Scanning RSSI of BLE peripherals in background - ios

Question

UPDATE 15/6 - 1 - ALTERNATIVE METHOD
As per Paulw's suggestion, I changed the strategy and instead of discovering for peripherals, I connected with peripheral and it worked fine when either peripheral or central are in foreground. The problem occurs when:
1) I setup notifications on delegate calls.
2) Both peripheral and central are in background.
3) As I bring them closer, I see no delegate calls.
4) As soon as I bring either peripheral or central in foreground, I get didDiscover call and everything works fine even if I now put app in both phones in background.
In short, the central does not discover peripheral if both are in background state.
This is the code for both parts:
func startAsPeripheral() {
peripheralManager = CBPeripheralManager(delegate: self, queue: DispatchQueue.global(qos: .background))
}
func startAsCentral() {
centralManager = CBCentralManager(delegate: self, queue: DispatchQueue.global(qos: .background))
}
}
extension BluetoothManager: CBCentralManagerDelegate {
func centralManagerDidUpdateState(_ central: CBCentralManager) {
LogManager.shared.log(object: "Central state update")
LogManager.shared.log(object: central.state)
if central.state != .poweredOn {
LogManager.shared.log(object: "Central is not powered on")
} else {
LogManager.shared.log(object: "Central scanning for: " + self.serviceUUID.uuidString);
self.centralManager.scanForPeripherals(withServices: [self.serviceUUID], options: [CBCentralManagerScanOptionAllowDuplicatesKey : false])
}
}
func centralManager(_ central: CBCentralManager, didDiscover peripheral: CBPeripheral, advertisementData: [String : Any], rssi RSSI: NSNumber) {
self.centralManager.stopScan()
self.peripheral = peripheral
self.peripheral.delegate = self
self.centralManager.connect(self.peripheral, options: nil)
let name = advertisementData[CBAdvertisementDataLocalNameKey] as? String
LogManager.shared.log(object: "Peripheral (" + (name ?? "unknown") + "): " + RSSI.stringValue)
}
func centralManager(_ central: CBCentralManager, didConnect peripheral: CBPeripheral) {
if peripheral == self.peripheral {
LogManager.shared.log(object: "Connected.")
peripheral.discoverServices([serviceUUID])
}
}
func centralManager(_ central: CBCentralManager, didDisconnectPeripheral peripheral: CBPeripheral, error: Error?) {
if peripheral == self.peripheral {
LogManager.shared.log(object: "Disconnected")
self.peripheral = nil
LogManager.shared.log(object: "Central scanning for: " + self.serviceUUID.uuidString);
self.centralManager.scanForPeripherals(withServices: [self.serviceUUID], options: [CBCentralManagerScanOptionAllowDuplicatesKey: false])
}
}
}
extension BluetoothManager: CBPeripheralDelegate {
func peripheral( _ peripheral: CBPeripheral, didDiscoverServices error: Error?) {
LogManager.shared.log(object: "Services: \(peripheral.services!)")
for service in peripheral.services! {
peripheral.discoverCharacteristics(nil, for: service)
}
}
func peripheral( _ peripheral: CBPeripheral, didDiscoverCharacteristicsFor service: CBService, error: Error?) {
LogManager.shared.log(object: "Characteristics: \(service.characteristics!)")
}
}
extension BluetoothManager: CBPeripheralManagerDelegate {
func peripheralManagerDidUpdateState(_ peripheral: CBPeripheralManager) {
switch peripheral.state {
case .unknown:
LogManager.shared.log(object: "Bluetooth Device is UNKNOWN")
case .unsupported:
LogManager.shared.log(object: "Bluetooth Device is UNSUPPORTED")
case .unauthorized:
LogManager.shared.log(object: "Bluetooth Device is UNAUTHORIZED")
case .resetting:
LogManager.shared.log(object: "Bluetooth Device is RESETTING")
case .poweredOff:
LogManager.shared.log(object: "Bluetooth Device is POWERED OFF")
case .poweredOn:
LogManager.shared.log(object: "Bluetooth Device is POWERED ON")
let service = serviceUUID
let myService = CBMutableService(type: service, primary: true)
let writeCharacteristics = CBMutableCharacteristic(type: characteristricUUID, properties: .write, value: nil, permissions: .writeable)
myService.characteristics = [writeCharacteristics]
peripheralManager.add(myService)
peripheralManager.startAdvertising([CBAdvertisementDataLocalNameKey : "Spacedemic", CBAdvertisementDataServiceUUIDsKey : [service]])
LogManager.shared.log(object: "Started Advertising")
default:
LogManager.shared.log(object: "Unknown State")
}
}
}
ORIGINAL QUESTION
I have an app used for contact tracing. I configure it to work as both central and peripheral, both should work in background AND foreground. This is for contact tracing, so users won't have the application in foreground state in most cases. Advertising works fine as what I have observed, both in background and foreground. The problem here is that when central is in background, it sometimes gets into didDiscover only one time and sometimes never. Thus if a peripheral is in range, I am unable to detect the RSSI from central or if the user moves away from the central. In contrast, when in foreground, I get frequent updates with RSSI in didDiscover method.
How can I solve this? Or is there a workaround to get the desired results?