How to save an activity state using save instance state

Question

I ve been working on the Android SDK platform  and it is a little unclear how to save an application s state  So given this minor re-tooling of the  Hello  Android  example   package com android hello   import android app Activity  import android os Bundle  import android widget TextView   public class HelloAndroid extends Activity      private TextView mTextView   null         Called when the activity is first created        Override   public void onCreate Bundle savedInstanceState        super onCreate savedInstanceState        mTextView   new TextView this        if  savedInstanceState    null           mTextView setText  Welcome to HelloAndroid           else          mTextView setText  Welcome back                setContentView mTextView           I thought it would be enough for the simplest case  but it always responds with the first message  no matter how I navigate away from the app   I m sure the solution is as simple as overriding onPause or something like that  but I ve been poking away in the documentation for 30 minutes or so and haven t found anything obvious

User · Answer

Here is a comment from Steve Moseley s answer  by ToolmakerSteve  that puts things into perspective  in the whole onSaveInstanceState vs onPause  east cost vs west cost saga       VVK - I partially disagree  Some ways of exiting an app don t trigger   onSaveInstanceState  oSIS   This limits the usefulness of oSIS  Its   worth supporting  for minimal OS resources  but if an app wants to   return the user to the state they were in  no matter how the app was   exited  it is necessary to use a persistent storage approach instead    I use onCreate to check for bundle  and if it is missing  then check   persistent storage  This centralizes the decision making  I can   recover from a crash  or back button exit or custom menu item Exit  or   get back to screen user was on many days later      ToolmakerSteve Sep   19  15 at 10 38

User · Answer

Simple quick to solve this problem is using IcePick  First  setup the library in app build gradle  repositories     maven  url  https   clojars org repo      dependencies     compile  frankiesardo icepick 3 2 0    provided  frankiesardo icepick-processor 3 2 0      Now  let s check this example below how to save state in Activity  public class ExampleActivity extends Activity      State String username     This will be automatically saved and restored     Override public void onCreate Bundle savedInstanceState        super onCreate savedInstanceState       Icepick restoreInstanceState this  savedInstanceState           Override public void onSaveInstanceState Bundle outState        super onSaveInstanceState outState       Icepick saveInstanceState this  outState           It works for Activities  Fragments or any object that needs to serialize its state on a Bundle  e g  mortar s ViewPresenters   Icepick can also generate the instance state code for custom Views   class CustomView extends View      State int selectedPosition     This will be automatically saved and restored     Override public Parcelable onSaveInstanceState         return Icepick saveInstanceState this  super onSaveInstanceState             Override public void onRestoreInstanceState Parcelable state        super onRestoreInstanceState Icepick restoreInstanceState this  state              You can put the calls to Icepick into a BaseCustomView and inherit from it      All Views extending this CustomView automatically have state saved restored

User · Answer

Really onSaveInstanceState   is called when the Activity goes to background   Quote from the docs   This method is called before an activity may be killed so that when it comes back some time in the future it can restore its state   Source

User · Answer

you can use the Live Data and View Model For Lifecycle Handel From JetPack  see this Reference    https   developer android com topic libraries architecture livedata

User · Answer

There is a way to make Android save the states without implementing any method  Just add this line to your Manifest in Activity declaration   android configChanges  orientation screenSize    It should look like this    lt activity     android name   activities MyActivity      android configChanges  orientation screenSize  gt   lt  activity gt    Here you can find more information about this property    It s recommended to let Android handle this for you than the manually handling

