getSensorList() vs. getDefaultSensor() in Android

问题:

I'm writing a game for Android and want to be able to use the accelerometer for input.

I see two ways of getting a sensor, one way is to use the first element of SensorManager.getSensorList(Sensor.TYPE_ACCELEROMETER) and the other is SensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER).

The getDefaultSensor doc says it could return a "composite" sensor, so if I want a "raw" sensor I should use getSensorList.

Any idea what the difference between a composite or raw sensor is? Does this even apply to the accelerometer? Anyone have experience with devices that contain multiple or composite accelerometers? (Or some other sensor?)

回答1:

Google's documentation is way ahead of their implementation here. I browsed through the code repository (which seems to be 2.3.1-ish source) and found:

public Sensor getDefaultSensor(int type) {
    // TODO: need to be smarter, for now, just return the 1st sensor
    List<Sensor> l = getSensorList(type);
    return l.isEmpty() ? null : l.get(0);
}

So there is no real difference (and I don't think they can really add one later) between the sensors returned from getDefaultSensor() and from getSensorList().

回答2:

An update: They have updated the getDefaultSensor method in Lollipop, and there now is a difference:

public Sensor getDefaultSensor(int type) {
    // TODO: need to be smarter, for now, just return the 1st sensor
    List<Sensor> l = getSensorList(type);
    boolean wakeUpSensor = false;
    // For the following sensor types, return a wake-up sensor. These types are by default
    // defined as wake-up sensors. For the rest of the SDK defined sensor types return a
    // non_wake-up version.
    if (type == Sensor.TYPE_PROXIMITY || type == Sensor.TYPE_SIGNIFICANT_MOTION ||
            type == Sensor.TYPE_TILT_DETECTOR || type == Sensor.TYPE_WAKE_GESTURE ||
            type == Sensor.TYPE_GLANCE_GESTURE || type == Sensor.TYPE_PICK_UP_GESTURE) {
        wakeUpSensor = true;
    }

    for (Sensor sensor : l) {
        if (sensor.isWakeUpSensor() == wakeUpSensor) return sensor;
    }
    return null;
}

So if there are multiple sensors available for the specified type, getDefaultSensor will return a non-wakeup version (unless the default type is one of those 6 above actually defined as a wakeup sensor)

By the way, Sensor.TYPE_TILT_DETECTOR, Sensor.TYPE_WAKE_GESTURE, Sensor.TYPE_GLANCE_GESTURE and Sensor.TYPE_PICK_UP_GESTURE are hidden in the SDK, as they are intended to be used only for the system UI. There's more details on them in the Sensor.java source