Creating a Contact Tracing Scanner using Swift for macOS

20 Oct 2020

Following on from my previous experiement which highlighted what an Exposure Notification-enabled device actually emits using Web Bluetooth, I decided to explore how I could do the same using Swift and Interface Builder for macOS.

The latest application release can be downloaded from GitHub, and simply unzipped/executed without any additional dependencies.

Using Core Bluetooth I was able to scan for devices which were advertising the registered service UUID 0xfd6f - and in-turn parse the service data. The process overall, was very similair to the one used when building the Web Bluetooth solution.

// ...

let exposureNotificationServiceUuid = CBUUID(string: "FD6F")

open func centralManagerDidUpdateState(_ central: CBCentralManager) {
  switch central.state {
    case .poweredOn:
      central.scanForPeripherals(withServices: [exposureNotificationServiceUuid], options: [CBCentralManagerScanOptionAllowDuplicatesKey: true])
    case .poweredOff:
      central.stopScan()
    default:
      break
  }
}

public func centralManager(_ central: CBCentralManager, didDiscover peripheral: CBPeripheral, advertisementData: [String : Any], rssi RSSI: NSNumber) {
  if let serviceAdvertisementData = advertisementData[CBAdvertisementDataServiceDataKey] as? NSDictionary {
    if let exposureNotificationServiceData = serviceAdvertisementData.object(forKey: exposureNotificationServiceUuid) as? Data {
      let hex = exposureNotificationServiceData.map { String(format: "%02hhx", $0) }.joined()
      updateDevice(Device(uuid: peripheral.identifier.uuidString, rollingProximityId: "\(hex.prefix(32))", metadata: "\(hex.suffix(8))", rssi: "\(RSSI)", lastSeen: Date()))
    }
  }
}

// ...

To clean-up the listing and ensure that only active devices where present, I used a scheduled timer which would remove any old devices every 5 seconds.

Timer.scheduledTimer(withTimeInterval: 5, repeats: true) { timer in
  self.removeDevicesNotSeenInLastSeconds(10)
  self.tableView?.reloadData()
}

private func removeDevicesNotSeenInLastSeconds(_ seconds: Double) {
  devices = devices.filter({ Date().timeIntervalSince1970 - $0.lastSeen.timeIntervalSince1970 < seconds })
}

For a deeper understanding of how the Exposure Notification system works, please visit my previous post.