Swift launch thread in background - ios

Question

I'm trying to send a process in a background thread using the following code:
let qualityOfServiceClass = QOS_CLASS_BACKGROUND
let backgroundQueue = dispatch_get_global_queue(qualityOfServiceClass, 0)
dispatch_async(backgroundQueue, {
print("running in the background queue")
btDiscovery
})
but the class is only processing while begin in foreground...any idea ?
EDIT1:
btDiscovery is a class which performs a BLE device scan every X seconds:
let btDiscoverySharedInstance = Beacon();
class Beacon: NSObject, CBCentralManagerDelegate {
private var centralManager: CBCentralManager?
private var peripheralBLE: CBPeripheral?
....
func centralManagerDidUpdateState(central: CBCentralManager) {
switch (central.state) {
case CBCentralManagerState.PoweredOff:
print("BLE powered off")
self.clearDevices()
case CBCentralManagerState.Unauthorized:
// Indicate to user that the iOS device does not support BLE.
print("BLE not supported")
break
case CBCentralManagerState.Unknown:
// Wait for another event
print("BLE unknown event")
break
case CBCentralManagerState.PoweredOn:
print("BLE powered on")
self.startScanning()
break
case CBCentralManagerState.Resetting:
print("BLE reset")
self.clearDevices()
case CBCentralManagerState.Unsupported:
print("BLE unsupported event")
break
}
}
func startScanning() {
print("Start scanning...")
if let central = centralManager {
central.scanForPeripheralsWithServices(nil, options: nil)
}
}
func centralManager(central: CBCentralManager, didDiscoverPeripheral peripheral: CBPeripheral, advertisementData: [String : AnyObject], RSSI: NSNumber) {
print("Discovered peripheral \(RSSI) dBM name: \(peripheral.name)")
print("UUID: \(peripheral.identifier.UUIDString)")
...
sleep(delayPolling)
self.startScanning()
}
when the app is launched and remains in foreground, the scan is performed correctly every "delayPolling" seconds.
but as soon as I put my app is background, the scan is paused. it restarts only when it comes back again in foreground.
I would need to leave this scan running in background every time (even if we set a lower priority to this thread).
EDIT2:
by reading the documentation https://developer.apple.com/library/ios/documentation/NetworkingInternetWeb/Conceptual/CoreBluetooth_concepts/CoreBluetoothBackgroundProcessingForIOSApps/PerformingTasksWhileYourAppIsInTheBackground.html
I can see that
When an app that implements the central role includes the UIBackgroundModes key with the bluetooth-central value in its Info.plist file, the Core Bluetooth framework allows your app to run in the background to perform certain Bluetooth-related tasks. While your app is in the background you can still discover and connect to peripherals, and explore and interact with peripheral data. In addition, the system wakes up your app when any of the CBCentralManagerDelegate or CBPeripheralDelegate delegate methods are invoked
I selected the corresponding options in my Info.plist file:
but my app is not running my thread in background.

Answer 1

I realize this is an old question, but scanning in the background requires that you supply a Service UUID.
central.scanForPeripheralsWithServices(nil, options: nil)
needs to be
central.scanForPeripheralsWithServices(serviceUUID, options: nil)