Android onSensorChanged(SensorEvent event) - Pedometer sensor (Nexus 5)

Question

I am using the pedometer on a nexus 5, and a oneplus one phone. They both have a sensor named "Pedometer", and the vendor is "QTI", and the sensorType is 33171009

Under andorid's sensor documentation, there is no documentation for this type of sensor.

The SensorEventListener calls public void onSensorChanged(SensorEvent event) with a sensor event, where the sensor name is "pedometer", and the values are an array of 16 float values. Since there is no documentation on this type of sensor, I do not know what each of these values mean.

It would've been helpful if the object SensorEvent also told us what each value is. Rather, you have to look up the values array in the documentation to see what each value represents.

Nonetheless, this particular sensor (Pedometer) is not mentioned anywhere in the android sensor documentation (at least from what I have discovered, if anybody knows where this exists that would be very helpful).

Digging into the source code, I find the instantiation of this sensor (pedometer) inside SystemSensorManager.java via a native method nativeGetNextSensor(Sensor sensor, int next).

The array is of size 16 because of this method:

   static int getMaxLengthValuesArray(Sensor sensor, int sdkLevel) {
        // RotationVector length has changed to 3 to 5 for API level 18
        // Set it to 3 for backward compatibility.
        if (sensor.mType == Sensor.TYPE_ROTATION_VECTOR &&
                sdkLevel <= Build.VERSION_CODES.JELLY_BEAN_MR1) {
            return 3;
        }
        int offset = sensor.mType;
        if (offset >= sSensorReportingModes.length) {
            // we don't know about this sensor, so this is probably a
            // vendor-defined sensor, in that case, we don't know how many value
            // it has
            // so we return the maximum and assume the app will know.
            // FIXME: sensor HAL should advertise how much data is returned per
            // sensor
            return 16;
        }
        return sSensorReportingModes[offset];
    }

Does anybody know what each of these 16 float values represent?

Answer 1

There's a reason that SensorEvent doesn't tell you what the values are- it doesn't know. It can't know. SensorEvent is a generic class- it knows nothing about the types of values it can be returning. This allows OEMs to add new types of sensors that Google didn't think of when creating the API while still using the same framework. This is actually what makes it such a cool API. Want to hook up a thermometer? You can do that. A mass spectrometer? You can do that. A Geiger counter? You can do that. If you were limited to the predefined types this would be impossible, unless you waited for Google to update Android itself.

As for what those 16 values mean- its going to depend on the hardware. There's going to be a C or C++ driver somewhere feeding that data to the Android framework, and that driver is likely not open sourced (but maybe you're lucky and it is). I'd suggest going to QTI's website, finding the pedometers they sell, and looking at the hardware documentation. While its going to be for C or C++, it will likely be fairly simple to see what data it exports that would be sent up to the java layer.Th