What's the sure-fire way to check if a sensor exists and IS WORKING?

I've seen many methods on SO:

PackageManager manager = getPackageManager();
boolean hasLightSensor = manager.hasSystemFeature(PackageManager.FEATURE_SENSOR_LIGHT);

And another:

SensorManager sm = (SensorManager) getSystemService(SENSOR_SERVICE);
Sensor light = sm.getDefaultSensor(Sensor.TYPE_LIGHT);
if (light != null) {
...
}

And another:

SensorManager sm = (SensorManager) getSystemService(SENSOR_SERVICE);
Sensor light = sm.getDefaultSensor(Sensor.TYPE_LIGHT);
SensorEventListener listener = new SensorEventListener() {
     // create sensor listener
}
boolean hasLightSensor = sm.registerListener(listener, light, SensorManager.SENSOR_DELAY_NORMAL);

There are quite a few classes and methods involved in Android sensor error handling. I've found that the best way to get clear answers is by looking at the source code, and sometimes that goes down a rabbit hole.

SensorManager is an abstract class that defines the methods to get the sensor list. getDefaultSensor()calls getSensorList(). If no sensors are found, getDefaultSensor will return null. The Android Sensor Best Practices page therefore recommends checking that sensors exist before using them, as follows:

if (sensorManager.getDefaultSensor(Sensor.TYPE_PRESSURE) != null) {
    // Success! There's a pressure sensor.
} else {
    // Failure! No pressure sensor.
}

You may also need to verify the return value of the registerListener() call. The registerListenerImpl methods are implemented in SystemSensorManager, which is a concrete class that extends SensorManager. registerListener will:

• return false if any of the input arguments are invalid
• throw an IllegalStateException if MAX_LISTENER_COUNT has been exceeded
• return false if the sensor cannot be added to the SensorEventQueue associated with the provided SensorEventListener

This last point is important. The BaseEventQueue class (base class of SensorEventQueue) provides a method called addSensor(), which can return false if:

• the sensor has already been added to the queue and is active
• the sensor cannot support the provided batch report latency
• the sensor could not be enabled

This last point is very important, since a return value of false may indicate a sensor failure. To find out more about what can cause the sensor to not enable, you need to look into the Native Sensor Manager code in C++. Suffice it to say, you should always check the return value of your registerListener() calls. Don't forget to unregister when you're done with the sensor.