core-bluetooth

Core Bluetooth

Scan for, connect to, and exchange data with Bluetooth Low Energy (BLE) devices. Covers the central role (scanning and connecting to peripherals), the peripheral role (advertising services), background modes, and state restoration. Targets Swift 6.3 / iOS 26+. Use accessorysetupkit for privacy-preserving accessory discovery and setup; use this skill for direct Core Bluetooth GATT communication.

Contents

• Setup
• Central Role: Scanning
• Central Role: Connecting
• Discovering Services and Characteristics
• Reading, Writing, and Notifications
• Peripheral Role: Advertising
• Background BLE
• State Restoration
• Common Mistakes
• Review Checklist
• References

Setup

Info.plist Keys

Key	Purpose
NSBluetoothAlwaysUsageDescription	Required. Explains why the app uses Bluetooth
UIBackgroundModes with bluetooth-central	Background scanning and connecting
UIBackgroundModes with bluetooth-peripheral	Background advertising

Bluetooth Authorization

Core Bluetooth has no explicit permission request API. Add NSBluetoothAlwaysUsageDescription, create the manager when the app is ready for Bluetooth access, then check manager.authorization and manager.state. Treat .denied and .restricted as terminal until the user changes Settings; wait for .poweredOn before scanning, connecting, advertising, or publishing services.

Central Role: Scanning

Creating the Central Manager

Always wait for the poweredOn state before scanning.

import CoreBluetooth

final class BluetoothManager: NSObject, CBCentralManagerDelegate {
    private var centralManager: CBCentralManager!
    private var discoveredPeripheral: CBPeripheral?

    override init() {
        super.init()
        centralManager = CBCentralManager(delegate: self, queue: nil)
    }

    func centralManagerDidUpdateState(_ central: CBCentralManager) {
        guard central.state == .poweredOn else { return }
        startScanning()
    }
}

Scanning for Peripherals

Scan for specific service UUIDs to save power. Pass nil to discover all peripherals (not recommended in production).

let heartRateServiceUUID = CBUUID(string: "180D")

func startScanning() {
    centralManager.scanForPeripherals(
        withServices: [heartRateServiceUUID],
        options: [CBCentralManagerScanOptionAllowDuplicatesKey: false]
    )
}

func centralManager(
    _ central: CBCentralManager,
    didDiscover peripheral: CBPeripheral,
    advertisementData: [String: Any],
    rssi RSSI: NSNumber
) {
    guard RSSI.intValue > -70 else { return } // Filter weak signals

    // IMPORTANT: Retain the peripheral -- it will be deallocated otherwise
    discoveredPeripheral = peripheral
    centralManager.stopScan()
    centralManager.connect(peripheral, options: nil)
}

Central Role: Connecting

func centralManager(
    _ central: CBCentralManager,
    didConnect peripheral: CBPeripheral
) {
    peripheral.delegate = self
    peripheral.discoverServices([heartRateServiceUUID])
}

func centralManager(
    _ central: CBCentralManager,
    didDisconnectPeripheral peripheral: CBPeripheral,
    timestamp: CFAbsoluteTime,
    isReconnecting: Bool,
    error: Error?
) {
    if isReconnecting {
        // System is automatically reconnecting
        return
    }
    // Handle disconnection -- optionally reconnect
    discoveredPeripheral = nil
}

Discovering Services and Characteristics

Implement CBPeripheralDelegate to walk the service/characteristic tree.

extension BluetoothManager: CBPeripheralDelegate {
    func peripheral(
        _ peripheral: CBPeripheral,
        didDiscoverServices error: Error?
    ) {
        guard let services = peripheral.services else { return }
        for service in services {
            peripheral.discoverCharacteristics(nil, for: service)
        }
    }

    func peripheral(
        _ peripheral: CBPeripheral,
        didDiscoverCharacteristicsFor service: CBService,
        error: Error?
    ) {
        guard let characteristics = service.characteristics else { return }
        for characteristic in characteristics {
            if characteristic.properties.contains(.notify) {
                peripheral.setNotifyValue(true, for: characteristic)
            }
            if characteristic.properties.contains(.read) {
                peripheral.readValue(for: characteristic)
            }
        }
    }
}

Reading, Writing, and Notifications

Reading a Value

func peripheral(
    _ peripheral: CBPeripheral,
    didUpdateValueFor characteristic: CBCharacteristic,
    error: Error?
) {
    guard let data = characteristic.value else { return }

    switch characteristic.uuid {
    case CBUUID(string: "2A37"):
        if let heartRate = parseHeartRate(data) {
            print("Heart rate: \(heartRate) bpm")
        }
    case CBUUID(string: "2A19"):
        let batteryLevel = data.first.map { Int($0) } ?? 0
        print("Battery: \(batteryLevel)%")
    default:
        break
    }
}

private func parseHeartRate(_ data: Data) -> Int? {
    guard data.count >= 2 else { return nil }
    let flags = data[0]
    let is16Bit = (flags & 0x01) != 0
    if is16Bit {
        guard data.count >= 3 else { return nil }
        return Int(data[1]) | (Int(data[2]) << 8)
    } else {
        return Int(data[1])
    }
}

