Develop Reference

Activity backstack handling

Suppose I have activities A, B, C, D, E.
when I open application with activity A and switch to activity B and further to activity C. Suppose, somehow B gets destroyed by system. and when I press back button, I came out of app. What I really wanted was to be able to go to B if it exists otherwise A (landing page). In normal cases it's working smoothly but it's some cases where I have to interact with other apps/interfaces for eg. open url in browser. After sometime if I go back to app, I see latest page as it is but when I press back button app exits.
I have searched for solutions but couldn't find one so posting it as a question.
I know the idea of passing extra in intent and starting new intent on back press but these would not work in some cases or requires creating a backstack handler of own.

There is no guarantee that your activities will stay in memory and this is an intentional behaviour.
Your option is use to onRestoreInstanceState and onSaveInstanceState so that user do not lose critical data.
#Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
// Get data
super.onRestoreInstanceState(savedInstanceState);
}
#Override
protected void onSaveInstanceState(Bundle outState) {
// Save data
super.onSaveInstanceState(outState);
}

One way which I have used in the past to validate activity state is to add e.g. a boolean attribute in your activity, set this true when activity starts, if your activity is destroyed after some time this will be false. Check state by overriding the activities onBackPressed method?
e.g.
onBackPressed(){
if status{
/*activity live*/
}
else{
/*activity destroyed/*
}
}
This might work If I understand you issue correctly.

Dealing with SavedInstances, and restoration of activity

basically my app has 2 activities.Say "A" and "B"
.
A launches B.
Activity B plays music and also has a notification.
Case 1:when the view is still on activity B` i press home button and then i click on the the notification, activity B is opened with its view intact and with its music playing (because in the manifest i am using android:launchMode= "singleTop" and hence another instance of the activity is not created) this part is as desired......
but
Case 2:when the view is on activity B and i press back button ,activity A appears and then i click on the the notification, activity B is opened with the view lost and music also stops(not desired)......i am making a guess that it happens because when i press the back button the activity is destroyed ,so i have to progamatically restore its view right??so to restore its view i override two methods .....
#Override
public void onSaveInstanceState(Bundle savedInstanceState) {
// Save UI state changes to the savedInstanceState.
// This bundle will be passed to onCreate if the process is
// killed and restarted.
savedInstanceState.putBoolean("boolMusicPlaying", boolMusicPlaying);
savedInstanceState.putInt("swapnumber", swapnumber);
savedInstanceState.putString("seekbarprogress", progress2);
savedInstanceState.putInt("position.seekbar",seekbar.getProgress());
savedInstanceState.putString("seekmaxString", max2);
savedInstanceState.putInt("seekmaxInt",seekMax);
savedInstanceState.putParcelableArrayList("songfetails",songdetails);
super.onSaveInstanceState(savedInstanceState);
}
//make a note ....even if i don't override onDestroy() and don't call on SaveInstanceState explicitly, then too i am not getting any desired effect.......
#Override
public void onDestroy()
{ Bundle savedState = new Bundle();
onSaveInstanceState(savedState);//because of this line....the app is crashing......
super.onDestroy();
}
but it didn't help.....
and in on create i am putting a check if saved instances is null or not....create view accordingly...(i.e from saved instances or from fresh) but it didn't help...
also this line is giving a crash ...onSaveInstanceState(savedState);
even if i don't override ondestroy() and kill the app manually from task killer,and then try to open activity B,then too the saved instances thing should work right,because the method OnSaveInstanceState will be automatically called then,right???please help

Basically if you have pressed a back button,restoration of activity should be done using shared preferences/or a database but if you haven't pressed the back button and then you want to restore the state of the activity (because the activity was destroyed by the system ) then the bundle savedinstances can be used ...

From Android documentation:
When your activity is destroyed because the user presses Back or the activity finishes itself, the system's concept of that Activity instance is gone forever because the behavior indicates the activity is no longer needed. However, if the system destroys the activity due to system constraints (rather than normal app behavior), then although the actual Activity instance is gone, the system remembers that it existed such that if the user navigates back to it, the system creates a new instance of the activity using a set of saved data that describes the state of the activity when it was destroyed. The saved data that the system uses to restore the previous state is called the "instance state" and is a collection of key-value pairs stored in a Bundle object.
To fix second problem you must restore data form some other source (service that plays music, restore info from application context or singleton, etc.)
For more info check this.

