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In my app i have start and stop button, when user press start i call startScan method

bluetoothAdapter.getBluetoothLeScanner().startScan(getLeScanCallback());

When user press stop i call stopScan however it doesn't seem to do anything. BluetoothAdapter keeps scanning for new devices.

bluetoothAdapter.getBluetoothLeScanner().stopScan(getLeScanCallback());

Here is my getLeScanCallback method:

private ScanCallback getLeScanCallback(){
    ScanCallback leScanCallback = new ScanCallback() {
        @Override
        public void onScanResult(int callbackType, ScanResult result) {
            super.onScanResult(callbackType, result);
            boolean duplicate = false;
            for(Device d : devices) {
                if(d.getAddress().equals(result.getDevice().getAddress()))
                    duplicate = true;
            }
            if(!duplicate) {
                Device device = new Device();
                device.setName(result.getDevice().getName());
                device.setAddress(result.getDevice().getAddress());
                device.setStatus(getString(R.string.disconnected));
                devices.add(device);
                deviceListAdapter.notifyDataSetChanged();
            }
        }

        @Override
        public void onBatchScanResults(List<ScanResult> results) {
            super.onBatchScanResults(results);
        }

        @Override
        public void onScanFailed(int errorCode) {
            super.onScanFailed(errorCode);
        }
    };

    return leScanCallback;
}

It gets called even after stopScan() was called. What am I doing wrong or in other words, how to stop scanning for BLE devices?

Michał Witanowski
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4 Answers4

28

Every time you call getLeScanCallback you create new instance of ScanCallback and lost reference to previous instance.

stopScan stops ongoing scan regardless what instance you pass BUT if it is a different instance (than the one you used to start) it will not remove old instance of callback so still some events will be delivered to it before it stops.

RadekJ
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  • This seems to be a completely incorrect answer. You don't "start" a scan with the ScanCallback instance. All the start and stop calls get its own instance, and that particular instance is called upon completion or failure. The code is fine, you just have to be prepared on that the system might deliver results after the stopScan. – BjornW Feb 27 '19 at 21:54
  • If `stopScan` is called with new callback there is still some events delivered to previous callback before scan is stopped. If you will use the same callback instance, it will remove it instantly and no events will be delivered to it (even though stopping scan might not be instant). – RadekJ Feb 28 '19 at 08:34
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    @BjornW That's not correct. Look at the source code of `BluetoothLeScanner`, if the callback you pass to `stopScan` is not the same as the one passed to `startScan`, then `stopScan` has no effect – alexc May 21 '19 at 00:23
  • @alexc actually I don't even understand my previous comment, reading it now :) I remember having to fix this myself as well, and I looked at the underlying source and android debug output and figured it out, but it must be badly documented.. – BjornW May 22 '19 at 10:45
8

Scan results will not stop immediately, but will eventually stop. There is a queue of pending results that you can flush with:

scanner.flushPendingScanResults();

But even then, the documentation is clear that it will send whatever it has found to the callback. This happens even if you have stopped scanning. Poor API design, but it is what it is.

The easiest I think is to set a flag to ignore the scan results after you have stopped them.

spekary
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    That is very good to know. When you say "The documentation is clear that..." can you point us toward the relevant docs? Thanks – HughHughTeotl Jul 09 '18 at 15:25
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    Last sentence was the best line for my implementation. "The easiest I think is to set a flag to ignore the scan results after you have stopped them." – jhwblender Dec 29 '22 at 15:37
2

I haven't noticed any issues with your code, it seems to be fine. Perhaps there's an issue with your getLeScanCallback() method?

Instead of having a method to return your ScanCallback, you could declare it as a member variable so you would have leScanCallback = new ScanCallback() and then you could just refer to this member variable instead when trying to stop the scan.

bluetoothAdapter.getBluetoothLeScanner().stopScan(leScanCallback);

Depending on what version of Android you're supporting, it might be worthwhile adding the startLeScan and stopLeScan methods which Rajesh Panchal suggested for pre Lollipop Android devices.

Mark O'Sullivan
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-1

replace

bluetoothAdapter.getBluetoothLeScanner().startScan(getLeScanCallback());
bluetoothAdapter.getBluetoothLeScanner().stopScan(getLeScanCallback());

with 

bluetoothAdapter.getBluetoothLeScanner().startLeScan(getLeScanCallback()); 
bluetoothAdapter.getBluetoothLeScanner().stopLeScan(getLeScanCallback());
Rajesh Panchal
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