I have an Activity with a lot of fragments transactions in it, and sometimes my users need to open the preferences Activity for a few seconds. I do it like this :
Intent preferencesIntent = new Intent(this, PreferencesView.class);
this.startActivity(preferencesIntent);
Each time I do this, onDestroy() of my main Activity is called. So when my user are coming back, the main Activity is in it initial stage. But I would like the main Activity to be like when they leave it.
I know that I should save/restore my data and just let the Activity recreate itself. But this would be very complicated, and my users are in my main Activity most of the time.
So is there a way to tell Android not to kill my main Activity while it is not visible?
Thank you in advance!
You've alreay written the answer: save/restore state. This is how android works.
Your activity can be destroyed configuration change or something else.
You can't prevent Android to stop an Activity.
The only way I see is to load the preferences in a Fragment in the main Activity.
Related
When the app is started,in onCreate of MainActivity, some loading data from sharedpreferences and some numerical computations are done. But it is not necessary doing this every time it's coming back from others activities in the app. How can I avoid this?
onCreate is called when a new activity is created for the first time. If you navigate to another activity and come back to previous one then onCreate will NOT be called instead onResume will be called. Have a look at the documentation explaining the lifecycle of an activity
https://developer.android.com/reference/android/app/Activity
To avoid OnDestory() and OnCreate() beeing called all the time when navigating between my main activity and some sub activities, I have set the singleTop option in my manifest. Unfortunatly this causes that the UI in my main activity is gone when returning from a sub activity. Do I really need to redraw my UI manually?
I am still wondering how to handle these basic navigation features. Is it unusual to perform application initialization tasks (e.g. start services) in OnCreate() of the main activity?
I must be missing something from your explanation I guess because the very basic point that you provided is not correct. When you launch another (sub) activity from your activity then your starting activity is not destroyed but it is stopped. So your onStart is called when you navigate back.
So maybe you can explain a little bit more the background of your requirements.
Use android:launchMode="singleTask" for your main activity so the system will not re-create it if you call it via Intent. You can also clear the stack and go back to your main activity by using
Intent intent = new Intent(SubActivity.this, MainActivity.class);
intentHome.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
intentHome.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP);
startActivity(intentHome);
I have Activity A and I am calling Activity B from Activity A using setResultForActivity.
Now in Activity B when I press Done button I am firing finish() and it returns to
Activity A and it return down to onActivityResult. Now the issue is after when I fired finish() in Activity B , Activity A's onCreate doesn't get called and thats why
some of the custom listeners in my ListView isn't working , it seems that they are not bind.
so the whole activity respond pretty weirdly , can anyone has solution to this ?
Why a fourth answer? Because in my view, the others aren't fully correct.
Truth is, Activity A could have been destroyed in the meantime, or not. This depends on whether Android needs memory or not. So it is possible that Activity A´s onCreate() is called (along with the other lifecycle callbacks), or not. In the latter case, onActivityResult() is called before onResume().
While for configuration changes, the most efficient way to preserve the Activity's state is via nonConfigurationState, if you want to prepare for a restart of your Activity after it has been destroyed, you can use the InstanceState mechanism, because while Android destroys your Activity A, it will keep its intent and saved instance state to re-crearte it.
This stresses the absolute necessity to place initialization exactly in the callback where it belongs.
To test whether your Activity logic works regardless of whether Android destroyed it or not, you can use the DevTools setting "Development Settings" -> "Immediately destroy activities". The DevTools app is available on AVDs and can also be downloaded from Google Play.
Just place your onCreate() stuff in onResume() of Activity A except setContentView().
Just have a read on Android Activity Lifecycle : http://developer.android.com/training/basics/activity-lifecycle/stopping.html. onCreate() is only called when the activity is first created. You can do your list thing in the onResume() method.
Activity A's onCreate won't get called because the activity has not been destroyed. When an Activity regains focus from another activity, it's onStart and onResume get called, so I would put your bound listeners in those. They will also be called when onCreate is normally called.
After your child activity is finished() it return to execute onActivityResult which is in your case in Activity A. The onCreate method is not supposed and does not get called when killing of you sub-activity, a.k.a Activity B.
