How to notify a queue in Swift (GCD)

Question

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

My code:

func getWavesByMostRecent(closure: @escaping ([Wave]?) -> Void)         


        

Answer 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.

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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 {...}.