Advertising BLE service data from macOS

[avaidyam]

I'm fairly sure you should be able to do this on macOS (not iOS perhaps?). Here's an example. Specifically, CBAdvertisementDataServiceDataKey should be what you need.

[grishka]

I modified my test app to pass a Data for that key (the docs say that its value is a [CBUUID:Data] dictionary so that's what I was trying), it's still complaining that it's not an allowed key.

The service as seen by Android doesn't have any service data either:

[avaidyam]

Ah, I see. It looks like the nearby library actually recognizes this limitation too with a workaround:

/**
 * Starts advertising service data in a way that is supported by CoreBluetooth.
 *
 * Since CoreBluetooth doesn't support setting the @c CBAdvertisementDataServiceDataKey key, the
 * service list is advertised using @c CBAdvertisementDataServiceUUIDsKey and the associated data is
 * advertised using @c CBAdvertisementDataLocalNameKey. Since @c CBAdvertisementDataLocalNameKey
 * does not support binary data, the value is base64 encoded and truncated if the resulting value is
 * longer than 22 bytes. This also means we can only support advertising a single service.
 *
 * @param serviceData A dictionary that contains service-specific advertisement data.
 * @param completionHandler Called on a private queue with @c nil if successfully started
 *                          advertising or an error if one has occured.
 */
- (void)startAdvertisingData:(NSDictionary<CBUUID *, NSData *> *)serviceData
           completionHandler:(nullable GNCStartAdvertisingCompletionHandler)completionHandler;

Another relevant snippet:

  // Apple devices don't support advertising service data, so Apple devices advertise a base64
  // encoded local name, while other devices advertise service data. Here we attempt to reconstruct
  // service data by decoding the local name. If successful, this is possibly a Nearby advertisement
  // on an Apple device.
  NSString *localName = advertisementData[CBAdvertisementDataLocalNameKey];

  // ...  

  // A Nearby Apple advertisement should only have a single service, so simply grab the first one if
  // it exists.
  NSArray<CBUUID *> *serviceUUIDs = advertisementData[CBAdvertisementDataServiceUUIDsKey];

Yet another relevant snippet:

  // Maps service UUIDs to their service data.
  //
  // For each platform should follow to set the service UUID(key) and service
  // data(value):
  //
  // iOS    : 16 bit service UUID (type=0x03) + LocalName data (type=0x08)
  // Windows: Service data (type=0x16)
  // Android: 16 bit service UUID (type=0x03) + Service data (type=0x16)
  absl::flat_hash_map<Uuid, nearby::ByteArray> service_data;

Can you see if this method works instead?

[grishka]

I tried this:

Didn't trigger neither the notification nor the MDNS service.

[avaidyam]

It looks like iOS/macOS BLE advertisement was only added to the google/nearby project last month by @bourdakos1 actually, so perhaps the Android/Play services system library isn't ready for it yet. Of note, it looks like migration of the Nearby Share code to the public project only started 2 weeks ago as well.

[grishka]

Okay then. Maybe they're preparing to ship their (inevitably worse lol) Nearby Share app for macOS.

Another thing to keep in mind is that google/nearby is their universal cross-platform implementation in C++ that is maybe used by projects like Chromium, but Play Services uses another, closed-source (as far as I can tell) pure-Java implementation.

[grishka]

I decompiled the latest (as of now) version of Play Services and found this:

So this fallback is implemented, but it's behind what looks like a server flag, that defaults to false:

(I renamed things here and there to make the code easier to understand)

[grishka]

In case someone wants to look at it themselves, the bulk of code dealing with the reception of these BLE advertisements is in com.google.android.gms.nearby.sharing.discovery.FastInitiation

[EionRobb]

Would using private apis be off the menu? The BluetoothHCILESetAdvertisingParameters
https://github.com/seemoo-lab/internalblue/blob/master/macos/IOBluetoothExtended/IOBluetoothExtended/IOBluetoothHostController.h#L126C8-L126C40

eg https://github.com/RadiusNetworks/scanbeacon-gem/blob/f3f9ff9123593ff821b3dba7f253d9735e10082f/ext/core_bluetooth/core_bluetooth.m#L213-L222 ?

[grishka]

I'm totally fine with using private APIs, I'll test this one, although I don't have my hopes up

[grishka]

Oh these APIs are the ones I've already tested and they didn't work on modern macOS versions
#4 (comment)