How to notify a queue in Swift (GCD)

问题

I'm using GCD to notify main thread (have 2 async calls inside the function)

My code:

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)
        }
    })
}

But .notify closure just doesn't work and I cannot call my "main" closure right. What am I doing wrong? Any advice would be appreciated.

回答1:

Try this change:

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 ?? "")
    }
})

As noted in matt's comment defer is a good tool to do something always when leaving.

---

This is another issue, but updating an Array from multiple thread simultaneously would cause some problems. It rarely happens, so it can be a hard-to-fix bug.

I'm not sure if GeoFire calls its callback in the main thread or not, but if not, you should better enclose all the callback code in DispatchQueue.main.async {...}.

回答2:

dispatchGroup.leave() is still in the closure, instead it should be at the end of the for loop like this:

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)
        }
    })
}

来源：https://stackoverflow.com/questions/51349678/how-to-notify-a-queue-in-swift-gcd