Develop Reference

Navigation Component "Back" via device reloads Fragment

The two screens relevant to my issue amount to MainFragment (a master view) and ProfileFragment (a detail view). The launch path from login -> main -> profile works fine. The return path isn't working as well. I get stranded at main when navigating backward with via gesture or device button.
On the initial "back" from the profile, I'm returned to main and see main's onViewCreated, onResume lifecycles. I expect back from main would return to login or even exit the app, but instead it reloads main a dozen times before crashing.
For any subsequent "back" from main, the logs reveal main's onAttach, onCreate, onViewCreated, onResume lifecycles recurring each time. This seems suspicious although it's a pretty vanilla navigation graph, a single Activity hosting four basic Fragments. They're all navigated with basic actions using their generated, type-safe Directions classes. I haven't overridden onBackPressed in any of these classes.
Combinations of app:popUpTo="#+id/loginFragment" and/or app:popUpToInclusive="true" haven't helped.
Do I need to implement back or "up" in order to work as expected? Any thoughts are appreciated.

On closer inspection, it's working the way it's coded. Of course it is!
The preceding LoginFragment checks a SharedPreference value and navigates to main. I confirmed that happens so quickly I thought I never returned to login screen, when in fact I did.
I'll investigate removing login screen from the stack after authentication.

How to return to the same activity instance after resuming it?

The problem is somewhat odd and after having trying to figure it out for about a day now, I am posting it here.
I have an application where an activity A(main activity) launches other activities(B,C or D).
The issue here happens when activity A has started Activity B and 'home' button is pressed.
Case 1 - When I test my application in debug mode on my device (HTC Desire) after pressing the 'home' button, I again click the application icon, it returns to the same activity (activity B), which is what is should do. No issues here.
Case 2 - When I export the signed package, and then install the application on the same device, then if I click the application icon after pressing the 'home' button, then a new instance of activity A (main activity) is launched ON TOP of activity B. I got to know this because when I press 'back' from that activity, it returns to activity B and pressing 'back' again shown activity A.
The behavior ceases to exist if the application is quit in the same order it was started, that is, if I press 'back' from activity B, then 'back' from activity A (exit).
After this everything runs fine.
I have tested it many times with different settings but I can't seem to figure out why the behavior is like this.
Any help is appreciated.

I think giving Activity A the 'single top' flag in your manifest should fix this.

Regarding Case 1:
When launching your intent from Activity A to start Activity B, add the flag FLAG_ACTIVITY_CLEAR_WHEN_TASK_RESET
This will ensure that when you go home and launch the the app again, Activity A will be shown.
Regarding Case 2:
I'm not exactly sure how this would occur. It seems like it thinks you have two versions of the app, the signed one and the unsigned one but keeps them both in the same task stack. You may want to consider using singleTask or singleInstance for your Activity if you only want one instance. See the doc on tasks and back stack for more details.

I would agree with Noel regarding the likely cause of Case 2. Without task reparenting or it being set to a launchmode preventing multiple instances of an activity, there is a chance that launching it from Home isn't deemed the same stack as launching it from Eclipse (assuming this to be the case).
In my talent calculator app I have the whole application set allowTaskReparenting=true to ensure nothing is left in other stacks (primarily email as it can email launch urls). I then have my main activity set to launchMode="singleTask" as I only ever want one instance of this to exist no matter what launches it or with whatever intent.
My only other activity is for loading and saving and that has noHistory="true" to make sure it is removed and never returned to. That basically means it only exists while you're in it, and can never return to it.
clearTaskOnLaunch="true" will also ensure only the main Activity remains in the stack when it's launched from Home, but this isn't always the case if you have other ways to get into your activity. If it's only ever launched from Home then set this.
Hope that all helps.

Do you start you application manually or using Eclipse or another IDE? When starting from Intellij IDEA I had exactly the same problems. Then I stopped and ran it manually and behaviour was as expected.

startActivity doesn't work from a Fragment in HoneyComb

The onCreate() method is called but the new Activity never appears. No errors are logged.
Follow up - There was no problem with calling startActivty() from a Fragment (we had a bug in the second Activity that caused it to exit immediately).
startActivity() from a Fragment works exactly like startActivty() from outside a Fragment.

A fragment should not be calling startActivity(), IMHO. A fragment should be telling its activity to display something, and the activity should be deciding how to do that. In some devices, that might be launching another activity. In some devices, that might be by adding a fragment. That decision should be made at the activity level, as it is the activities that are deciding what fragments go in what activities, based upon screen size.
Here is a sample application where a click on an item in a ListFragment causes either a separate DetailsFragment to be updated (for large/xlarge screens) or pops a new activity (for normal screens).

The problem was a bug in the second Activity that caused it to exit immediately. So to answer my own question, there is no problem with calling startActivity from a Fragment. It works exactly like calling startActivity from outside Fragment.

Does pressing Back always cause Activity to finish()?

