Develop Reference

Get accelerometer values in time

I have to get the accelerometer values in time.
I saw the Sensor class and the SensorEventListener but this listener notify only when the value changes and I need a periodic notification (also if the value do not change).
Otherwise I would like to continuously read the accelerometer value but it is not possible according to this thread: Get current SensorEvent value
What can I do?

Why do you need to continuously "read" the sensor values even when they are the same as before? If you need to calculate some output on a regular basis, then a better solution might be to set up a timer that triggers on the required intervals at which time you use the values from a set of variables that are updated whenever the sensor values change. That way even if they don't change, the timer will initiate the calculations.
Kaamel

How to check the sensor data every 0.5 seconds in Android

There is an onSensorChanged(SensorEvent event) listener for android sensor. But this is triggered whenever this is a tiny change in the sensor data. How can I check the sensor reading every 0.5s instead of whenever the sensor data has changed?

I'm afraid you can not set the interval that the callback will be triggered since even passing SensorManager.SENSOR_DELAY_NORMAL parameter will cause it to trigger more than once every half second. Check the values for SENSOR_DELAY below.
SENSOR_DELAY_UI (60,000 microsecond delay)
SENSOR_DELAY_FASTEST (0 microsecond delay)
SENSOR_DELAY_NORMAL) (200,000 microseconds delay)
So, you will receive callbacks every 0.2 seconds and implement your way to DoYourProcedure() once every 2.5 ticks.

use Handler.postDelay(runnable, timeInMillis)

Actively query the proximity sensor once?

I need to get the current value of the proximity sensor (rather than implementing a continuous listener). On some devices, the first reported value will be a default value (e.g. "FAR") that isn't necessarily accurate, and actual values will only start appearing after the second or third reading. At the moment, I've implemented a 1-second Handler and use the last reported value (after the second has elapsed) as the "true" value, but this solution seems crude (and slow). Is there a better approach that works on all 4.0+ devices? I could simply count up until I've received 3 readings, but on some devices (e.g. GNex), the first value will be correct, and the value will only change after that if there is actually a change in the sensor.

You can do what I did:
You probably have an if statement on the listener - one logic flow for near and one for far.
Instead of waiting on the handler - do this:
if(near) {
myHandler.removeCallbacks(yourRunnableForFar);
myHandler.postDelayed(yourRunnableForNear,100);
else {
myHandler.removeCallbacks(yourRunnableForNear);
myHandler.postDelayed(yourRunnableForFar,100);
}
Notice that the inaccurate first reading(s) will immediately be followed by an accurate one, so the last one "wins".
This code works well if you didn't register sensors other than proximity. If you have a flow of readings from other sensors, than use a static flag (such as the boolean near) to trigger the handler calls only on state change.
Elaboration:
yourRunnableForFar and yourRunnableForNear - are placeholders that implement Runnable to hold your app logic on what to do when the proximity sensor returns "near" (event.values[0] == 0) or "far" (not 0).
myHandler is just any Handler you might created, or declare one just for this with Handler myHandler = new Handler(Looper.getMainLooper());
You might want to acquire a proximity lock on near, and release it and clear the listener on far. But this is app logic that might be completely different from app to app.

Android Chronometer - One second "ticks" late

The app is a sports timer for cycling, skiing etc, where racers start at regular intervals. e.g. 1 minute.
In my implementation of OnChronometerTickListener I notice that the calls occur at intervals significantly longer than 1000 mS. I use the elapsed time (between the Tick and the Chronometer's base) to count down the last 5 seconds for each interval. Due to the late callback, I can get ticks at, say,
55,500
56,600
57,750
58,870
59,980
61,110
I can therefor skip a whole second when I use m_Elapsed % 1000.
I have even seen the text in the Chronometer get behind and have to skip a second.
I have no problem with accuracy when I do calculations based on the Chronometer's base time and current system time.
Do I have to write my own Chronometer using finer callbacks?
Or is there some other fix?

Android Chronometer

How to start the chronometer with a specific time other than default 00:00? Is it possible to set chronometerObj.setBase(startTime) ?
ch.setBase(SystemClock.elapsedRealtime()-anylongvalue);
ch.start(); can I set start time, if I put anylongvalue?

In general:
mChronometer.setBase(SystemClock.elapsedRealtime() - (nr_of_min * 60000 + nr_of_sec * 1000)))

Chronometer object, when instantiated, defaults to the base time being set to now ('now' as in the value you get from SystemClock.elapsedRealtime()).
You can change the base time (00:00 time) by calling setBase(<some other value>).
Presumably, although I haven't tried the experiment, you could see the elapsed time since last system boot using setBase(0).
So you can use chronometer to see the elapsed time since any arbitrary call you made in the past to SystemClock.elapsedRealtime(). The trick is that you need to have stored that value somewhere you can dependably get it back despite app and phone state changes. (See Android: chronometer as a persistent stopwatch. How to set starting time? What is Chronometer "Base"? for example.)
Many answers suggest persisting that arbitrary time-in-the-past in an intent. but, at best, this only keeps the timer counting up while the phone stays on.
I already am using a database and store my starting time in there. I created a one-column table for it and store a single record in it. My starting time for the chronometer survives a phone reboot.