Develop Reference

nRF52840 - Android Kotlin application : Problem with 16-bit UUIDs

I'm developing a frame exchange sequence between an nRF52840 and an Android smartphone using the BLE protocol.
The service exposed by nRF52840 has the following 16-bit UUID : EB7A
In my Android application, if I only want to retrieve BLE devices that have this service, I need to initialize the filter like this :
private var scanFilters: List<ScanFilter> = arrayListOf(
ScanFilter.Builder().setServiceUuid(ParcelUuid(UUID.fromString("0000eb7a-0000-1000-8000-00805f9b34fb"))
)
However, if I want to write about a feature of this service, I have to use the following UUID:
0000eb7a-0000-0000-0000-000000000000
I don't understand why I can't use the same UUID for filtering as for read/write operations, can you help me?

I don't know whether you have figured it out. If yes, would be interesting to know what you have discovered. I am quite new to android and nRF dev myself, but I have 2 theories for your question:
If you are using a vendor-specific UUID for the filter, I guess you are not able to modify it, which is why you have to use a different UUID.
Have you tried deriving the UUID as a String for example and then using it in a filter (not sure if this would work thou)
public static String SOME_SERVICE = "0000eb7a-0000-1000-8000-00805f9b34fb"
ScanFilter = newScanFilter.Builder().setServiceUuid(UUID.fromString(SOME_SERVICE);

Web Bluetooth with a Custom GATT Service?

First, the source code for the BLE Peripheral Simulator is a tremendous resource for anyone investigating Web Bluetooth.
There is a an approved list of GATT Services.
When I modified the Android source code to use a custom defined GATT service (easy to do by simply specifying your own UUID in the Android code) pairing from the web page to the Android app running the GATT sever fails.
The Web Bluetooth documentation does state, "If device’s advertised Service UUIDs have a non-empty intersection with the set of Service UUIDs, add device to result and abort these sub-steps [for pairing]."
Does this mean only services in the approved GATT list are supported? If so, what is the rational? It seems that such a restriction would limit innovation.

I am working with web-bluetooth also and i was able to talk with custom services, yet to read data from them you have to have device docs explaining how to talk to that services. I think this approved list is like generally used list which doesn't require unique logic to get data. However there is difference since some characteristics are blacklisted for web-bluetooth yet available to fetch data from using ios/android stuff.

If I had done some more research on Stack Overflow I would have realized the UUID spec requires lowercase a-f.
Type of Character generated by UUID
The following values work for a custom defined service and characteristic:
private static final UUID SERVICE_UUID = UUID.fromString("29143321-ef6c-4761-947c-c858f9a2e8f1");
private static final UUID CHARACTERISTIC_UUID = UUID.fromString("92f3131b-ffa8-4dd1-a12b-641d65a78857");

How do Bluetooth SDP and UUIDs work? (specifically for Android)

My understanding is that the SDP is a list of UUIDs that other devices can fetch.
According to this PDF from MIT, "A more general way to think of
SDP is as an information database." Does this mean I can add multiple values to SDP? Since Android has BluetoothDevice.fetchUuidsWithSdp(), how do I set the UUIDs of a device?
Also, what does each section of an UUID mean? UUIDs look like 00000000-0000-1000-8000-00805F9B34FB, but what information does this convey?

An UUID identifies a service that is available on a particular device. So if you call BluetoothDevice.fetchUUidsWithSdp() your BroadcastReceiver will receive the relevant Intent ACTION_UUID containing the device and the service UUID.
The bluetooth specification defines some common UUIDs.
If you don't want to connect to one of these well known services but intent to implement your own bluetooth application, then you have to just generate your own UUID (use uuidgen from a unix console or an online generator) that identifies your application/service.
You can create an UUID instance in java like this UUID uuid = UUID.fromString("785da8ea-1220-11e5-9493-1697f925ec7b");.
So if you create the server side for your bluetooth application on Android you typically do this
BluetoothAdapter adapter = BluetoothAdapter.getDefaultAdapter();
BluetoothServerSocket serverSocket = adapter.listenUsingRfcommWithServiceRecord("YourHumanReadableServiceName", uuid);
And this is where you "set" your UUID. The Android bluetooth API creates the SDP-entry consisting of YOUR application's UUID and name for you. Other devices can now retrieve this entry. Androids bluetooth stack will now associate a bluetooth channel to your BluetoothServerSocket. If you want to connect to this ServerSocket, the connecting side usually connects doing this:
// you will most likely already have this instance from a discovery or paired device list
BluetoothDevice serverDevice = adapter.getRemoteDevice(bluetoothMacAddress);
// connect to your ServerSocket using the uuid
BluetoothSocket socket = serverDevice.createRfcommSocketToServiceRecord(uuid);
socket.connect();
Android will again do the heavy lifting for you: It checks the SDP-Records on the remote device, looks up the bluetooth channel that corresponds to your service's UUID and connects using this information.
There is a common code snippet spooking around here on SO that advices you to use "reflection" to get to a hidden API looking similar to this code:
try {
// this is the way to go
socket = device.createRfcommSocketToServiceRecord(uuid);
socket.connect( );
} catch ( IOException exception ) {
// don't do that! You will bypass SDP and things will go sideways.
Method m = device.getClass().getMethod("createRfcommSocket", new Class[] {int.class});
socket = (BluetoothSocket) m.invoke(device, 1);
socket.connect();
}
Most people try this and it "just works" in their dev environment but you should know what you do using this. You actively bypass the SDP lookup that retrieves the right bluetooth channel to be used with your service and you will end up connecting to channel 1. If you have more than one Service running on the device, things WILL go sideways in this cases and you will end up in debugging hell ;-)
I developed a small middleware called Blaubot to create small networks using bluetooth/wifi/nfc and experienced all sorts of problems on the devices I used to test with (12 models). It was often the case that the bluetooth stack was not fully functional anymore in cases where it got some load or after many connects/disconnects (which you usually will have, if you are developing your app). In these cases the device.createRfcommSocketToServiceRecord(uuid) would occasionally fail and only turning the bluetooth adapter off and on again helped to bring the bluetooth adapters back to life (in some cases only after a full power cycle). If this happens and you use the reflection method, you will probably not have much fun with bluetooth.
But if you know this and keep concurrent calls to the BluetoothAdapter within bounds, bluetooth connections and the adapters will be pretty stable.

