onServicesDiscovered never called while connecting to GATT Server

Question

I have a bluetooth headset which is paired with my Nexus 5X (running Android 7.1) and I would like to connect to a GATT Server of the headset. I tried it with the following code:

private BluetoothGattCallback btleGattCallback = new BluetoothGattCallback() {
    @Override
    public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState) {
        Log.d(TAG, "onConnectionStateChange: " + status + ", " + newState);

        if(newState == STATE_CONNECTED) {
            Log.d(TAG, "Device connected");
            boolean ans = gatt.discoverServices();
            Log.d(TAG, "Discover Services started: " + ans);
        }
    }

    @Override
    public void onServicesDiscovered(BluetoothGatt gatt, int status) {
            Log.d(TAG, "Number of Services: " + gatt.getServices().size());
    }
};

public void onDeviceClicked(BluetoothDevice device) {
    BluetoothGatt gatt = device.connectGatt(this, false, btleGattCallback);
    Log.d(TAG, "Connected to GATT: " + gatt.connect());
}

If I click on the headset in my UI onDeviceClicked is called and it comes to this Log output:

<!-- language: lang-none -->
Connected to GATT: true
onConnectionStateChange: 0, 2    // GATT_SUCCESS, STATE_CONNECTED
Device connected
Discover Services started: true

As you can see onServicesDiscovered is never fired. I tried to call connectGatt with TRANSPORT_LE (ref) but then I get a onConnectionStateChange: 133, 0. I also found this question which is why I added the gatt.connect() method as mentioned in answer two.

Do you have any ideas why I don't get the onServicesDiscovered callback?

In your code example you reference `mBluetoothGatt.discoverServices();` and a local variable `BluetoothGatt gatt = device.connectGatt(this, false, btleGattCallback);`? Is it possible they are not the same instance or device? — ayvazj, Jan 07 '17 at 07:56
Oh sorry that was for testing reasons. I changed it in the code above but it doesn't work either. — Cilenco, Jan 07 '17 at 09:57

ayvazj · Answer 1 · 2017-01-08T22:00:58.327

BLE on Android can be a little finicky.

Make sure you are calling mBluetoothGatt.discoverServices() on the UI thread.

if(newState == STATE_CONNECTED) {
    Log.d(TAG, "Device connected");
    new Handler(Looper.getMainLooper()).post(new Runnable() {
        @Override
        public void run() {
            boolean ans = mBluetoothGatt.discoverServices();
            Log.d(TAG, "Discover Services started: " + ans);
        }
    });
}

Also try making BluetoothGatt gatt a field variable instead of a local variable.

If you are doing any significant work, try using a library that masks all of the idiosyncrasies so you can focus on the high level logic. https://github.com/Polidea/RxAndroidBle.

Here is an example of how to read a characteristic.

        connectionObservable
                .flatMap(rxBleConnection -> rxBleConnection.readCharacteristic(characteristicUuid))
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(bytes -> {
                    readOutputView.setText(new String(bytes));
                    readHexOutputView.setText(HexString.bytesToHex(bytes));
                    writeInput.setText(HexString.bytesToHex(bytes));
                }, this::onReadFailure);

Or with Java 7 syntax

        connectionObservable
                .flatMap(new Func1<RxBleConnection, Observable<byte[]>>() {
                    @Override
                    public Observable<byte[]> call(RxBleConnection rxBleConnection) {
                        return rxBleConnection.readCharacteristic(characteristicUuid);
                    }
                })
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Subscriber<byte[]>() {
                    @Override
                    public void onCompleted() {

                    }

                    @Override
                    public void onError(Throwable e) {
                        onReadFailure(e);
                    }

                    @Override
                    public void onNext(byte[] bytes) {
                        readOutputView.setText(new String(bytes));
                        readHexOutputView.setText(HexString.bytesToHex(bytes));
                        writeInput.setText(HexString.bytesToHex(bytes));
                    }
                });

This does not help either but the API looks good :) Can you give me an example how to read e.g the battery level? I'm a bit confused with the new lambdas and the method referencing in the examples... — Cilenco, Jan 07 '17 at 22:05
Lambda and method reference takes a little getting used to but it saves a lot of boiler plate, I updated my answer with an example using Java 7 syntax. Lambda and method reference takes a little getting used to but it saves a lot of boiler plate. — ayvazj, Jan 08 '17 at 21:58
Thank you for the code :) But now I get `STARTED RxBleRadioOperationConnect` and after that `onConnectionStateChange newState=0 status=133` and everything is on the UI Thread... Any ideas for this? Is it in some way possible to set `TRANSPORT_BREDR` as the transport method? — Cilenco, Jan 08 '17 at 22:43
status=133 is a general error from the native (C/C++) BLE stack. It is most often caused by not closing open BLE connections which leaves you in a state where you can not longer open a connection because you've maxed out your connection limits. — ayvazj, Oct 28 '17 at 07:19
You don't necessarily need to run it on a UI thread, you just need all the calls to be on the SAME thread. I have crete an executor Thread and let all the callbacks\etc run from it. works like a charm :) — Re'em, Jan 18 '18 at 14:22

score 9 · Accepted Answer · answered Jan 07 '17 at 20:36

9

Something that has been really useful for me is to wait for about 600ms after the connection has been established and then start the service discovery.

answered Jan 07 '17 at 20:36

Roberto Betancourt

2,375
3
27
35

1

Wow that worked! Thank you `onServicesDiscovered` is now called but if I call `gatt.getServices()` it returns an empty list. Any ideas why? – Cilenco Jan 07 '17 at 21:40
1