Writing a Value

func writeValue(_ data: Data, to characteristic: CBCharacteristic,
                on peripheral: CBPeripheral,
                preferResponse: Bool = true) {
    let type: CBCharacteristicWriteType
    if preferResponse, characteristic.properties.contains(.write) {
        type = .withResponse
    } else if characteristic.properties.contains(.writeWithoutResponse),
              peripheral.canSendWriteWithoutResponse {
        type = .withoutResponse
    } else if characteristic.properties.contains(.write) {
        type = .withResponse
    } else {
        return
    }

    guard data.count <= peripheral.maximumWriteValueLength(for: type) else { return }
    peripheral.writeValue(data, for: characteristic, type: type)
}

// Confirmation callback for .withResponse writes.
func peripheral(
    _ peripheral: CBPeripheral,
    didWriteValueFor characteristic: CBCharacteristic,
    error: Error?
) {
    if let error {
        print("Write failed: \(error.localizedDescription)")
    }
}

// Resume queued .withoutResponse writes here.
func peripheralIsReady(toSendWriteWithoutResponse peripheral: CBPeripheral) {}

Subscribing to Notifications

// Subscribe
peripheral.setNotifyValue(true, for: characteristic)

// Unsubscribe
peripheral.setNotifyValue(false, for: characteristic)

// Confirmation
func peripheral(
    _ peripheral: CBPeripheral,
    didUpdateNotificationStateFor characteristic: CBCharacteristic,
    error: Error?
) {
    if characteristic.isNotifying {
        print("Now receiving notifications for \(characteristic.uuid)")
    }
}

Peripheral Role: Advertising

Publish services from the local device using CBPeripheralManager.

final class BLEPeripheralManager: NSObject, CBPeripheralManagerDelegate {
    private var peripheralManager: CBPeripheralManager!
    private let serviceUUID = CBUUID(string: "12345678-1234-1234-1234-123456789ABC")
    private let charUUID = CBUUID(string: "12345678-1234-1234-1234-123456789ABD")

    override init() {
        super.init()
        peripheralManager = CBPeripheralManager(delegate: self, queue: nil)
    }

    func peripheralManagerDidUpdateState(_ peripheral: CBPeripheralManager) {
        guard peripheral.state == .poweredOn else { return }
        setupService()
    }

    private func setupService() {
        let characteristic = CBMutableCharacteristic(
            type: charUUID,
            properties: [.read, .notify],
            value: nil,
            permissions: [.readable]
        )

        let service = CBMutableService(type: serviceUUID, primary: true)
        service.characteristics = [characteristic]
        peripheralManager.add(service)
    }

    func peripheralManager(
        _ peripheral: CBPeripheralManager,
        didAdd service: CBService,
        error: Error?
    ) {
        guard error == nil else { return }
        peripheralManager.startAdvertising([
            CBAdvertisementDataServiceUUIDsKey: [serviceUUID],
            CBAdvertisementDataLocalNameKey: "MyDevice"
        ])
    }
}

Background BLE

Background Central Mode

Add bluetooth-central to UIBackgroundModes. In the background:

• Scanning must specify one or more service UUIDs; nil scans are foreground-only
• Scan options, including CBCentralManagerScanOptionAllowDuplicatesKey, have no effect

Background Peripheral Mode

Add bluetooth-peripheral to UIBackgroundModes. In the background:

• Without this mode, published service contents are disabled while suspended
• The local name is not advertised
• Service UUIDs move to the overflow area and require explicit service scans

State Restoration

State restoration allows the system to re-create your central or peripheral manager after your app is terminated and relaunched for a BLE event.

Central Manager State Restoration

// 1. Create with a restoration identifier
centralManager = CBCentralManager(
    delegate: self,
    queue: nil,
    options: [CBCentralManagerOptionRestoreIdentifierKey: "myCentral"]
)

// 2. Implement the restoration delegate method
func centralManager(
    _ central: CBCentralManager,
    willRestoreState dict: [String: Any]
) {
    if let peripherals = dict[CBCentralManagerRestoredStatePeripheralsKey]
        as? [CBPeripheral] {
        for peripheral in peripherals {
            // Re-assign delegate and retain
            peripheral.delegate = self
            discoveredPeripheral = peripheral
        }
    }
    let restoredServices = dict[CBCentralManagerRestoredStateScanServicesKey]
        as? [CBUUID]
    let restoredOptions = dict[CBCentralManagerRestoredStateScanOptionsKey]
        as? [String: Any]
    // Resume scanning with restoredServices/restoredOptions if still needed.
}

