Develop Reference

What will alarm manager do if I change phone's time?

I set multiple alarms and in order to control them I change my phone's time. They will work normally ? What In other words, will alarms collapse if I change my phone's time?

Well it all depends on the type you have passed in the set method of alarm Manger.
if you have used ELAPSED_REALTIME or ELAPSED_REALTIME_WAKEUP, alarm will not trigger
ELAPSED_REALTIME
Added in API level 1
int ELAPSED_REALTIME
Alarm time in SystemClock.elapsedRealtime() (time since boot, including sleep). This alarm does not wake the device up; if it goes off while the device is asleep, it will not be delivered until the next time the device wakes up.
if you have user RTC or RTC_WAKEUP, alarm will be triggered according the device time
RTC
Added in API level 1
int RTC
Alarm time in System.currentTimeMillis() (wall clock time in UTC). This alarm does not wake the device up; if it goes off while the device is asleep, it will not be delivered until the next time the device wakes up.

I don't know what you mean by collapse. But the alarm is only aware of the device's time and so if you do change your device time, it should work against that instead.

Background wifi scanning in Android

I'd started for about a month or two and I've to develop an Indoor Positioning System based on Wifi fingerprinting. I need an app that periodically scans wifi APs and send the result data to a Server.
So far I created an app that it's able to scan wifi APs and get the results when different connections are detected. I'm doing this in main activity using a broadcast receiver. The app is also able to send the data to the server.
What I want now is move this process to a periodic process in background, even when the smartphone is in sleep mode.
I have already read some topics about how to do it but none was clear. My question is what is the best way to do this? Using a Service/IntentService with a Timer/TimerTask?
Thanks.
Edit: Thanks!! AlarmManager and Services work fine!

I think an AlarmManager fits your needs, use setRepeating to set something to repeat every X time
Schedule a repeating alarm. Note: for timing operations (ticks,
timeouts, etc) it is easier and much more efficient to use Handler. If
there is already an alarm scheduled for the same IntentSender, it will
first be canceled.
Like set(int, long, PendingIntent), except you can also supply a
period at which the alarm will automatically repeat. This alarm
continues repeating until explicitly removed with
cancel(PendingIntent). If the stated trigger time is in the past, the
alarm will be triggered immediately, with an alarm count depending on
how far in the past the trigger time is relative to the repeat
interval.
If an alarm is delayed (by system sleep, for example, for non _WAKEUP
alarm types), a skipped repeat will be delivered as soon as possible.
After that, future alarms will be delivered according to the original
schedule; they do not drift over time. For example, if you have set a
recurring alarm for the top of every hour but the phone was asleep
from 7:45 until 8:45, an alarm will be sent as soon as the phone
awakens, then the next alarm will be sent at 9:00.
If your application wants to allow the delivery times to drift in
order to guarantee that at least a certain time interval always
elapses between alarms, then the approach to take is to use one-time
alarms, scheduling the next one yourself when handling each alarm
delivery.
Note: as of API 19, all repeating alarms are inexact. If your
application needs precise delivery times then it must use one-time
exact alarms, rescheduling each time as described above. Legacy
applications whose targetSdkVersion is earlier than API 19 will
continue to have all of their alarms, including repeating alarms,
treated as exact.
Parameters
type One of ELAPSED_REALTIME, ELAPSED_REALTIME_WAKEUP, RTC,
or RTC_WAKEUP.
triggerAtMillis time in milliseconds that the alarm
should first go off, using the appropriate clock (depending on the
alarm type).
intervalMillis interval in milliseconds between
subsequent repeats of the alarm.
operation Action to perform when the
alarm goes off; typically comes from IntentSender.getBroadcast().
As the note says
Note: as of API 19, all repeating alarms are inexact. If your
application needs precise delivery times then it must use one-time
exact alarms, rescheduling each time as described above. Legacy
applications whose targetSdkVersion is earlier than API 19 will
continue to have all of their alarms, including repeating alarms,
treated as exact.
But i don't think you care much if it's not precise.
Or you can use setInexactRepeating
Schedule a repeating alarm that has inexact trigger time requirements;
for example, an alarm that repeats every hour, but not necessarily at
the top of every hour. These alarms are more power-efficient than the
strict recurrences traditionally supplied by setRepeating(int, long,
long, PendingIntent), since the system can adjust alarms' delivery
times to cause them to fire simultaneously, avoiding waking the device
from sleep more than necessary.
Your alarm's first trigger will not be before the requested time, but
it might not occur for almost a full interval after that time. In
addition, while the overall period of the repeating alarm will be as
requested, the time between any two successive firings of the alarm
may vary. If your application demands very low jitter, use one-shot
alarms with an appropriate window instead; see setWindow(int, long,
long, PendingIntent) and setExact(int, long, PendingIntent).
As of API 19, all repeating alarms are inexact. Because this method
has been available since API 3, your application can safely call it
and be assured that it will get similar behavior on both current and
older versions of Android.
Parameters
type One of ELAPSED_REALTIME, ELAPSED_REALTIME_WAKEUP, RTC,
or RTC_WAKEUP.
triggerAtMillis time in milliseconds that the alarm
should first go off, using the appropriate clock (depending on the
alarm type). This is inexact: the alarm will not fire before this
time, but there may be a delay of almost an entire alarm interval
before the first invocation of the alarm.
intervalMillis interval in
milliseconds between subsequent repeats of the alarm.
Prior to API 19,
if this is one of INTERVAL_FIFTEEN_MINUTES, INTERVAL_HALF_HOUR,
INTERVAL_HOUR, INTERVAL_HALF_DAY, or INTERVAL_DAY then the alarm will
be phase-aligned with other alarms to reduce the number of wakeups.
Otherwise, the alarm will be set as though the application had called
setRepeating(int, long, long, PendingIntent). As of API 19, all
repeating alarms will be inexact and subject to batching with other
alarms regardless of their stated repeat interval. operation Action to
perform when the alarm goes off; typically comes from
IntentSender.getBroadcast().
While my answer aim is to say a general way to repeat an action every X time, as other noticed, you will need Wifi Lock, Wake lock and use RTC_WAKEUP as AlarmManager type.
RTC_WAKEUP:
Alarm time in System.currentTimeMillis() (wall clock time in UTC),
which will wake up the device when it goes off.

