How can I list connected devices?

Question

How can I list the connected devices?

Steps:

• I start the device scan
• I find the device by name and connect to it to check the services and characteristics
• I make some changes on the app and run it again
• This time the scan doesn’t include my device, as it says on the introduction: “However when device is connected it won’t broadcast and needs to be disconnected from central to be scanned again.”
• Ok, so how can I find the connected device so I can disconnect it, or use it?

I tried let connectedDevices = await manager.connectedDevices(null) or () - gives an error - Attempt to invoke interface method ‘int.com.facebook.react.bridge.ReadableArray.size()’ on a null object reference let connectedDevices = await manager.connectedDevices([]) - gives an empty array

The connectedDevices method needs a serviceUUIDs parameter but how can I get the connected devices regardless of their serviceUUIDs?

[amb-silvertree]

This is still an issue: I specifically run into it during development all the time with the following set up on Android:

1. use react-native-ble-plx to discover and connect to my BLE device.
2. As a developer using Metro for react native, press ‘r’ to reload my app.
3. Now the app has lost the connection state normally returned by connectedDevices(). I get 0 devices listed even though my BLE device is happily still connected to my Android phone.

I can verify this is the problem because I use this (unacceptable for production) quick workaround: if you disable and enable Bluetooth on your Android phone, that will disconnect from your BLE hardware, and now your app state will match the hardware.

While dev mode is the easiest to reproduce this in, it’s relatively easy to see how this would happen in production: literally any crash in your app or killing of your app by a 3rd party service or Android OS will cause this bad state to exist

Until this issue is resolved I have resorted to a dual- library approach: using this library and ALSO react-native-ble-manager together. This is the only solution as of December 2021 that doesn’t require a user to disable and enable Bluetooth on Android.

My shim:

import BleManager from 'react-native-ble-manager'
import { NativeModules, NativeEventEmitter } from 'react-native'

const BleManagerModule = NativeModules.BleManager

// This shim replaces react-native-ble-plx's BleManager.connectedDevices because the latter does not return devices connected that 
// might have been connected before the app was started (or, more likely, before the app was crashed and restarted.)
// This functionality means that we can't use the react-native-ble-plx's BleManager.connectedDevices because our connected device
// won't show up on it. 
const getConnectedDeviceIds = async (serviceUUIDs) => {
    const connectedPeripherals = await BleManager.getConnectedPeripherals(serviceUUIDs)
    return connectedPeripherals.map(peripheral => peripheral.id)
}
export { getConnectedDeviceIds }

And an excerpt of how I use it (in my React Native provider load function):

      // SHIM: Sync the PLX bleManager wearables list with the devices connected in hardware
      // since the PLX bleManager doesn't return them right now.
      const ids = await getConnectedDeviceIds([SILVERTREE_SERVICE_UUID])
      for (id of ids){
        // Sync up PLX's state with the hardware state:
        const connectedDevice = await bleManager.connectToDevice(id, { refreshGatt: 'OnConnected',  autoConnect: false });
        
      } 
      // Now our app state should match the hardware state.

[nicolas-wl]

@nbastoWM Did u ever solve this? I’m getting the same error “Attempt to invoke interface method ‘int com.facebook.react.bridge.ReadableArray.size()’ on a null object reference”. However only on android, on iOS it works fine.