Peripheral Manager State Restoration

peripheralManager = CBPeripheralManager(
    delegate: self,
    queue: nil,
    options: [CBPeripheralManagerOptionRestoreIdentifierKey: "myPeripheral"]
)

func peripheralManager(
    _ peripheral: CBPeripheralManager,
    willRestoreState dict: [String: Any]
) {
    let services = dict[CBPeripheralManagerRestoredStateServicesKey]
        as? [CBMutableService]
    let advertisement = dict[CBPeripheralManagerRestoredStateAdvertisementDataKey]
        as? [String: Any]
    // Reconnect app state to restored services/advertisement as needed.
}

Common Mistakes

DON'T: Scan or connect before poweredOn

// WRONG: Scanning immediately -- manager may not be ready
let manager = CBCentralManager(delegate: self, queue: nil)
manager.scanForPeripherals(withServices: nil) // May silently fail

// CORRECT: Wait for poweredOn in the delegate
func centralManagerDidUpdateState(_ central: CBCentralManager) {
    if central.state == .poweredOn {
        central.scanForPeripherals(withServices: [serviceUUID])
    }
}

DON'T: Lose the peripheral reference

Core Bluetooth does not retain discovered peripherals. If you don't hold a strong reference, the peripheral is deallocated and the connection fails silently.

// WRONG: No strong reference kept
func centralManager(_ central: CBCentralManager,
                    didDiscover peripheral: CBPeripheral, ...) {
    central.connect(peripheral) // peripheral may be deallocated
}

// CORRECT: Retain the peripheral
func centralManager(_ central: CBCentralManager,
                    didDiscover peripheral: CBPeripheral, ...) {
    self.discoveredPeripheral = peripheral // Strong reference
    central.connect(peripheral)
}

DON'T: Scan for nil services in production

// WRONG: Discovers every BLE device in range -- drains battery
centralManager.scanForPeripherals(withServices: nil)

// CORRECT: Specify the service UUIDs you need
centralManager.scanForPeripherals(withServices: [targetServiceUUID])

DON'T: Assume connection order or timing

// WRONG: Assuming immediate connection
centralManager.connect(peripheral)
discoverServicesNow() // Peripheral not connected yet

// CORRECT: Discover services in the didConnect callback
func centralManager(_ central: CBCentralManager,
                    didConnect peripheral: CBPeripheral) {
    peripheral.delegate = self
    peripheral.discoverServices([serviceUUID])
}

DON'T: Write without checking properties and flow control

// WRONG: May fail, report an error, or provide no confirmation
peripheral.writeValue(data, for: characteristic, type: .withResponse)

// CORRECT: Check properties, length, and .withoutResponse flow control
if characteristic.properties.contains(.write),
   data.count <= peripheral.maximumWriteValueLength(for: .withResponse) {
    peripheral.writeValue(data, for: characteristic, type: .withResponse)
} else if characteristic.properties.contains(.writeWithoutResponse),
          peripheral.canSendWriteWithoutResponse,
          data.count <= peripheral.maximumWriteValueLength(for: .withoutResponse) {
    peripheral.writeValue(data, for: characteristic, type: .withoutResponse)
}

Review Checklist

• NSBluetoothAlwaysUsageDescription added to Info.plist
• All BLE operations gated on centralManagerDidUpdateState returning .poweredOn
• Discovered peripherals retained with a strong reference
• Scanning uses specific service UUIDs (not nil) in production
• CBPeripheralDelegate set before calling discoverServices
• Characteristic properties checked before read/write/notify
• Write payloads stay within maximumWriteValueLength(for:)
• .withoutResponse writes honor canSendWriteWithoutResponse
• Background mode (bluetooth-central or bluetooth-peripheral) added if needed
• State restoration identifier set if app needs relaunch-on-BLE-event support
• willRestoreState delegate method implemented when using state restoration
• Scanning stopped after discovering the target peripheral
• Disconnection handled with optional automatic reconnect logic
• Write type matches characteristic properties (.withResponse vs .withoutResponse)

References

• Extended patterns (reconnection strategies, data parsing, SwiftUI integration): references/ble-patterns.md
• Core Bluetooth framework
• CBCentralManager
• CBPeripheral
• CBPeripheralManager
• CBService
• CBCharacteristic
• CBUUID
• CBCentralManagerDelegate
• CBPeripheralDelegate
• NSBluetoothAlwaysUsageDescription
• CBManagerAuthorization
• scanForPeripherals(withServices:options:)
• startAdvertising(_:)
• writeValue(_:for:type:)
• maximumWriteValueLength(for:)
• canSendWriteWithoutResponse
• Configuring background execution modes