Move your code from Activity to Service. Then, start the service periodically using AlarmManager. Service does the scanning and stops self when it is done. Then it is started again when timer elapses.
EDIT: Also look at cpu lock and wifi lock.
EDIT2: Also when creating alarm, use RTC_WAKEUP constant so that your action is triggered even if the device is in sleep mode.

Alarm Manager Accuracy

How to use alarm manager at accurate time interval?
I used alarm manager but it response inaccurate.
Can anyone help me?
I used this code
PendingIntent sender;
AlarmManager am;
long firstTime;
Intent itnt = new Intent();
itnt.setAction("abts.medismo.medismo.ALARMRECEIVER");
sender = PendingIntent.getBroadcast(context, 0,itnt, 0);
am = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE);
am.cancel(sender);
firstTime = SystemClock.elapsedRealtime();
am.setInExactRepeating(AlarmManager.ELAPSED_REALTIME,firstTime, Integer.parseInt(sep[3]), sender);

You should use ELAPSED_REALTIME_WAKEUP, because if the phone is asleep it wont fire until it wakes up again.

There are two issues here.
The first, as others have pointed out, is that your alarms are not "wakeup" alarms. This means that if the device is asleep when the trigger time is reached, the alarm will not cause the device to wake up and process the alarm immediately. Instead, it will be processed as soon as something else causes the device to become active, such as the user picking it up and pushing the power button to start using it.
The second issue is that you are using setInexactRepeating(). As the name implies, these alarms are never going to be delivered at precisely predictable times. As the documentation says, "In addition, while the overall period of the repeating alarm will be as requested, the time between any two successive firings of the alarm may vary."
This is often enough for most apps, and it is more efficient in terms of battery use. However, if you truly need the alarm to be delivered at a specific time, you need to set the alarm time yourself instead of relying on periodic alarms. In addition, as of API version 19, you should use the new setExact() APIs if you truly need precise delivery.

I believe your observed inaccuracy may be caused by the subtle variation of flags for the alarms.
AlarmManager.ELAPSED_REALTIME and AlarmManager.ELAPSED_REALTIME_WAKEUP utilize elapsedRealtime(), which is the number of milliseconds since the device last booted up, good for measuring elapsed time for timers and the like.
AlarmManager.RTC and AlarmManager.RTC_WAKEUP utilize currentTimeMillis(), which is the system clock:
time in milliseconds since January 1, 1970 00:00:00 UTC. This method shouldn't be used for measuring timeouts or other elapsed time measurements, as changing the system time can affect the results.
All four of the AlarmManager clock flags I've listed will include time spent in sleep, but only the *_WAKEUP flags will wake the device to sound the alarm; the default flags will wait until the device is otherwise awakened and then sound the alarm immediately.
Make sure you select the flag that matches your use-case, and ensure that the value of your delay {Integer at sep[3]?} is the amount of time to wait between alarms in milliseconds; as you have it, your Integer is being unboxed and cast to a long; it works, but unless there's a reason for it, I recommend switching to a primitive long; so a 15 minute delay would be:
int delay = 900000L // 15 minutes * 60 seconds * 1000 milliseconds
http://developer.android.com/reference/android/app/AlarmManager.html
http://developer.android.com/reference/android/os/SystemClock.html

What happens to undelivered AlarmManager repeating alarms when phone wakes up?

