IOS Developpement : Background BLE scanning

Question

I'm currently trying to develop a Background Bluetooth related app, and I'm having some trouble scanning in background.

I followed the entire Apple IOS Bluetooth Background tutorial, but I cant figure out why scanning doesn't work in background (my info.plist is correctly set up to manage BLE Background tasks, both central and peripheral).

What I've tested so far :

1 - When my two devices are in range and in foreground, scanning works well.

2 - When the peripheral is in background, and the central comes active in foreground, scanning also works, peripheral is well detected.

Here comes the troubles

3 - first I launch my peripheral device, and I put the app in the background. Then on the peripheral device, I power off the bluetooth, I launch the app, I put it in background, and I power on the bluetooth. In my code, the -(void)centralManagerDidUpdateState:(CBCentralManager *)central calls the scan [centralmgr scanForPeripheralsWithServices:[NSArray arrayWithObject:servicesUUID] options:scanOptions];, function which works perfectly in the foreground. But nothing happened, the peripheral is never detected by the central.

4 - I launch the central, bluetooth activated, and I put it in background. Then I took my peripheral device, I went out of the bluetooth range (in the specific case I went to my supermarket to buy some beers to drown my sorrow), I launch it there -also bluetooth activated- I put it in background and I came back in the proximity of my peripheral. Once again nothing happened, the centrals did not detect the peripheral.

Furthermore, after the cases 3 and 4, if I just put the central on the foreground (the peripheral stays in background), detection restart, without doing anything else. As if the scan was successfully triggered in the case 4, but waiting the app to come active to actually start detecting other devices.

Does somebody have an idea on how to proceed/overcome the problem?

Thank you!

Ps: I appologize for my English, I'm a bit rusty

Answer 1

Hi for details you can see this answer

but the short answer is one of your Apps must be in Foreground to connect to each other Either your central device or the peripheral

Reason: for central you an only scan with providing the service UUIDs of the peripheral

and in background a peripheral cannot adverise its UUIDs so even they are there they just don't know of each others presence

Moreover some more notable things about background scanning are

you should be aware that advertising while your app is in the background operates differently than when your app is in the foreground. In particular, when your app is advertising while in the background:

The CBAdvertisementDataLocalNameKey advertisement key is ignored, and the local name of peripheral is not advertised.

All service UUIDs contained in the value of the CBAdvertisementDataServiceUUIDsKey advertisement key are placed in a special “overflow” area; they can be discovered only by an iOS device that is explicitly scanning for them.

If all apps that are advertising are in the background, the frequency at which your peripheral device sends advertising packets may decrease.

See Apple Docs