User · Answer

You need to override onSaveInstanceState Bundle savedInstanceState  and write the application state values you want to change to the Bundle parameter like this    Override public void onSaveInstanceState Bundle savedInstanceState      super onSaveInstanceState savedInstanceState        Save UI state changes to the savedInstanceState       This bundle will be passed to onCreate if the process is      killed and restarted    savedInstanceState putBoolean  MyBoolean   true     savedInstanceState putDouble  myDouble   1 9     savedInstanceState putInt  MyInt   1     savedInstanceState putString  MyString    Welcome back to Android         etc      The Bundle is essentially a way of storing a NVP   Name-Value Pair   map  and it will get passed in to onCreate   and also onRestoreInstanceState   where you would then extract the values from activity like this    Override public void onRestoreInstanceState Bundle savedInstanceState      super onRestoreInstanceState savedInstanceState        Restore UI state from the savedInstanceState       This bundle has also been passed to onCreate    boolean myBoolean   savedInstanceState getBoolean  MyBoolean      double myDouble   savedInstanceState getDouble  myDouble      int myInt   savedInstanceState getInt  MyInt      String myString   savedInstanceState getString  MyString        Or from a fragment    Override public void onViewStateRestored  Nullable Bundle savedInstanceState        super onViewStateRestored savedInstanceState          Restore UI state from the savedInstanceState         This bundle has also been passed to onCreate      boolean myBoolean   savedInstanceState getBoolean  MyBoolean        double myDouble   savedInstanceState getDouble  myDouble        int myInt   savedInstanceState getInt  MyInt        String myString   savedInstanceState getString  MyString        You would usually use this technique to store instance values for your application  selections  unsaved text  etc

User · Answer

This is a classic  gotcha  of Android development  There are two issues here    There is a subtle Android Framework bug which greatly complicates application stack management during development  at least on legacy versions  not entirely sure if when how it was fixed   I ll discuss this bug below  The  normal  or intended way to manage this issue is  itself  rather complicated with the duality of onPause onResume and onSaveInstanceState onRestoreInstanceState   Browsing across all these threads  I suspect that much of the time developers are talking about these two different issues simultaneously     hence all the confusion and reports of  this doesn t work for me    First  to clarify the  intended  behavior  onSaveInstance and onRestoreInstance are fragile and only for transient state  The intended usage  afaict  is to handle Activity recreation when the phone is rotated  orientation change   In other words  the intended usage is when your Activity is still logically  on top   but still must be reinstantiated by the system  The saved Bundle is not persisted outside of the process memory gc  so you cannot really rely on this if your activity goes to the background  Yes  perhaps your Activity s memory will survive its trip to the background and escape GC  but this is not reliable  nor is it predictable    So if you have a scenario where there is meaningful  user progress  or state that should be persisted between  launches  of your application  the guidance is to use onPause and onResume  You must choose and prepare a persistent store yourself   BUT - there is a very confusing bug which complicates all of this  Details are here   http   code google com p android issues detail id 2373  http   code google com p android issues detail id 5277  Basically  if your application is launched with the SingleTask flag  and then later on you launch it from the home screen or launcher menu  then that subsequent invocation will create a NEW task     you ll effectively have two different instances of your app inhabiting the same stack     which gets very strange very fast  This seems to happen when you launch your app during development  i e  from Eclipse or Intellij   so developers run into this a lot  But also through some of the app store update mechanisms  so it impacts your users as well    I battled through these threads for hours before I realized that my main issue was this bug  not the intended framework behavior  A great writeup and workaround  UPDATE  see below  seems to be from user  kaciula in this answer   Home key press behaviour  UPDATE June 2013  Months later  I have finally found the  correct  solution  You don t need to manage any stateful startedApp flags yourself   you can detect this from the framework and bail appropriately  I use this near the beginning of my LauncherActivity onCreate   if   isTaskRoot          Intent intent   getIntent        String action   intent getAction        if  intent hasCategory Intent CATEGORY LAUNCHER   amp  amp  action    null  amp  amp  action equals Intent ACTION MAIN             finish            return

User · Answer

The onSaveInstanceState bundle  and onRestoreInstanceState bundle  methods are useful for data persistence merely while rotating the screen  orientation change   They are not even good while switching between applications  since the onSaveInstanceState   method is called but onCreate bundle  and onRestoreInstanceState bundle  is not invoked again  For more persistence use shared preferences  read this article

