Background:
I am using the RxAndroidBle library and have a requirement to quickly (as possible) connect to multiple devices at a time and start communicating. I used the RxBluetoothKit for iOS, and have started to use RxAndroidBle on my Pixel 2. This had worked as expected and I could establish connections to 6-8 devices, as required, in a few hundred milliseconds. However, broadening my testing to phones such as the Samsung S8 and Nexus 6P, it seems that establishing a single connection can now take upwards of 5-6 seconds instead of 50-60 millis. I will assume for the moment that that disparity is within the vendor-specific BT implementations. Ultimately, this means that connecting to, e.g., 5 devices now takes 30 seconds instead of < 1 second.
Question:
What I understand from the documentation and other questions asked, RxAndroidBle queues all scanning, connecting, and communication requests and executes them serially to be safe and maintain stability based on the variety of Bluetooth implementations in the Android ecosystem. However, is there currently a way to execute the requests (namely, connecting) in parallel to accept this risk and potentially cut my total time to establish multiple connections down to whichever device takes the longest to connect?
And side question: are there any ideas to diagnose what could possibly be taking 5 seconds to establish a connection with a device? Or do we simply need to accept that some phones will take that long in some instances?
However, is there currently a way to execute the requests (namely, connecting) in parallel to accept this risk and potentially cut my total time to establish multiple connections down to whichever device takes the longest to connect?
Yes. You may try to establish connections using autoConnect=true which would prevent locking the queue for longer than few milliseconds. The last connection should be started with autoConnect=false to kick off a scan. Some stack implementations are handling this quite OK but your mileage may vary.
And side question: are there any ideas to diagnose what could possibly be taking 5 seconds to establish a connection with a device?
You can check the Bluetooth HCI snoop log. Also you may try using a BLE sniffer to check what is actually happening "on-air" (e.g. an nRF51 Development Kit).
Or do we simply need to accept that some phones will take that long in some instances?
This is also an option since usually there is little one can do about connecting time. From my experience BLE stack/firmware implementations are wildly different from each other.