Android quick settings tile that [dis]connects a Bluetooth headset

Motivation

Contributing code to an open source project

The three virtues of a good programmer

App requirements

• A quick settings tile that allows you to connect and to disconnect simply by tapping the quick settings icon
• The app remembers the device to connect and to disconnect
• You can select a device for the app to remember in a simple list UI
• Don't turn off Bluetooth itself; connect/disconnect only the specific device

• Use Kotlin
• Use Kotlin Coroutine for async operations
• Use AndroidX ViewModel
• May not use a DI framework but make the modules as friendly as possible to write automated tests
• i.e. Avoid fat activity

Coding highlights

1. Both A2DP and HEADSET had to be disconnected to disconnect the headset

suspend fun onReady() {
    coroutineScope {
      while (true) {
        delay(1000L)
        if (headsetProxy != null && a2dpProxy != null) break
      }
    }
  }

2. You must release the profile objects before leaving

3. You cannot [dis]connect Bluetooth headphones by Android SDK

val connect = BluetoothHeadset::class.java.declaredMethods.findLast {
  it.name.equals("connect")
}
connect?.setAccessible(true)
connect?.invoke(headsetProxy, device)

4. You cannot collect two StateFlows in a launch block

selectedIndexJob = CoroutineScope(Dispatchers.Main).launch {
  viewModel.selectedIndex.collect { index ->
    if (index < 0) return@collect
    listAdapter.notifyItemChanged(index)
    sharedPreferencesAdapter.setLastSelectedAddress(viewModel.getDevice(index).address)
  }
  viewModel.previousIndex.collect { index ->
    if (index < 0) return@collect
    listAdapter.notifyItemChanged(index)
  }
}

selectedIndexJob = CoroutineScope(Dispatchers.Main).launch {
  viewModel.selectedIndex.collect { index ->
    if (index < 0) return@collect
    listAdapter.notifyItemChanged(index)
    sharedPreferencesAdapter.setLastSelectedAddress(viewModel.getDevice(index).address)
  }
}
previousIndexJob = CoroutineScope(Dispatchers.Main).launch {
  viewModel.previousIndex.collect { index ->
    if (index < 0) return@collect
    listAdapter.notifyItemChanged(index)
  }
}

5. AndroidX ViewModel and constructor injection

6. lifecycleScope.launchWhenStarted ???

"In the previous example that used launchWhenStarted to collect the flow, when the coroutine that triggers the flow collection suspends as the View goes to the background, the underlying producers remain active."

Conclusion