User · Answer

I think I found the answer  Let me tell what I have done in simple words   Suppose I have two activities  activity1 and activity2 and I am navigating from activity1 to activity2  I have done some works in activity2  and again back to activity 1 by clicking on a button in activity1  Now at this stage I wanted to go back to activity2 and I want to see my activity2 in the same condition when I last left activity2   For the above scenario what I have done is that in the manifest I made some changes like this    lt activity android name   activity2            android alwaysRetainTaskState  true                  android launchMode  singleInstance  gt   lt  activity gt    And in the activity1 on the button click event I have done like this   Intent intent   new Intent    intent setFlags Intent FLAG ACTIVITY REORDER TO FRONT   intent setClassName this  com mainscreen activity2    startActivity intent     And in activity2 on button click event I have done like this   Intent intent new Intent    intent setClassName this  com mainscreen activity1    startActivity intent     Now what will happen is that whatever the changes we have made in the activity2 will not be lost  and we can view activity2 in the same state as we left previously   I believe this is the answer and this works fine for me  Correct me if I am wrong

User · Answer

To get activity state data stored in onCreate    first you have to save data in savedInstanceState by overriding SaveInstanceState Bundle savedInstanceState  method   When activity destroy SaveInstanceState Bundle savedInstanceState  method gets called and there you save data you want to save  And you get same in onCreate   when activity restart  savedInstanceState wont be null since you have saved some data in it before activity get destroyed

User · Answer

There are basically two ways to implement this change    using onSaveInstanceState   and onRestoreInstanceState    In manifest android configChanges  orientation screenSize     I really do not recommend to use second method  Since in one of my experience it was causing half of the device screen black while rotating from portrait to landscape and vice versa    Using first method mentioned above   we can persist data when orientation is changed or any config change happens  I know a way in which you can store any type of data inside savedInstance state object   Example  Consider a case if you want to persist Json object  create a model class with getters and setters    class MyModel extends Serializable  JSONObject obj   setJsonObject JsonObject obj    this obj obj     JSONObject getJsonObject   return this obj         Now in your activity in onCreate and onSaveInstanceState method do the following  It will look something like this    override onCreate Bundle savedInstaceState   MyModel data   MyModel savedInstaceState getSerializable  yourkey   JSONObject obj data getJsonObject      Here you have retained JSONObject and can use       Override protected void onSaveInstanceState Bundle outState    super onSaveInstanceState outState     Obj is some json object  MyModel dataToSave  new MyModel    dataToSave setJsonObject obj   oustate putSerializable  yourkey  dataToSave

User · Answer

Not sure if my solution is frowned upon or not  but I use a bound service to persist ViewModel state  Whether you store it in memory in the service or persist and retrieve it from a SQLite database depends on your requirements  This is what services of any flavor do  they provide services such as maintaining application state and abstract common business logic    Because of memory and processing constraints inherent on mobile devices  I treat Android views in a similar way to a web page  The page does not maintain state  it is purely a presentation layer component whose only purpose is to present application state and accept user input  Recent trends in web app architecture employ the use of the age-old Model  View  Controller  MVC  pattern  where the page is the View  domain data is the model  and the controller sits behind a web service  The same pattern can be employed in Android with the View being  well     the View  the model is your domain data  and the Controller is implemented as an Android bound service  Whenever you want a view to interact with the controller  bind to it on start resume and unbind on stop pause   This approach gives you the added bonus of enforcing the Separation of Concern design principle in that all of you application business logic can be moved into your service which reduces duplicated logic across multiple views and allows the view to enforce another important design principle  Single Responsibility

