如何在 Swift (GCD) 中通知队列

Question

我正在使用 GCD 通知主线程（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，我无法正确调用 "main" 闭包。我究竟做错了什么？如有任何建议，我们将不胜感激。

Answer 1

试试这个改变：

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 matt 的评论所述，defer 是一个很好的工具，可以在离开时做一些事情。

这是另一个问题，但同时从多个线程更新数组会导致一些问题。它很少发生，因此它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它的回调，但如果没有，你最好将所有回调代码封装在DispatchQueue.main.async {...}.

中

Answer 2

dispatchGroup.leave() 仍在闭包中，而应该像这样在 for 循环的末尾：

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

How to notify a queue in Swift (GCD)

grand-central-dispatch

swift