@Cilenco you also have to verify that the service discovery was actually successful by checking that status == GATT_SUCCESS. Check this for more info: https://developer.android.com/reference/android/bluetooth/BluetoothGattCallback.html#onServicesDiscovered(android.bluetooth.BluetoothGatt, int). If it hasn't, then maybe you need to wait for longer before starting the service discovery. If it still doesn't work, then maybe your peripheral has other problems. – Roberto Betancourt Jan 09 '17 at 02:07

score 1 · Answer 3 · answered May 13 '19 at 14:01

Had the same problem, but waiting 600 ms wasn't enough. This is probably due to the BLE module used. I fixed the problem by calling my method

discoverServices();

after calling

device.connectGatt(this,false,gattCallback)

I'm basically just calling discoverServices every 5 seconds (this is arbitrarily chosen)

private void discoverServices() {
    if(!gattConnected) { //just a boolean
        new Handler(Looper.getMainLooper()).post(new Runnable() {
            @Override
            public void run() {
                gatt.discoverServices();
            }
        });
        new Handler(Looper.getMainLooper()).postDelayed(new Runnable() {
            @Override
            public void run() {
                discoverServices();
            }
        }, 5000);
    }
}

In the onServicesDiscovered(...) method of my gattCallback I make gattConnected true. This worked for me.

Reading all answers above you can be surprised how they even worked for someone. This answer is the only one that gives you a fallback solution if services can't be found after the first try. — Vlad Hudnitsky, May 07 '20 at 17:13

score 0 · Answer 4 · edited Apr 03 '20 at 01:08

This may help

I will explain in two steps: connecting and discovering services

connecting: connect from mainthread

set auto-reconnect to false

if version greater than or equals to M, set Transport type

else directly use reflection and handle it properly

Handler(applicationContext.mainLooper).post {
                    Log.d(TAG, " Post is called inside mainlooper")
                    mBluetoothGatt = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
                        Log.d(TAG, " Is Or Greater than M $mBluetoothDevice")
                        mBluetoothDevice!!.connectGatt(this, false,
                                onGhattListener, TRANSPORT_LE)
                    } else {
                        Log.d(TAG, " Less than M")
                        try {
                            Log.d(TAG, " Trying TRANPORT LE with reflection")
                            val m = mBluetoothDevice!!.javaClass.getDeclaredMethod("connectGatt", Context::class.java, Boolean::class.javaPrimitiveType, BluetoothGattCallback::class.java, Int::class.javaPrimitiveType)
                            m.isAccessible = true
                            val transport = mBluetoothDevice!!.javaClass.getDeclaredField("TRANSPORT_LE").getInt(null)
                            m.invoke(mBluetoothDevice, this, false, onGhattListener, transport) as BluetoothGatt
                        } catch (e: Exception) {
                            e.printStackTrace()
                            Log.d(TAG, " Catch to call normal connection")
                            mBluetoothDevice!!.connectGatt(this, false,
                                    onGhattListener)
                        }
                    }
                    Log.d(TAG, "mBluetooth gatt is $mBluetoothGatt")
                    mBluetoothGatt?.let {
                        refreshDeviceCache(mBluetoothGatt!!)
                    }
                }

discover services : in onGhattListener , if device is connected fire discoverServices() from main thread

override fun onConnectionStateChange(gatt: BluetoothGatt, status: Int,
                                         newState: Int) {
        Log.d(TAG, "onConnectionStateChange $gatt and status $status and newstate $newState")
        when (newState) {
            BluetoothGatt.STATE_CONNECTED -> {
                Handler(Looper.getMainLooper()).post {
                    gatt.discoverServices()
                }
            }
            BluetoothGatt.STATE_DISCONNECTED -> {

            }
            BluetoothGatt.STATE_CONNECTING -> {

            }
            BluetoothGatt.STATE_DISCONNECTING -> {

            }
        }
    }

this may solve your problem

call refresh method with reflection

fun refreshDeviceCache(gatt: BluetoothGatt): Boolean {
    try {
        val localMethod = gatt.javaClass.getMethod("refresh")
        if (localMethod != null) {
            return localMethod.invoke(gatt) as Boolean
        }
    } catch (localException: Exception) {
        Log.e(TAG, "An exception occured while refreshing device")
        localException.printStackTrace()
    }
    return false
}

Thanks for your recommendation to set `autoConnect` to `false` - this solved my troubles! — Tom Larcher, Oct 03 '19 at 05:09

score 0 · Answer 5 · answered Jan 21 '21 at 23:43

0

I have also face the same issue. Start scanning in the onStart() method. It worked for me

Earlier I was starting the scan on onCreate method. That's why it did not work for me

answered Jan 21 '21 at 23:43

user15055948

1

score -1 · Answer 6 · answered Aug 14 '18 at 14:27

In fact，I solve this problem by Execution the method mBluetoothGatt.discoverServices() several times（10 or more），

    int i = 10;
    while (i > 0)
    {
        if (!mIsBLE_Finded)  //如果服务发现失败,继续执行discoverServices方法
        {
            i--;
            mBluetoothGatt.discoverServices();
            System.out.println("BLEService-->" + "尝试次数:" + i);
        }
        else //在10次的尝试中，存在某次服务发现成功了
        {
            i = -1;
        }
    }

onServicesDiscovered never called while connecting to GATT Server

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