Develop Reference

Possibility of not calling Activity.onResume()

My app does a lot of setting up in onCreate and then in onResume.
Is it possible that if a device rotation occurs while onCreate is being performed then onResume won't be called?
I'm asking because I receive many crash logs happening in onPause(). The crashes occur, because in onResume() I ALWAYS register some listeners and in onPause they are ALWAYS unregistered. Exception is thrown when a listener that hasn't been registered is unregistered.
So: is it possible that onResume() is not called at all before onPause()?

is it possible that onResume() is not called at all before onPause()?
No. Never.
onResume() Called just before the activity starts interacting with the user. At this point the activity is at the top of the activity stack, with user input going to it.
Always followed by onPause(). So if onPause() of an Activity is being called means onResume() must called.
Read A summary of the activity lifecycle's callback methods for more details.

Is it possible that if a device rotation occurs while onCreate is being performed then onResume won't be called?
Yes possibly, although it seems highly unlikely. Even if you are doing a lot of stuff in onCreate in relative terms you should never be doing so much that it takes several seconds. In other words, for a user to start your Activity and then rotate the device is going to take some finite 'human reaction' time.
If you believe this is happening then...
Reduce the amount of stuff going on in onCreate
NEVER assume that your listeners are registered - check first. Example, when you register a listener set a boolean and only unregister if it's true and set the boolean to false...
Psuedo code:
// To register
if (!isListenerRegistered) {
register(...);
isListenerRegistered = true;
}
// To unregister
if (isListenerRegistered) {
unregister(...);
isListenerRegistered = false;
}

No, http://developer.android.com/training/basics/activity-lifecycle/pausing.html
the sequence is always onCreate(), onStart(), onResume(), then the user sees the activity in the foreground(totally visible). Once another activity starts, this one goes to onPause(), then onStop().
When the device is rotated the activity gets reloaded, so it goes through the destruction steps depending on when the rotation occured. After onDestroy() is called, the creation starts again with onCreate(). Add log messages in your callbacks to see this:
Log.d("DEBUG", "In Activity A's onCreate()");
You can filter your logcat via the log tag, in this case "DEBUG" and see the lifecycle for yourself. Or just step through with the debugger.

How to handle power button (Android)

I am a developing game using andengine. When I press the power button, the onPause() method is not being called. How do I fix the issue?
#Override
protected void onPause() {
//this method is not calling when press power button
super.onPause();
this.mEngine.onPause() ;
}

Weird, haven't heard of it not being called. Let me research a bit.
Okay, pressing power should always call onPause(), there seem to be some kind of bug in your code. Could you add some log calls to follow every step. and print the DDMS log.
Other possible problems:
It seems some devices misbehave and call onPause() twice when pressing the power button.
In this case try using the powerManager to see if the screen is actually turned of.
PowerManager pm = (PowerManager) getSystemService(Context.POWER_SERVICE);
if (pm.isScreenOn()) {
//now do stuff
}
Maybe onPause() is called but onCreate() is called right after ?
When you turn off the device, the lock screen is displayed. This typically forces the display to a particular orientation. If the screen is not currently in that orientation, it will need to be changed, and the top activity's configuration appropriately changed to match.
You can add config changes like this:
android:configChanges="keyboardHidden|orientation" android:configChanges="keyboardHidden|screensize"
To make sure this doesn't happen.

cancel the events of lock/unlock on my android application

When i unlock my device to see my application, the process in onResume() is launched. How do I cancel the events of unlock, to avoid onResume() from processing?

OnResume is always called when your Activity was in background (e.g. other App, Lockscreen, Homescreen... is shown).
Look at the Activity Life Cycle to check if you can move your code from onResume() maybe to onStart() to fix your issue.

To prevent unlock your device use below flag in your activity.
WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON
Code
getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
Note :
As stated on the Activity lifecycle docs, onCreate and onResume will always both be called the first time an Activity is started. When going back to Activity, at least onResume will be called

Strange lifecycle behavior in my Screen Saver App

I just developed a screen saver app and I found a strange behavior in its lifecycle.
My work flow is like this:
start my RegisterService, where I call registerReceiver method to register a BroadcastReceiver, which can receive ACTION_SCREEN_OFF.
2.In the onReceive method of this BroadcastReceiver, I start an activity as the screensaver.
3.In the activity, I write Log.i() statement to track its running.
My question is:
When the screen times out, or when I press the POWER key, the screen turns off, and the system will send ACTION_SCREEN_OFF message. As I expect, my receiver starts the screen saver activity. However, I find this Activity calls onCreate(), onResume(), onPause(), onResume() sequentially according to the output in logcat.
It seems as if some a activity comes at front of my screensaver and finishes immediately, so my screensaver calls onPause() and then onResume().
Any idea? This problem handicaps me in programming, please help. Thanks!

Well based on a brief study of the PowerManagerService.java source code, when it's time to turn the screen off, the system initiates an animation (look at line 2183 for the class source) to do that. That means that your activity will pause and then will resume after the animation has ended.
I cannot be 100% sure for this, since I haven't tested it in my environment but this is the only logical explanation I found for your situation.
Hope this helps...

I can recommend you something very easy that might work for you, if you do not want the pause behavior why don't you try to Override the method onPause() and just do nothing :P don't call super.onPause() and that will terminate the behavior of it.
Other thing that might work for you, declare a static variable, add 1 on the "onResume()" method and return to "0" when "onStop()" is called. now just evaluate when the "onResume()" is called and if the variable is "0" then is the first time, anything else do nothing.
I hope this helps cause there is no much information on your question to be more specific.

How to handle Android activity lifecycle on wake up (alarm app)

I'm having a couple of problems with an alarm app I am developing.
The first thing that I think is a bit weird is that when an alarm goes of and wakes the phone up. These things happend.
oncreate
onresume
onpause
onresume
Why are they run in that order? The last two should not be called? And this is what's causes me big trouble, because when i press home or back on the phone, onPause is run, which I want to call finish() from. And that part works as it should, but that does not work when the phone wakes upp from sleep bacause of the onPause call...
Any ideas?

I also have a similar problem: i create an Activity and a Service in the background. When the service receive some data from the net it will bring-up the activity.
Everything works fine until the activity in in the onStop phase and the screen goes black.
When I have this situation and I request the activity to restart, I have:
onRestart
onStart
onResume
onPause
onNewIntent
onResume
As you can see I have an onResume, onPause and another onResume and the activity came to the user.
Seems that is because we use a singleTop or singleInstance Activity (maybe also appear on singleTask). The problem is that when you send an Intent to an already created singleTop Activity, your new intent active the onNewIntent() method. from the API you can see that:
An activity will always be paused
before receiving a new intent, so you
can count on onResume() being called
after this method.
so maybe this is why you receive onResume-onPause-onResume calls.
However this is a strange behavior!

I would suggest you look at how the official alarm application handles the events:
https://android.googlesource.com/platform/packages/apps/DeskClock

Do you really want to call finish() in onPause()? onPause() is called when the current activity is not in the foreground any more. Maybe you want to call finish() in your onStop() method instead. See the Activity lifecycle for details.