User · Answer

Saving state is a kludge at best as far as I m concerned  If you need to save persistent data  just use an SQLite database  Android makes it SOOO easy   Something like this   import java util Date  import android content Context  import android database Cursor  import android database sqlite SQLiteDatabase  import android database sqlite SQLiteOpenHelper   public class dataHelper        private static final String DATABASE NAME    autoMate db       private static final int DATABASE VERSION   1       private Context context      private SQLiteDatabase db      private OpenHelper oh        public dataHelper Context context            this context   context          this oh   new OpenHelper this context           this db   oh getWritableDatabase               public void close             db close            oh close            db   null          oh   null          SQLiteDatabase releaseMemory                public void setCode String codeName  Object codeValue  String codeDataType            Cursor codeRow   db rawQuery  SELECT   FROM code WHERE codeName        codeName        null           String cv                 if  codeDataType toLowerCase   trim   equals  long      true               cv   String valueOf codeValue                     else if  codeDataType toLowerCase   trim   equals  int      true                        cv   String valueOf codeValue                     else if  codeDataType toLowerCase   trim   equals  date      true                        cv   String valueOf   Date codeValue  getTime                       else if  codeDataType toLowerCase   trim   equals  boolean      true                        String valueOf codeValue                     else                       cv   String valueOf codeValue                      if codeRow getCount    gt  0    exists-- update                       db execSQL  update code set codeValue        cv                      where codeName        codeName                           else    does not exist  insert                       db execSQL  INSERT INTO code  codeName  codeValue  codeDataType  VALUES                               codeName                                    cv                                    codeDataType                                public Object getCode String codeName  Object defaultValue              Check to see if it already exists         String codeValue               String codeDataType               boolean found   false          Cursor codeRow    db rawQuery  SELECT   FROM code WHERE codeName        codeName        null           if  codeRow moveToFirst                          codeValue   codeRow getString codeRow getColumnIndex  codeValue                 codeDataType   codeRow getString codeRow getColumnIndex  codeDataType                 found   true                     if  found    false                        return defaultValue                    else if  codeDataType toLowerCase   trim   equals  long      true                        if  codeValue equals        true                                return  long 0                            return Long parseLong codeValue                     else if  codeDataType toLowerCase   trim   equals  int      true                        if  codeValue equals        true                                return  int 0                            return Integer parseInt codeValue                     else if  codeDataType toLowerCase   trim   equals  date      true                        if  codeValue equals        true                                return null                            return new Date Long parseLong codeValue                      else if  codeDataType toLowerCase   trim   equals  boolean      true                        if  codeValue equals        true                                return false                            return Boolean parseBoolean codeValue                     else                       return  String codeValue                        private static class OpenHelper extends SQLiteOpenHelper            OpenHelper Context context                super context  DATABASE NAME  null  DATABASE VERSION                       Override         public void onCreate SQLiteDatabase db                db execSQL  CREATE TABLE IF  NOT EXISTS code                  id INTEGER PRIMARY KEY  codeName TEXT  codeValue TEXT  codeDataType TEXT                         Override         public void onUpgrade SQLiteDatabase db  int oldVersion  int newVersion                        A simple call after that  dataHelper dh   new dataHelper getBaseContext     String status    String  dh getCode  appState    safetyDisabled    Date serviceStart    Date  dh getCode  serviceStartTime   null   dh close    dh   null

User · Answer

To answer the original question directly  savedInstancestate is null because your Activity is never being re-created   Your Activity will only be re-created with a state bundle when    Configuration changes such as changing the orientation or phone language which may requires a new activity instance to be created  You return to the app from the background after the OS has destroyed the activity     Android will destroy background activities when under memory pressure or after they ve been in the background for an extended period of time   When testing your hello world example there are a few ways to leave and return to the Activity    When you press the back button the Activity is finished  Re-launching the app is a brand new instance  You aren t resuming from the background at all  When you press the home button or use the task switcher the Activity will go into the background  When navigating back to the application onCreate will only be called if the Activity had to be destroyed     In most cases if you re just pressing home and then launching the app again the activity won t need to be re-created  It already exists in memory so onCreate   won t be called   There is an option under Settings -  Developer Options called  Don t keep activities   When it s enabled Android will always destroy activities and recreate them when they re backgrounded  This is a great option to leave enabled when developing because it simulates the worst case scenario    A low memory device recycling your activities all the time     The other answers are valuable in that they teach you the correct ways to store state but I didn t feel they really answered WHY your code wasn t working in the way you expected

User · Answer

The savedInstanceState is only for saving state associated with a current instance of an Activity  for example current navigation or selection info  so that if Android destroys and recreates an Activity  it can come back as it was before   See the documentation for onCreate and onSaveInstanceState  For more long lived state  consider using a SQLite database  a file  or preferences   See Saving Persistent State