I've heard that pressing the back button will essentially cause the current Activity to finish(). Is this always the case? Seems like it would be with the way it pops the Activity off the stack.
The one situation I'm not so sure about is when the root Activity in a Task has back pressed. I'm currently experiencing a very weird effect, described as follows:
On loading my application, the first Activity is for initialization, and once it finishes, it calls my main Activity (a TabActivity). This first init activity has android:noHistory="true" set in the Manifest so pressing Back from my main Activity won't go back to that. It goes to the Launcher. When I click on my App in the Launcher a second time, the initialization activity loads again, and loads the main Activity when done. Almost immediately after, it loads a second instance of my main Activity. But ONLY after the Application has already been run once, and was exited by pressing BACK from the main Activity. It does it every subsequent time until I force quit the app or load a new version from the IDE.
Based on this, I am suspecting some kind of Activity instance is lying around and being reused, since it only happens on the second+ time I run the application (and exit with BACK -- using HOME just returns to the last state of the app, no big deal). Anyone have any thoughts??

I've heard that pressing the back button will essentially cause the current Activity to finish(). Is this always the case?
No it is not. The most activities have this behaviour but not all. For example you could create a Dialog and set it setCancelable(false) and it won't close if you click BACK button.
Furthermore you could customize activity behaviour on BACK button pressed by overriding onBackPressed
Called when the activity has detected the user's press of the back key. The default implementation simply finishes the current activity, but you can override this to do whatever you want.
About your application behaviour..Did you verify if the activity launcher is finished after it loads your main activity? I mean if the onDestroy() method is called. Maybe after it runs the main activity it remains there and when you click back you just go back to the old Launcher...
hope this helps..

Read through the Activity and Task design guidelines on the Android developer site; they explain how the Home and Back buttons work. Obviously, if you override the default behavior (as mentioned by hara above), the back button will not finish the activity.
On your specific issue, check your logcat. You should be able to see there whether it is bringing an old process back to life or starting up a new one. If that is unclear, insert a couple of log statements into onCreate, onPause, onDestroyed, etc., so that you can see exactly what is happening with your process.

You can control BACK-BUTTON by writing the following code.
public boolean onKeyDown(int keyCode, KeyEvent event)
{
if (keyCode == KeyEvent.KEYCODE_BACK ) {
//preventing default implementation previous to
//android.os.Build.VERSION_CODES.ECLAIR
return false;
}
return super.onKeyDown(keyCode, event);
}

Are you running your activities with any special flags, such as singleInstance or singleTop? Those could be causing the oddities you're seeing. The easiest way to track down what's causing your problem is to absolutely fill it with debugging messages. For example:
In your initialisation activity, add a log in the beginning of onCreate to get the name of the activity such as this.toString(). More on why you want this line later.
When it launches the main tabbed activity, get the name of the launching activity and a message saying it's launched the tabbed one.
Override the onPause(), onStop() and onDestroy() callbacks and add debugging lines with this.toString() and also a message telling you which callback it is.
What this will do is tell you whether you've got multiple instances of the initialisation activity lying around. To this by comparing the name of the activities calling your main activity with the ones that were just created and the ones that went through to onDestroy.
If you don't know how to debug, use Log.d(LOG_TAG, "Your message here");. And then define a constant LOG_TAG String somewhere. After that, show the LogCat perspective in Eclispe by going to Window, show perspective (or view, don't remember exactly), other, Android, LogCat. The purpose of having a LOG_TAG constant is that you can set up LogCat to filter to that String and only show you those messages. It will make it easier to see them among the mass of system log messages.

The short answer to the original question is 'no'. This is largely because, unfortunately, not every developer follows the guidelines referenced by previous answers.
Yet the guidleines themselves mention exceptions, when the Back key should not call finish(). the most prominent exception is the Web browser, which has its own "back stack" for each window, so it must have its own custom handling of the Back key.

If there are no fragments on the back stack and a developer has not overridden onBackPressed, the activity will finish when the back button is pressed.
Here is the source code for Android 4.4.2 Activity.onBackPressed():
public void onBackPressed() {
if (!mFragments.popBackStackImmediate()) {
finish();
}
}

just override onbackpressed().. on back press this method will get execute remove super and do what u want to do.

Weird Home button behavior

I'm experiencing kind of strange behavior of my application after hard Home button is pressed.
When you press Home, everything is OK - my app goes to the background, showing Home screen. But if you try to choose my app in the main menu or in the list of last tasks it behaves like it was not started before and does not show the last activity you were on - it just starts from scratch, namely, shows the splash screen and starts next corresponding activities. Moreover, old activities of this app remain on the activities stack, and previous instance of the app is not terminated - so if you press Back for a few times you'll just run into those activities which were undoubtedly started during the previous session of work with my app. Splash screen activity is filtered by "android.intent.action.MAIN" filter and "android.intent.category.LAUNCHER" category.
The strange thing is that all of that happens despite the fact that I do not intercept any Back key hits, or override any onPause or onResume methods. What's happening contradicts with my understanding of Android app lifecycle - I was sure that when you hit Home an app just goes to the background, and when you choose it in the menu later - it just unwinds and does not start anew. (Of course, unless stuff like that is stated in the app manifest or corresponding methods are overridden or something else).
I also checked it for some other lifecycle events - such as changing orientation or flipping hard keyboard out - and none of those led to such strange results. It appears that the problem occurs when you try to start the app from main menu or menu of last applications.
I hope you will be able to help me. Any advice on what to pay attention to or where to search for solution would be really great.
Regards, Alex

You need to set android:launchMode="singleTask" in your LAUNCHER activity in your manifest file.
For more info on the launchMode attribute see here
Note that:
The default mode is "standard".
and:
Every
time there's new intent for a
"standard" activity, a new instance of
the class is created to respond to
that intent.