.waveID, "is out of range")
                    }
                } else {
                print(error?.localizedDescription ?? "")
                }
            })
            dispatchGroup.leave()
        }
        dispatchGroup.notify(queue: .main) {
            print("THERE ARE SO MANY WAVES:", waves.count)
            closure(waves)
        }
    })
}

如何在Swift（GCD）中通知队列

2 个答案: