Develop Reference

Confusion between onPause() onStop() onResume()

I am developing an Android app in which I want to check if the user has minimized the application or just come from another activity.
In detail, if the user have started another app, went to the home screen or locked the screen, I want to show the activity where the user will enter the password to access the app. But where or how to check this exactly?
https://developer.android.com/guide/components/activities/activity-lifecycle.html
I was trying onResume() but according to documentation onResume() can be fired if the user’s navigating to another activity and coming back.

I'm not very clear on what you are trying to achieve.
The life cycle diagram is quite clear if you are wondering which lifecycle method it would hit when something happens.
Basically, it's the same to minimise the app and go to another activity. But if you are referring to coming from another activity in your own app, you can distinguish your own activity by adding extra information to the intent you use.
Basically, it's like this:
Intent intent = new Intent(FirstActivity.this,SecondActivity.class);
intent.putExtra(key,value);
startActivity(intent);
And in your SecondActivity, you can always retrieve that data like this:
Bundle bundle = getIntent().getExtras();
if ( bundle != null && bundle.containsKey(key) ) {
value = bundle.getInt(key); // not nessecarily getInt(), you should use according to your value type
// use the value to tell if it is from your own app
} else {
// it is not from your own app
}
You can use this mechanism combined with the lifecycle methods. For example, if you use the latter code in your onCreate() method, then whenever the Activity is created, if will check who creates it, which sounds like your what you might want.

As soon as your activity becomes visible it will call OnStart() and as soon as it is ready for the interaction(such as touch ,click etc event). it calls onResume, at this stage your app is running and it is completely in foreground. When your activity start another activity or a dialog box then it calls onPause it means activity is visible but user can not interact with the Activity UI. in case we start another Activity which completely hides the previous activity then its onStop method is called

onPause: Called when another activity comes into the foreground.
onStop: Called when that other activity is completely visible.
onResume: Called when your activity is navigated back to from the onPause state.
Maybe your app was already in the onStop state, so then it would call onRestart.

Kill Overlapping Threads from Intent in Android mp3 app

I am having a problem (I think) with overlapping threads playing music in my android mp3 player. I have a class SongList.java that lists and plays music when an item is clicked and a ArtistList.java that intents SongList to play songs by that specific artist. Two or more songs will play at the same time if they are not in the same activity.
How do I tell my playSong method to stop all threads but the most recent one?

You should have just one single Thread or service doing playback.
SongList and ArtistList should not start a new service. Rather you should have a PlaybackService you can pass a SongList or ArtistList as parameter / argument. If doing so, you should just have one single Thread doing playback and never run into that kind of trouble.
Have a look here:
http://developer.android.com/guide/topics/media/mediaplayer.html#mpandservices
They implement a Service that uses the build in MediaPlayer for audio playback. Next you can bind your MusicPlaybackService to an activity.
Here is an introduction:
http://developer.android.com/guide/components/bound-services.html
You can specify methods that the activity can invoke on your service with the help of Binder. So you could provide a method like playSongs(SongList songs) or playArtists(ArtistList list) that could be invoked by the activity (i.e. by clicking on a certain button). Since the service runs the playback (by using MediaPlayer) the service is responsible for Threading and playback. So if you call playSongs(SongList songs) of the service, the Service should stop MediaPlayer and restart MediaPlayer with the desired music file (provided by SongList). With this approach your MusicPlaybackService manages the playback and guarantees that only one music file is played simultaneously.

Activity can't be resumed

Over the past months I have been developing a versatile real-time game engine, and I have learned a lot, but I still feel very naive when it comes to the application life cycle. Specifically, I am trying to implement an Activity which can be shuffled in to the background by the user, then properly resumed.
My current architecture is as such: I have a an XML menu launcher activity which can create a real-time Game activity using intent. Relevant data in this Game activity is referenced through static data structures and variables. The Game activity creates worker threads in the onSurfaceCreate() callback of my SurfaceView object.
When a user presses the back button, the activity is destroyed, and they are sent back to the XML menu in the launcher activity. Fine, good for now. When the user presses the home button, the Activity is sent to the background. OK, great. My problems arise when the user tries to find their way back in the Game activity once is has been sent to the background. When the user touches the launcher icon, the Game is destroyed and the menu is re-launched. Also, when the user resumes the game through the task manager, the onSurfaceCreate() callback fires and the worker threads are started, but the game is frozen.
So, I have two questions. First, how do I resume my activity through the launcher icon, instead of re-launching the game? Second, when I resume my activity, what actions are necessary to restart my worker threads while persisting the game data?
Thanks!
EDIT: By request, I am including some code below. My onSurfaceCreate is a little bit complicated, it sends a message to another thread, which then implements the callback. I have verified that this implementation function fires when the Game activity is resumed.
protected void surfaceCreate()
{
Log.e(TAG, "surfaceCreate");
Thread gameThread = createGameThread();
gameThread.start();
}
protected final void onResume()
{
super.onResume();
resume();
}
protected final void onPause()
{
super.onPause();
pause();
}
These cryptic pause() and resume() methods simply set a boolean variable which prevents some game logic from being executed, they do nothing to hinder the worker threads, which should continue looping.
EDIT: Thanks Mohammad, for solving my first (although smaller) problem. It turns out that the launcher icon behaves differently when not connected by USB to the IDE. The second problem remains unresolved.
EDIT: All working! The second problem turned out to be an issue unique to my code, I apologize for that. I hope that this question can still be useful for those dealing with the launcher and IDEs.

There are multiple possible problems/solutions listed in this post. This includes misunderstanding of activity lifecycle and launchMode settings.
First, how do I resume my activity through the launcher icon, instead
of re-launching the game?
You are most likely missing definition for your onResume() and onPause() methods. Examples are here:
http://developer.android.com/training/basics/activity-lifecycle/pausing.html
Straight from the API:
onPause() Called as part of the activity lifecycle when an
activity is going into the background, but has not (yet) been killed.
onResume() Called after onRestoreInstanceState(Bundle),
onRestart(), or onPause(), for your activity to start interacting with
the user.
Check the Android Activity Lifecycle out:
http://developer.android.com/reference/android/app/Activity.html
One theory on your issue may be is that you're most likely going to onStop() (by hitting the home button) and the Android OS is looking for your onRestart(), but can't find it hence freezing/restarting.
Second, when I resume my activity, what actions are necessary to
restart my worker threads while persisting the game data?
Careful how you use the word restart. You want to pause and resume the application (not restart). Save data in onPause() (use a database or any other save feature you'd like). Load data in onResume(). Although, the activity should resume as normal if you just fill up these methods.
Now, if you want to save state for when you restart the application, you should save states in onStop() and/or onDestroy(). You should load states in onStart(). In order to save states, you can check this out:
https://stackoverflow.com/a/151940/2498729
For those using Eclipse (or any other IDE to run/test your application):
From what have you described you probably have overridden
android:launchMode in AndroidManifest.xml or if you are testing by
"run as" from Eclipse try exiting the application after installing and
auto-starting. Then start again from the emulator and test the Home
button behavior. I suppose this is because Android does not put
Activities on the OS stack when started from Eclipse and then the Home
button behavior is not as usual. If this does not solve your problem,
try reading
http://developer.android.com/guide/topics/fundamentals.html#lmodes.
I had launchMode set in my StartupActivity. THen I removed that (it
was set to "singleTask", it behaves like I want it; the app is
"resumed" to the Activity I expect, ie not StartupActivity but
MainActivity.
Source:
https://stackoverflow.com/a/1619461/2498729
http://developer.android.com/guide/topics/manifest/activity-element.html
According to this:
https://stackoverflow.com/a/3002890/2498729
You should change your andoird:launchMode to "Standard" (or "default").
Standard: A > B > HOME > B (what you want)
SingleTask: A > B > HOME > A (what you don't want)

Start Activity on background

I have an app that responds to Internet message by creating a new Activity and user has 20 seconds to respond.
The problem is when the app is running on background.
I can show a notification, but when the user returns to the App the new Activity isn't started.
Is there any way to start an Activity even when the App isn't on foreground (without the activity getting focus) or any easy workaround where the activity would start right after returning to the app? (which would be worse solution, cause I would have to rework the sync timer :))
Thanks

Take a look at the Activity life cycle
Here you have the onResume() method you can override and do something before the Activity itself is shown. From there you could do some kind of check that you are returning from a notification or a check on that the user has to answer something now and launch a new Activity from the onResume() method.

How do I make a music player activity keep running?

I have a music player Activity in my app.
The problem is that, if the user switch to other Activity inside my app, the music keeps playing, but if later he returns to the music player Activity, everything resets(song's title, duration...) except the music itself.
Is there a way to make an activity keep running?(not by using a service).

Take a look here it might help you.
How to mute and unmute it on the onPause and onResume
Use the onPause() and onResume() for that.