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如何在swift(gcd)中通知队列

grand-central-dispatch swift

Max Kraev · 技术社区 · 7 年前

我正在使用gcd通知主线程(函数中有两个异步调用)

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 closure不起作用,我不能正确调用我的“main”closure。我做错什么了?任何建议都将不胜感激。

2 回复 | 直到 7 年前

OOPer 7 年前

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 ?? "")
    }
})

如马特评论中所述 defer 是一个很好的工具,总是在离开的时候做一些事情。

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。这种情况很少发生,所以很难修复。

我不确定 GeoFire 是否在主线程中调用其回调,但如果不是,则最好将所有回调代码括在 DispatchQueue.main.async {...} 是的。

-3

Michaelcode 7 年前

dispatchGroup.leave()仍在闭包中,而应该位于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)
        }
    })
}