User · Answer

My colleague wrote an article explaining application state on Android devices including explanations on activity lifecycle and state information  how to store state information  and saving to state Bundle and SharedPreferences and take a look at here   The article covers three approaches   Store local variable UI control data for application lifetime  i e  temporarily  using an instance state bundle   Code sample     Store state in state bundle   Override public void onSaveInstanceState Bundle savedInstanceState         Store UI state to the savedInstanceState       This bundle will be passed to onCreate on next call   EditText txtName    EditText findViewById R id txtName     String strName   txtName getText   toString       EditText txtEmail    EditText findViewById R id txtEmail     String strEmail   txtEmail getText   toString       CheckBox chkTandC    CheckBox findViewById R id chkTandC     boolean blnTandC   chkTandC isChecked       savedInstanceState putString    Name     strName     savedInstanceState putString    Email     strEmail     savedInstanceState putBoolean    TandC     blnTandC      super onSaveInstanceState savedInstanceState       Store local variable UI control data between application instances  i e  permanently  using shared preferences   Code sample     store state in SharedPreferences   Override protected void onPause       super onPause          Store values between instances here   SharedPreferences preferences   getPreferences MODE PRIVATE     SharedPreferences Editor editor   preferences edit        Put the values from the UI   EditText txtName    EditText findViewById R id txtName     String strName   txtName getText   toString       EditText txtEmail    EditText findViewById R id txtEmail     String strEmail   txtEmail getText   toString       CheckBox chkTandC    CheckBox findViewById R id chkTandC     boolean blnTandC   chkTandC isChecked       editor putString    Name     strName      value to store   editor putString    Email     strEmail      value to store   editor putBoolean    TandC     blnTandC      value to store      Commit to storage   editor commit        Keeping object instances alive in memory between activities within application lifetime using a retained non-configuration instance   Code sample     store object instance  private cMyClassType moInstanceOfAClass     Store the instance of an object  Override public Object onRetainNonConfigurationInstance       if  moInstanceOfAClass    null     Check that the object exists       return moInstanceOfAClass     return super onRetainNonConfigurationInstance

User · Answer

My problem was that I needed persistence only during the application lifetime  i e  a single execution including starting other sub-activities within the same app and rotating the device etc   I tried various combinations of the above answers but did not get what I wanted in all situations  In the end what worked for me was to obtain a reference to the savedInstanceState during onCreate   mySavedInstanceState savedInstanceState    and use that to obtain the contents of my variable when I needed it  along the lines of   if  mySavedInstanceState   null       boolean myVariable   mySavedInstanceState getBoolean  MyVariable        I use onSaveInstanceStateand onRestoreInstanceState as suggested above but I guess i could also or alternatively use my method to save the variable when it changes  e g  using putBoolean

User · Answer

Now it makes sense to do 2 ways in the view model  if you want to save the first as a saved instance  You can add state parameter in view model like this https   developer android com topic libraries architecture viewmodel-savedstate java  or you can save variables or object in view model  in this case the view model will hold the life cycle until the activity is destroyed   public class HelloAndroidViewModel extends ViewModel      public Booelan firstInit   false       public HelloAndroidViewModel             firstInit   false                   public class HelloAndroid extends Activity      private TextView mTextView   null    HelloAndroidViewModel viewModel   ViewModelProviders of this  get HelloAndroidViewModel class         Called when the activity is first created        Override   public void onCreate Bundle savedInstanceState        super onCreate savedInstanceState        mTextView   new TextView this          Because even if the state is deleted  the data in the viewmodel will be kept because the activity does not destroy     if  viewModel firstInit           viewModel firstInit   true         mTextView setText  Welcome to HelloAndroid          else         mTextView setText  Welcome back                setContentView mTextView

User · Answer

Kotlin code   save   override fun onSaveInstanceState outState  Bundle        super onSaveInstanceState outState apply           putInt  intKey   1          putString  stringKey    String Value           putParcelable  parcelableKey   parcelableObject             and then in onCreate   or onRestoreInstanceState         val restoredInt   savedInstanceState  getInt  intKey      1   default int     val restoredString   savedInstanceState  getString  stringKey       default string      val restoredParcelable   savedInstanceState  getParcelable lt ParcelableClass gt   parcelableKey      ParcelableClass     default parcelable   Add default values if you don t want to have Optionals

User · Answer

Meanwhile I do in general no more use  Bundle savedInstanceState  amp  Co   The life cycle is for most activities too complicated and not necessary   And Google states itself  it is NOT even reliable   My way is to save any changes immediately in the preferences    SharedPreferences p   p edit   put     commit     In some way SharedPreferences work similar like Bundles  And naturally and at first such values have to be read from preferences   In the case of complex data you may use SQLite instead of using preferences   When applying this concept  the activity just continues to use the last saved state  regardless of whether it was an initial open with reboots in between or a reopen due to the back stack

