Android scan does not discover any peripherals

Version

Tell us which versions you are using:

• react-native-ble-manager v6.6.6
• react-native v59.9
• Android v.8.1.0 on Nexus 6P phone

Expected behaviour

Would expect to see one or more Bluetooth peripherals to be discovered.

Actual behaviour

No peripherals are found whatsoever on Android Nexus 6P with Android 8.1.0. (Note that the ‘stopScan’ event does get triggered after the expected 5 seconds)

Works fine on iOS and on Android-based Kindle Fire HD10 7th generation.

Steps to reproduce

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

const eventEmitter = new NativeEventEmitter(NativeModules.BleManager)

export default class App extends React.Component {

  async componentDidMount() {
    
    eventEmitter.addListener(
      'BleManagerDiscoverPeripheral',
      async (args) => {
        console.log("found: " + args.id)
      }
    )

    eventEmitter.addListener(
      'BleManagerStopScan',
      () => {
        console.log("done")
      }
    )

    console.log("started")
    await BleManager.start( { forceLegacy: true } )
    console.log("scanning")
    await BleManager.scan([], 5)
  }

  render() {
    return null
  }
}

[Gra55h0pper]

Found the problem! 😃

I had missed the Note in the manual stating: “Android API >= 23 require the ACCESS_COARSE_LOCATION permission to scan for peripherals”. (and apparently, a lack of permissions does not cause the ‘scan’ method to trow an exception).

Here’s updated code with the fix:

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

const eventEmitter = new NativeEventEmitter(NativeModules.BleManager)

async function getBluetoothScanPermission() {
  const granted = await PermissionsAndroid.request(
    PermissionsAndroid.PERMISSIONS.ACCESS_COARSE_LOCATION,
    {
      title: 'Bluetooth Permission',
      message: 
        'In the next dialogue, Android will ask for permission for this ' +
        'App to access your location. This is needed for being able to ' +
        'use Bluetooth to scan your environment for peripherals.',
      buttonPositive: 'OK'
      },
  )
  if (granted !== PermissionsAndroid.RESULTS.GRANTED) {
    console.log('BleManager.scan will *NOT* detect any peripherals!')
  }
}

export default class App extends React.Component {

  async componentDidMount() {
    
    eventEmitter.addListener(
      'BleManagerDiscoverPeripheral',
      async (args) => {
        console.log("found: " + args.id)
      }
    )

    eventEmitter.addListener(
      'BleManagerStopScan',
      () => {
        console.log("done")
      }
    )

    console.log("started")
    await BleManager.start( { forceLegacy: true } )
    console.log("check location access permission")
    await getBluetoothScanPermission()
    console.log("scanning")
    await BleManager.scan([], 5)
  }

  render() {
    return null
  }
}

[Gra55h0pper]

@rayjadore the scan method just starts the scan. It doesn’t return anything (other than a promise). I would expect your code to return undefined on iOS also.

Use the BleManagerDiscoverPeripheral event to report peripherals found. Also, make sure to call BleManager.start({ forceLegacy: true }) first.

If it still doesn’t work, then perhaps we face the same problem.