mbl_mw_datasignal_log Crashes

Hi I'm investigating a case where mbl_mw_datasignal_log is called after sensor is disconnected unexpectedly and leads to app crashing. The line crashes happen is state->create_next(true); inside the mbl_mw_datasignal_log timeout. The exception can't be handled in our code because it is outside the logger callback. Is this a bug in the SDK or are we missing something? The code works fine if it is connected. This only happens if the sensor is disconnected.

The following code should show the crash described:

device.apiAccessQueue.async {
    if let signal = mbl_mw_sensor_fusion_get_data_signal(device.board,
        mbl_mw_metawearboard_set_time_for_response(device.board, 2000)
        // Simulates unexpected disconnection
        mbl_mw_datasignal_log(signal, nil, { (context, logger) in
            if let logger = logger {
                // Set Logger
            } else {
                // Handles nil


  • Changing if (force) to if (force && !pending_fns.empty()) in C++ SDK's AsyncCreator::create_next seems to fix the problem. Does this make sense?

    void AsyncCreator::create_next(bool force) {
        // if (force) {
        if (force && !pending_fns.empty()) {
        if ((force && !pending_fns.empty()) || (!force && pending_fns.size() == 1)) {
  • edited December 2

    Yes, the empty check should happen since the disconnect handler clears the pending_fns queue.

    Thanks for bringing that up. We'll add that in the next release

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