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androidbluetoothbluetooth-lowenergystm32android-bluetooth

Android BLE automatic reconnections after pairing

We want Android to automatically connect to our custom made BLE peripheral.

Our peripheral should regularly (but infrequently) advertise and attempt to Indicate some time-sensitive sensor data to the phone. Thus we want the phone to be ready to connect at any time.

Generally, you can pair a smart watch with an Android, and Android will then automatically connect to the smart watch whenever it is in range. So we believe our use case should be feasible.

I read a lot of answers that advise to set the "autoconnect" parameter to true when connecting. I have tried that and the reconnections don't persist through a reboot or even after disabling and re-enabling Bluetooth on Android. This answer by Brian says I should scan in the background, but Android made this unrealistic. If I use a foreground service, my users will hate the app. If I use a background service, I may miss the peripheral's attempts to connect during Android's Doze and the code becomes error prone.

Ideally, I want to do something like what Emil said in his answer here. Please read the follow up question and response.

However, we can't see our app through Android's Bluetooth settings. We can only connect to the peripheral and pair with it using our app (or nrf Connect). In desperation, I tried modifying the peripheral's advertising flags. Then I could see it in Android's Bluetooth settings. But when I try to pair using Android's settings, the attempt fails because the peripheral is not in "pairing mode".

We are building both the app and the peripheral, so we can change both. I want to know if our use case is possible and what we need to do to get it working. We are using the STM32WB for our peripheral.

Solution

  • Use a combination of these techniques:

    1. Bond the device. This might be needed due to the crappy Android Bluetooth LE API design that doesn't take the "address type" as an extra parameter when connecting to a device. When you connect using the Bluetooth device address, it looks up a device with this address in the bonding info, and uses the corresponding address type (random or public).

    2. Use connectGatt with autoConnect set to true. This means no timeout, as well as auto-reconnect if the connection drops. Even if it takes days or weeks until the peripheral starts advertising, it will still work.

    3. Listen to https://developer.android.com/reference/android/bluetooth/BluetoothAdapter#ACTION_STATE_CHANGED and restart your connections when Bluetooth is re-started.

    4. Use a Foreground Service in your application's process to prevent the OS from killing the process. Users can nowadays hide the annoying notification in Android settings if they want to.

    5. Listen to https://developer.android.com/reference/android/content/Intent#ACTION_BOOT_COMPLETED to start your app after boot, including your Foreground Service.

    6. Listen to https://developer.android.com/reference/android/content/Intent#ACTION_PACKAGE_REPLACED to automatically restart your app after an app update. See https://stackoverflow.com/a/2134443/556495 for some instructions.