Develop Reference

Do you need to remove an alarm before setting new one

I have an Android alarmManager being set and a broadcastReceiver to pick it up, whenever the app triggers onResume, I am forcing a new alarm to be set.
So I'm wondering for best practices and considering good resource management, should I be removing the first alarm (if it hasn't triggered) before I reset a new alarm ?
Potentially if an alarm doesn't trigger, each time the user re-opens the app, is it creating more resource usage even if the alarm is for the same time ?

Yes you should cancel it when it is appropriate.
I suspect you will have a lot of wasted cycles otherwise. For Example,
App opens at Time T.
Schedule an alarm for T'.
Next the user opens the app again at some time, X seconds prior to T'.
You will then schedule an alarm presumable at another time T''.
However you will get an alarm firing in X secs anyways. If you ignore it, you are simply wasting battery if the phone had to wake in order to deliver the alarm.

If you create the same PendingIntent for your alarm, then the previous one will automatically be canceled when you set it again.
See AlarmManager.set(), PendingIntent.

how to sense that the scheduled time by AlarmManager was passed in rebooting

for example,An AlarmManager to count numbers at 7AM using Calendar was set.
But user turns off the android at 6am and turns on 8am.
So program does'nt count.
I dont think i can avoid this situation if i reset AlarmManager on receiver of BOOT COMPLETED.
Please tell me the way to count exact number in this situation.

You'd have to do a bit of work. Save the time of your next alarm in permanent storage (file, shared preference, or database). Whenever an alarm occurs, update this value. Set a BOOT_COMPLETED listener. When the boot completed listener launches, have it get the current time and check if its later than the time of the next alarm you stored. If so, you missed it. If not, you're ok.
Now if you have to worry about missing multiple alarms, it gets more complicated, but the idea is the same.

How to make a robust scheduled event?

I want my application to trigger an event at a given time. At the moment I'm using AlarmManager. But this will be lost if the users phone is restarted, or the user uses a task killer or ends the application with the android task manager.
So what is the best way to do this. Should I just use an alarm and have it repeatedly set in a service so when it is deleted it come right back?

You can use awake alarm service on boot completed so that all the task will be re scheduled after reboot. You can store alarm id and details in database.

Might be overkill for what you need, but check out CommonWare's WakefulIntentService:
https://github.com/commonsguy/cwac-wakeful
Even if you don't want to fire up an entire IntentService when your alarm is triggered, his code should give you some idea of how you can persist your alarms across device reboots.

Notification when app not used for more than certain period

Is there a way to send a notification to the user that app has been downloaded but a certain task from the app is not yet complete even after certain period - say a month. One way is a background service which should come alive every month in this case, check the app state (in sharedprefs) and then send a notification. Is there some other easier way in Android without writing custom service.

Here's how I would do it. Schedule an alarm using the AlarmManager to go off a month from today. That alarm can trigger some code inside of a Receiver or otherwise to check whether the said event has occured. If it hasn't, you can then show a Dialog or whatever.

In order to wake up your app after some amount of time (in your example a month) you're going to have to set an alarm. You can use AlarmManager for that. If all you're going to do is check SharedPreferences, you can do that in a broadcast receiver. You can send your notification there.

Android, Is it poosible to use system timer as broadcast receiver?

In my application I need to refresh my data in each 5s or 10s or 1 minute, according to my user preference.
I can create thread and use timer class but i want to know is there any other way? for example timer of system sends flag (broadcast something) and i get it in application (in order to refresh my data)?
Thanks

If you want to refresh the data every serval seconds, you can setup a Hanlder to do this. However, this requires your activity keeps running.
If you want to do something at a specific time, like an Alarm. You can use the AlarmManager. This would lauch your activity or other components even it is not running now.
If you just want to moniter the change in the system time. Well, there is a Intent ACTION_TIME_TICK . You can register a BroadcastReceiver to listent to it and implement your own code when the systemt brocasts it. However, the "TICK" interval is "One Minute" only and can not be changed.

AlarmManager is what you are looking for. It can be used to set up periodic events that are delivered via a PendingIntent (which can be turned into an Intent broadcast).
I would be wary checking for new data on such a regular interval unless the device is always connected to a power source, especially if you are making the check over a network. With that small a poll interval going to the nework the battery will be flat in hours.