Detect whether Apple Pencil is connected to an iPad Pro

Question

Is there an API that allows you to determine whether the Apple Pencil is connected to an iPad Pro? Looking over the 9.1 SDK I don\'t see anything that directly does this. Or per

Answer 1

Took me quite a while to figure out that CBCentralManager's centralManager(_ central: CBCentralManager, didConnect peripheral: CBPeripheral) is only called when the connection is initiated via its connect(_ peripheral: CBPeripheral, options: [String : Any]? = nil) function (yes, reading the docs helps :]).

Since we have no callback for when devices have been connected to the device through the user (as is the case with Apple Pencil - I'd love to be proven wrong on this one btw), I had to resort to using a timer here.

This is how it works:

When you initialize ApplePencilReachability a timer is setup that checks for the availability of the pencil every second. If a pencil is found the timer gets invalidated, if bluetooth is turned off it also gets invalidated. When it's turned on again a new timer is created.

I am not particularly proud of it but it works :-)

import CoreBluetooth

class ApplePencilReachability: NSObject, CBCentralManagerDelegate {

  private let centralManager = CBCentralManager()
  var pencilAvailabilityDidChangeClosure: ((_ isAvailable: Bool) -> Void)?

  var timer: Timer? {
    didSet {
      if oldValue !== timer { oldValue?.invalidate() }
    }
  }

  var isPencilAvailable = false {
    didSet { 
      guard oldValue != isPencilAvailable else { return }
      pencilAvailabilityDidChangeClosure?(isPencilAvailable)
    }
  }

  override init() {
    super.init()
    centralManager.delegate = self
    centralManagerDidUpdateState(centralManager) // can be powered-on already?
  }
  deinit { timer?.invalidate() }

  func centralManagerDidUpdateState(_ central: CBCentralManager) {
    if central.state == .poweredOn {
      timer = Timer.scheduledTimer(withTimeInterval: 1, repeats: true) { 
        [weak self] timer in // break retain-cycle
        self?.checkAvailability()
        if self == nil { timer.invalidate() }
      }
    } else {
      timer = nil
      isPencilAvailable = false
    }
  }

  private func checkAvailability() {
    let peripherals = centralManager.retrieveConnectedPeripherals(withServices: [CBUUID(string: "180A")])
    let oldPencilAvailability = isPencilAvailable
    isPencilAvailable = peripherals.contains(where: { $0.name == "Apple Pencil" })
    if isPencilAvailable {
      timer = nil // only if you want to stop once detected
    }
  }

}

Answer 2

I can't find any actual documentation on the Apple Pencil's Bluetooth implementation (and I don't believe any exists), but the following code Works for Me™.

It checks for connected devices that advertise themselves as supporting the "Device Information" service and then if any of these have the name "Apple Pencil".

PencilDetector.h

@import CoreBluetooth

@interface PencilDetector : NSObject <CBCentralManagerDelegate>

- (instancetype)init;

@end

PencilDetector.m

#include "PencilDetector.h"

@interface PencilDetector ()

@end

@implementation PencilDetector
{
  CBCentralManager* m_centralManager;
}

- (instancetype)init
{
  self = [super init];
  if (self != nil) {
    // Save a reference to the central manager. Without doing this, we never get
    // the call to centralManagerDidUpdateState method.
    m_centralManager = [[CBCentralManager alloc] initWithDelegate:self
                                                            queue:nil
                                                          options:nil];
  }

  return self;
}

- (void)centralManagerDidUpdateState:(CBCentralManager *)central
{
  if ([central state] == CBCentralManagerStatePoweredOn)
  {
    // Device information UUID
    NSArray* myArray = [NSArray arrayWithObject:[CBUUID UUIDWithString:@"180A"]];

    NSArray* peripherals =
      [m_centralManager retrieveConnectedPeripheralsWithServices:myArray];
    for (CBPeripheral* peripheral in peripherals)
    {
        if ([[peripheral name] isEqualToString:@"Apple Pencil"])
        {
            // The Apple pencil is connected
        }
    }
  }
}

@end

In practice, the following, simpler, synchronous code, which doesn't wait for the central manager to be powered on before checking for connected devices seems to work just as well in my testing. However, the documentation states that you shouldn't call any methods on the manager until the state has updated to be CBCentralManagerStatePoweredOn, so the longer code is probably safer.

Anywhere you like

m_centralManager = [[CBCentralManager alloc] initWithDelegate:nil
                                                        queue:nil
                                                      options:nil];

// Device information UUID
NSArray* myArray = [NSArray arrayWithObject:[CBUUID UUIDWithString:@"180A"]];

NSArray* peripherals =
  [m_centralManager retrieveConnectedPeripheralsWithServices:myArray];
for (CBPeripheral* peripheral in peripherals)
{
  if ([[peripheral name] isEqualToString:@"Apple Pencil"])
  {
    // The Apple pencil is connected
  }
}