If we schedule an inexact repeating alarm with AlarmManager.setInexactRepeating to be fired every hour, for example, what happens if the phone is in standby for 4 hours and then it is woken up? Does the system deliver 4 alarms in a row for the same PendingIntent or does it deliver only a single alarm?
EDIT: reading the documentation with more attention, the set(int, long, android.app.PendingIntent) method documentation says: Alarm intents are delivered with a data extra of type int called Intent.EXTRA_ALARM_COUNT that indicates how many past alarm events have been accumulated into this intent broadcast. Recurring alarms that have gone undelivered because the phone was asleep may have a count greater than one when delivered.
So, when the phone comes out of standby, only one alarm will be delivered, with an intent extra that contains the number of missed alarms.

Reading the documentation with more attention, the set(int, long, android.app.PendingIntent) method documentation says: Alarm intents are delivered with a data extra of type int called Intent.EXTRA_ALARM_COUNT that indicates how many past alarm events have been accumulated into this intent broadcast. Recurring alarms that have gone undelivered because the phone was asleep may have a count greater than one when delivered.
So, when the phone comes out of standby, only one alarm will be delivered, with an intent extra that contains the number of missed alarms.

it depends if you use
pending intent like this
PendingIntent pending = PendingIntent.getBroadcast(context, 0, i,
PendingIntent.FLAG_CANCEL_CURRENT);
it wont trigger 4 times as it cancells the previous pending intent

Android Alarm What is the difference between four types of Alarm that AlarmManager provides and when to use what?

I want to know the difference between RTC, RTC_WAKEUP, ELAPSED_REALTIME, ELAPSED_REALTIME_WAKEUP.
I want to write an alarm application where I will set alarm and close my application and expect for alarm for the time set.
There will be multiple alarms. Right now I am writing for emulator but later will test on device. In emulator, once I set the alarm and close the emulator and restart it, then will it be cleared, as I find with RTC, RTC_WAKEUP and ELAPSED_REALTIME. I am confused. Should I used ELAPSED_REALTIME_WAKEUP? I have not seen any tutorial using ELAPSED_REALTIME_WAKEUP.
please explain.
Thanks.

ELAPSED_REALTIME
Alarm time in SystemClock.elapsedRealtime() (time since boot, including sleep). This alarm does not wake the device up; if it goes off while the device is asleep, it will not be delivered until the next time the device wakes up.
ELAPSED_REALTIME_WAKEUP
Alarm time in SystemClock.elapsedRealtime() (time since boot, including sleep), which will wake up the device when it goes off.
RTC
Alarm time in System.currentTimeMillis() (wall clock time in UTC). This alarm does not wake the device up; if it goes off while the device is asleep, it will not be delivered until the next time the device wakes up.
RTC_WAKEUP
Alarm time in System.currentTimeMillis() (wall clock time in UTC), which will wake up the device when it goes off.

Types of Alarms :
ELAPSED_REALTIME – Fires the pending intent after the specified length of time since device boot. If the device is asleep, it fires when the device is next awake.
ELAPSED_REALTIME_WAKEUP – Fires the pending intent after the specified length of time since device boot. It wakes up the device if it is asleep.
RTC – Fires the pending intent at a specified time. If the device is asleep, it will not be delivered until the next time the device wakes up.
RTC_WAKEUP – Fires the pending intent at a specified time, waking up the device if asleep.

There are two general clock types for alarms: "elapsed real time" and "real time clock" (RTC). Elapsed real time uses the "time since system boot" as a reference, and real time clock uses UTC (wall clock) time. This means that elapsed real time is suited to setting an alarm based on the passage of time (for example, an alarm that fires every 30 seconds) since it isn't affected by time zone/locale. The real time clock type is better suited for alarms that are dependent on current locale.
Source: https://developer.android.com/training/scheduling/alarms.html

From the site you can get the difference between the 4 constanst
Below is example of the setting alarm
Calendar mCalendar = Calendar.getInstance();
mCalendar.add(Calendar.SECOND, 20);
Intent intent_Timer = new Intent(TimerEvents.this, AlarmReceiver.class);
intent_Timer.putExtra("alarm_message", "Drax Rules!!!");
// In reality, you would want to have a static variable for the request
// code instead of 192837
PendingIntent sender = PendingIntent.getBroadcast(this, 192837,
intent_Timer, PendingIntent.FLAG_UPDATE_CURRENT);
// Get the AlarmManager service
AlarmManager am = (AlarmManager) getSystemService(ALARM_SERVICE);
am.set(AlarmManager.RTC_WAKEUP, mCalendar.getTimeInMillis(), sender);
Hope this will be helpful to you