Android: onSaveInstanceState in Back button

I am developing an application in which i am overriding the back button. I created a check box. On click of which i am calling intent for:
startActivityforResult();
And also maintaining the state of activity as :
#Override
public void onSaveInstanceState(Bundle savedInstanceState) {
super.onSaveInstanceState(savedInstanceState);
savedInstanceState.putBoolean("checkbox", checkbox.isChecked());
}
#Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
super.onRestoreInstanceState(savedInstanceState);
checkbox_state = savedInstanceState.getBoolean("checkbox");
}
which run fine and the state is maintained.
Means i am entering value in edit text. and on check box click calling new activity for result and while return on first activity the state is maintain.
Now but from second activity if i click on device back button the state is not maintained.
So what should i do to maintain the state on back button. I searched but not found satisfied solution. Please suggest me.

When You start the new Activity then the current Activity gets hidden and new Activity is placed on top of the stack. Now if you press the back button on the new Activity then the first activity should come up in its current state (If it wasn't destroyed, calling finish()) where it was left i.e. if the check box was checked then it should remain checked. You don't need to save the activity state unless the orientation is changed or the activity is destroyed.
Are you sure you are not doing any thing in the onActivityResult or onResume() method which effects the state of the check box? I would recommend to first comment out all the code in both the methods and see if your check box retains the state. Also can you also make sure that the code itself doesn't uncheck the checkbox before starting the new Activity?

Now but from second activity if i click on device back button the state is not maintained.
onSaveInstanceState() is mostly used for configuration changes (e.g., rotating the screen).
So what should i do to maintain the state on back button
Most likely, there is no "state" that needs to be "maintained", outside of your data model. Your activity needs to update its data model: files, database, preferences, ContentProvider, some singleton that is your in-memory data model manager, whatever.

when is onRestoreInstanceState called?

Sorry for my incomprehension, but I am new in the android development.
I have an application with activity A and activity B in it, and I go from activity A to activity B. When I left activity A, the onSaveInstanceState method was called, but when I went back to activity A (from activity B in the same application), the bundle in the onCreate method was null.
What can I do, to save the activity A's previous state? I only want to store the data for the application lifetime.
Can someone help me with this?
Here is my code for Activity A:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (savedInstanceState != null)
{
Log.v("Main", savedInstanceState.getString("test"));
}
else
{
Log.v("Main", "old instance");
}
}
#Override
public void onSaveInstanceState(Bundle savedInstanceState)
{
Log.v("Main", "save instance");
savedInstanceState.putString("test", "my test string");
super.onSaveInstanceState(savedInstanceState);
}
public void buttonClick(View view)
{
Intent intent = new Intent(this, Activity2.class);
startActivity(intent);
}
Here is my code for Activity B, when I press a button to go back to activity A:
public void onBack(View view)
{
NavUtils.navigateUpFromSameTask(this);
}

To answer your question, have a look at the android doc: https://developer.android.com/reference/android/app/Activity.html#onRestoreInstanceState(android.os.Bundle)
It says that onRestoreInstanceState is called after onStart() method in the activity lifecycle.

Saving and restoring state is meant to save the current temporary data that is obsolete when user exits the application.
When you minimize or leave the Activity by opening next one it might be killed by the system due to lack of resources and restarted with savedInstanceState when you get back to it. So use onSaveInstanceState() only for saving minimize-restore session data or data that should be preserved on rotation.
So if you start a new Activity in front and get back to the previous one (what you are trying to do), the Activity A might not be killed (just stopped) and restarted without going being destroyed. You can force killing it and restoring by checking Don't keep activities in developer options menu.
If you call finish() or remove the Activity from recent task list the savedInstanceState will not be passed to onCreate() since the task was cleared.
If the value must be persistent consider using SharedPreferences.