Please post some source code for us to work on and I will improve my answer! :)
I have 2 activities. Main Activity A & Activity B
I do not want Activity A to destroy. I am starting Activity B in a new task.
public static void startActivity(Class<?> startClass) {
Intent intent = new Intent(Constants.getActivity(), startClass);
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
Constants.getActivity().startActivity(intent);
}`
Constants.getActivity() returns the Context on current activity
startClass is the either activity "A" or activity "B"
Thing is they create/destroy the activities and they leak. Am I doing it wrong? How can I start activity "B" from activity "A" and vice versa keep them both in background when I dont need them.
First of all, what are you trying to do? You should always separate things you want to do in the background from your UI. Think of your Activities are simply a container to show the UI, everything else can be stored and restored from persistent storage or savedinstance bundles.
It is very crucial that you understand the difference between Activity lifecycle and Service lifecycle here.
I'm going to refer to my answer from another question here:
Your app will be in the background, onResume and onPause will both be called, provided that the OS have enough memory to keep the new app and all the old apps.
If you have a long running process that you need while the user not looking at it, use a service.
If you need the user to return the app in the same state, you need to do the work in onResume and onPause to save the state information and initialize your UI again when the user comes back. If you are really worried that it can get killed (well, it shouldn't lose the bundle I think), you can store them in SharePreferences for your app.
If you want to know when the app returns from that specific share intent, use startActivityForResult
You cannot keep an activity "alive" as you said. When an activity is paused, Android system is always able to claim its memory and unload it, at any time.
But if you want to switch from A to B, then maybe A and B can be different views inside a single activity. Maybe you'll want to have a look at http://developer.android.com/reference/android/widget/ViewFlipper.html
When you use tasks, cleanup is very important. You need to cleanup all tasks in the activity. readthis
If the activity does not have a lot of crazy initialization, just use finish and onCreates. Else be aware that onResume will be called often as you switch between activity's. Cleanup will be crucial here. If you dont cleanup, its possible one of your activities (with dead object reference from the other cleaned up activity) may come back up from the activity stack and throw exceptions. Its very difficult to debug this kinda exception.
I am making an application in which i want to let the control go to some activity and then to come back. When I have searched about that problem, I came to know that this problem could be solved by using OnPause() and OnResume(), but I don't know how to use them.
This is the code where I am transferring the control to some other activity. Please tell me how to use OnPause() and OnResume() in this regard.Thanks in advance.
Intent intent = new Intent(MCQ.this,ConfigActivity.class);
int ClassIndex = 2;
intent.putExtra("ClassIndex", ClassIndex);
startActivity(intent);
You are not making too much sense. When you call startActivity(), Android will put your current Activity to the background, and start (or bring to foreground) the new Activity specified in your Intent.
onPause() called when your Activity disappears from the screen, eg. the system either puts it in the background, or terminates it.
onResume() is called when your Activity becomes visible on the screen, eg. either after onCreate() if Android created a new instance of your Activity, or when the system brings your Activity back from the background.
There is no explicit passing of control between Activities. If it is on the top of the activity stack (visible on the device screen), than you can interact with it.
http://developer.android.com/reference/android/app/Activity.html#ActivityLifecycle
onPause() and onResume() are called by android.
Your job is to implement these methods. onPause and onStop are called at your launching activity by Android, and onCreate, onStart and onResume at the Activity your starting. If you want to go back, you usually just press the back button at your device.
You might want to take a look at the Android devGuide and especially at the Activity lifecycle.
I think you may be confused as to what onPause() and onResume() are used for.
onResume() is called when the Activity is about to show it's view to the user. You can use your intent in this method so the main Activity would instantly go to the ClassIndex activity. However, when you leave the ClassIndex activity, onResume() will be called again which will send you back to ClassIndex.
onPause() is called when the Activity is leaving. Whether this is for locking the screen or moving to another Activity. In this case, when you call startActivity in the main Activity, Android will call onPause() in the main activity before entering the new one.
What you may want in this case is to use startActivityForResult() and use it to determine if you're coming back from your ClassIndex activity or not. If not, then start it. If so, then move on to the main activity.
A better solution may-or-may-not be to start the app in ClassIndex and move to your main activity when the user is done with it. This would be the case if you want to move to ClassIndex every time the user enters the app.