Continue scanning for other BLE devices while connecting to one

Question

I want my app to scan continuously for up to 3 different BLE peripherals (identified by MAC address). When one device is found, I stop scanning for it and connect to it. Code:

static Observable<ScanResult> bleSharedScanner = MyApplication
        .getRxBleClient()
        .scanBleDevices(new ScanSettings.Builder()
                .setScanMode(SCAN_MODE)
                .build())
        .share();

static Observable<ScanResult> device1Scanner = bleSharedScanner
        .filter(scanResult -> scanResult.getBleDevice().getMacAddress().equalsIgnoreCase( device1MacAddress ) 
        );

Code for Device2 and Device3 is the same. Initially I subscribe to all 3 of these device scanners; when I find one of the devices I unsubscribe that subscription—meanwhile the scan for the other two continues.

So far this seems to be working on my test Android phone. But I noticed your response to a previous question: "…stop the scanSubscription before trying to connect—Android sometimes do not handle well scan and connection at the same time."

Is this a common problem? Is there a good workaround? I need to continue scanning for the other devices while interacting with the one found.

score 3 · Accepted Answer · answered Dec 15 '17 at 22:03

My experience shows that at least some Huawei devices have issues when you connect to a peripheral while the scanning is in progress (I confirmed it on P8 Lite and P9) with two different peripherals.

When the scanning was in progress I was receiving a COMMAND_DISALLOWED for the LE_CREATE_CONNECTION command. Communication between the host and BLE chip was extracted from HCI Snoop Log. The only workaround that worked for me was to stop the scan for a period of establishing the connection.

We do not have any statistics to prove it - unfortunately.

score 3 · Answer 2 · answered Dec 16 '17 at 02:24

The "correct" way is to simply connect to each device and set the autoConnect flag to true. Then you can have multiple connections pending and the connection attempt never times out compared to when you scan.

Only issue is that the device must either be bonded or has been scanned at least once since the latest Bluetooth restart to correctly make the stack learn the address type of the device (public or random).

Anyway, 99% of Android devices commonly used have Bluetooth chips that can scan and initiate at the same time. Huawei P8 lite and a cheap Asus tablet are the only exceptions I've found so far that can't do this.

Continue scanning for other BLE devices while connecting to one

2 Answers2