This happens because you are navigating the hard way. If you used the phone's default back button navigation, your onCreate would get the bundle passed in.
To circumvent this issue, I suggest you save your state to shared preferences as well as a back up. When the bundle is null, restore it from the shared preferences.

reference
onSaveInstanceState
... onPause()-> onSaveInstanceState() -> onStop() -> onDestory()
onRestoreInstanceState
onCreate()-> onStart()-> onRestoreInstanceState()-> onPostCreate(Bundle) ...
Or You can use LiveData. Save the states in it and observe.If the device rotates it'll update the views accordingly.

After onStart() which is after onCreate()

I used in this case a flag and SharedPreferences. This should work, and when you change the screen orientation.

using onResume and onPause

Suppose I have two activities, activity1 and activity2.
I want to navigate from activity1 to activity2, get some info from activity2 and insert back it to activity1 and also I don't want to lose activity1 previous state as I left.
how can I save its state ?

what you are describing is the perfect classic reason to use the Activity.startActivityForResult() method.
this what google wrote in this method documentation:
Launch an activity for which you would like a result when it finished. When this activity exits, your onActivityResult() method will be called with the given requestCode
so what you should do is: from your activity1 start activity for result, and from activity2 use the setResult(int resultCode, Intent data) method with the data you want your activity1 to get back, and call finish() (it will get back to onActivityResult() in the same state activity1 was before..).

Override onSaveInstanceState(Bundle) in activity1 to save whatever data you want then override onRestoreInstanceState(Bundle) in the same activity to get the values back. Using Bundle, you can store pretty much any data that you want. I'd recommend something like this:
public class MainActivity extends Activity {
...
public static final String DATA1_KEY = "data1";
private boolean value1;
...
#Override
protected void onSaveInstanceState (Bundle outState) {
outState.putBoolean(DATA1_KEY, value1);
}
#Override
protected void onRestoreInstanceState (Bundle savedInstanceState) {
value1 = savedInstanceState.getBoolean(DATA1_KEY);
}
}

You should override onSaveInstanceState(Bundle savedInstanceState)
Please check this answer for example code
Or use SharedPreferences. Check this code

If you want to keep your data alive only at runtime, the consider using static members. Then you can access and manipulate these members from any activites. for example:
public class FirstActivity extends Activity
{
public static String data = null;
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(...);
data = "This is a test!";
}
}
From your second activity you can access these static variables like
public class SecondActivity extends Activity
{
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(...);
if(FirstActivity.data!=null)
{
//You can use it:
System.out.println(FirstActivity.data);
}
}
}
Of course you can add getter/setter functions to make it safer and more elegant.
If you want to store them for a longer time, the please consider using:
SharedPreferences
SQLite
or store your data in an online database.

The user performs an action in your app that starts a new activity. The current activity is stopped when the second activity is created. If the user then presses the Back button, the first activity is restarted.
When your activity is stopped, the Activity object is kept resident in memory and is recalled when the activity resumes. You don’t need to re-initialize components that were created during any of the callback methods leading up to the Resumed state. The system also keeps track of the current state for each View in the layout, so if the user entered text into an EditText widget, that content is retained so you don't need to save and restore it.
Note: Even if the system destroys your activity while it's stopped, it still retains the state of the View objects (such as text in an EditText) in a Bundle (a blob of key-value pairs) and restores them if the user navigates back to the same instance of the activity.
CommonsWare here says:
When user "press the BACK button", then the Bundle from
onSaveInstanceState() (if any) is discarded, as the user has indicated
they are finished with the activity. The onSaveInstanceState() Bundle
is used in cases where the user has not said they are finished with
the activity (e.g., they accepted an incoming phone call) yet Android
elects to destroy the activity to free up RAM.
And documentation says:
Note that it is important to save persistent data in onPause() instead
of onSaveInstanceState(Bundle) because the later is not part of the
lifecycle callbacks, so will not be called in every situation as
described in its documentation.
In other words, put your save/restore code for non View objects in onPause() and onResume() instead of onSaveInstanceState(Bundle) and onRestoreInstanceState(Bundle).Finally I guess that you don't need to save any state if you only have View objects and if you have any other states you can use preferences,file or sqlite to save them in onPause() and retreive them in onResume().
You can see more details in these pages:
Stopping and Restarting an Activity
Saving Activity state in Android
Android wont save the current state of an activity