What should my UUID be?

I'm relatively new to Android and am creating a Bluetooth App on a Nexus 9 that will connect to a Bluetooth device application my coworker has written on an Arduino processor. I'm following this doc, which is very helpful:
http://developer.android.com/guide/topics/connectivity/bluetooth.html
However, to connect as a client I have to use this code, which uses this MY_UUID symbol.
// MY_UUID is the app's UUID string, also used by the server code
tmp = device.createRfcommSocketToServiceRecord(MY_UUID);
In the text the doc says: "The UUID passed here must match the UUID used by the server device when it opened its BluetoothServerSocket (with listenUsingRfcommWithServiceRecord(String, UUID)). Using the same UUID is simply a matter of hard-coding the UUID string into your application and then referencing it from both the server and client code."
This is confusing to me... does it mean it must match a UUID specified in the Arduino firmware?? My coworker who wrote the firmware doesn't know what that would be. When I sniff his firmware advertising, it has ID "RNBT-DFBC", but when I use that as a UUID I get an exception :
java.lang.IllegalArgumentException: RNBT-DFBC is not a valid Bluetooth address
And none of the sample Bluetooth projects I've looked at seem to explain the basis of this UUID value, they're just "magic numbers."
So... what on earth do I use as the parameter for my createRfcommSocketToServiceRecord() function? I feel like I am misunderstanding this, because a Bluetooth client cant possibly generally have such "intimate" knowledge of a server it wants to connect to. So sorry if it's a dumb question, but any help is appreciated.

Here is a nice way that tells you how to get UUID for your app.

You should use a pre-shared UUID that both your device and the Arduino device knows. For example:
public static final java.util.UUID MY_UUID
= java.util.UUID.fromString("DEADBEEF-0000-0000-0000-000000000000");
The String is in a format according to RFC 4122. You can find more information about the specifics for Android in the Android UUID documentation .

CoreBluetooth: the UUID of a peripheral is an inherent attribute of a device?

I use CoreBluetooth to connect my iPhone with a device equipped with Bluetooth 4.0.
I print its (as a peripheral) UUID :
<CBPeripheral: 0x1742fca80, identifier = B148AD69-1FC7-498C-016F-33BA3BE041A3, name = HMSoft, state = disconnected>
I wonder whether this identifier is an inherent attribute of a device.
Since I use the following code in android to get its UUID which is different from what I get using CoreBluetooth in iPhone:
BluetoothDevice device = intent.getParcelableExtra(BluetoothDevice.EXTRA_DEVICE);
for (ParcelUuid uuid : device.getUuids()) {
Log.d("UUID", uuid.getUuid().toString());
}
( I connect with the same device but the print UUID is different).

Identifier in CBPeripheral is a randomly generated Unique identifier. This gets varied over time. In case of a CBPeripheral, we need to know about two kinds of UUIDs. One is ServiceUUID and other is CharacteristicUUID. Each peripheral broadcast data over each service. A single service can have more than one characteristics. For eg. a device information service can have device name, device version etc as its characteristics.
See the following image for a better understanding CBPeripheral. The Apple docs speaks well on this.

My first answer, so I hope I won't make a fool of myself ;-)
BluetoothDevice.getUuids():
Returns the supported features (UUIDs) of the remote device. I.e. UUIDs of services advertised by the device, not UUID of the device itself.
I'm currently looking myself for an Android way to get the UUID of a discovered BLE device...