Use Bloothtooth LE while app is in background - ios

Question

I am building an iOS app with Xamarin, with this BLE plugin:
https://github.com/aritchie/bluetoothle
I'm just broadcasting a UUID via BLE, and it works. Here is my code:
var data = new Plugin.BluetoothLE.Server.AdvertisementData
{
LocalName = "MyServer",
};
data.ServiceUuids.Add(new Guid("MY_UUID_HERE"));
await this.server.Start(data);
The only problem is that it stops broadcasting once I put the app in the background. And resumes again when I open the app again.
How can I let it continue to broadcast once it's in the background? I read the documentation here:
https://developer.apple.com/library/content/documentation/NetworkingInternetWeb/Conceptual/CoreBluetooth_concepts/CoreBluetoothBackgroundProcessingForIOSApps/PerformingTasksWhileYourAppIsInTheBackground.html
And it says that I have to use the CBCentralManager class to obtain the preservation and restoration feature (so I can keep broadcasting the UUID at all times), but I'm having a hard time translating this to Xamarin/C#.
EDIT
After researching some more, I read that I need to create an instance of CBCentralManager and implement WillRestoreState in the delegate. I did this in the AppDelegate:
[Register("AppDelegate")]
public class AppDelegate : MvxApplicationDelegate, ICBCentralManagerDelegate
{
private IGattServer server = CrossBleAdapter.Current.CreateGattServer();
private CBCentralManager cbCentralManager;
public override bool FinishedLaunching(UIApplication application, NSDictionary launchOptions)
{
// irrelevant code...
this.Ble();
return true;
}
private async Task Ble()
{
try
{
await Task.Delay(5000); // wait for it to finish initializing so I can access BLE (it crashes otherwise)
var options = new CBCentralInitOptions();
options.RestoreIdentifier = "myRestoreIndentifier";
this.cbCentralManager = new CBCentralManager(this,null,options);
var data = new Plugin.BluetoothLE.Server.AdvertisementData
{
LocalName = "MyServer",
};
data.ServiceUuids.Add(new Guid("MY_UUID_HERE"));
await this.server.Start(data);
}
catch (Exception e)
{
}
}
public void UpdatedState(CBCentralManager central)
{
//throw new NotImplementedException();
}
[Export("centralManager:willRestoreState:")]
public void WillRestoreState(CBCentralManager central, NSDictionary dict)
{
//never gets called
}
But it didn't make a difference for me. And the WillRestoreState method never gets called... I don't mind using a different plugin/library if I have to at this point...
EDIT 2
I just realized that the app is still broadcasting while it is in the background, I just don't see the service UUID anymore (in the web portal of the beacon that I'm testing with), I only see the phone's identifier.

Answer 1

After doing tons of research, I found that it is simply an iOS restriction - you can not broadcast the UUID of a BLE service while your app is in the background. Background work is very restrictive in iOS.
EDIT to include Paulw11 comment (which is true):
You can advertise a service, but it is advertised in a way that only another iOS device that is specifically scanning for that service UUID can see.

Answer 2

Although you can not broadcast the UUID of a BLE service while your iOS app is in the background, for anyone trying to do something similar, you should look into iBeacon. It's Apple's protocol for letting iOS apps do bluetooth stuff while it's in the background.