User · Answer

What to save and what not to   Ever wondered why the text in the EditText gets saved automatically while an orientation change  Well  this answer is for you   When an instance of an Activity gets destroyed and the System recreates a new instance  for example  configuration change   It tries to recreate it using a set of saved data of old Activity State  instance state    Instance state is a collection of key-value pairs stored in a Bundle object      By default System saves the View objects in the Bundle for example     Text in EditText Scroll position in a ListView  etc    If you need another variable to be saved as a part of instance state you should OVERRIDE onSavedInstanceState Bundle savedinstaneState  method   For example  int currentScore in a GameActivity  More detail about the onSavedInstanceState Bundle savedinstaneState  while saving data   Override public void onSaveInstanceState Bundle savedInstanceState           Save the user s current game state     savedInstanceState putInt STATE SCORE  mCurrentScore           Always call the superclass so it can save the view hierarchy state     super onSaveInstanceState savedInstanceState          So by mistake if you forget to call   super onSaveInstanceState savedInstanceState  the default behavior   will not work ie Text in EditText will not save    Which to choose for restoring Activity state    onCreate Bundle savedInstanceState    OR  onRestoreInstanceState Bundle savedInstanceState    Both methods get the same Bundle object  so it does not really matter where you write your restoring logic  The only difference is that in onCreate Bundle savedInstanceState  method you will have to give a null check while it is not needed in the latter case  Other answers have already code snippets  You can refer them   More detail about the onRestoreInstanceState Bundle savedinstaneState    Override public void onRestoreInstanceState Bundle savedInstanceState           Always call the superclass so it can restore the view hierarchy     super onRestoreInstanceState savedInstanceState           Restore state members from the saved instance     mCurrentScore   savedInstanceState getInt STATE SCORE          Always call super onRestoreInstanceState savedInstanceState   so that System restore the View hierarchy by default   Bonus  The onSaveInstanceState Bundle savedInstanceState  is invoked by the system only when the user intends to come back to the Activity  For example  you are using App X and suddenly you get a call  You move to the caller app and come back to the app X  In this case the onSaveInstanceState Bundle savedInstanceState  method will be invoked   But consider this if a user presses the back button  It is assumed that the user does not intend to come back to the Activity  hence in this case onSaveInstanceState Bundle savedInstanceState  will not be invoked by the system  Point being you should consider all the scenarios while saving data   Relevant links   Demo on default behavior Android Official Documentation

User · Answer

Kotlin Solution  For custom class save in onSaveInstanceState you can be converted your class to JSON string and restore it with Gson convertion and for single String  Double  Int  Long value save and restore as following  The following example is for Fragment and Activity   For Activity   For put data in saveInstanceState   override fun onSaveInstanceState outState  Bundle            super onSaveInstanceState outState             for custom class-----         val gson   Gson           val json   gson toJson your custom class          outState putString  CUSTOM CLASS   json             for single value------         outState putString  MyString   stringValue          outState putBoolean  MyBoolean   true          outState putDouble  myDouble   doubleValue          outState putInt  MyInt   intValue          Restore data    override fun onRestoreInstanceState savedInstanceState  Bundle        super onRestoreInstanceState savedInstanceState         for custom class restore     val json   savedInstanceState  getString  CUSTOM CLASS       if   json   isEmpty              val gson   Gson           testBundle   gson fromJson json  Session  class java             for single value restore     val myBoolean  Boolean   savedInstanceState  getBoolean  MyBoolean      val myDouble  Double   savedInstanceState  getDouble  myDouble      val myInt  Int   savedInstanceState  getInt  MyInt      val myString  String   savedInstanceState  getString  MyString        You can restore it on Activity onCreate also   For fragment   For put class in saveInstanceState    override fun onSaveInstanceState outState  Bundle            super onSaveInstanceState outState          val gson   Gson           val json   gson toJson customClass          outState putString  CUSTOM CLASS   json          Restore data    override fun onActivityCreated savedInstanceState  Bundle             super onActivityCreated savedInstanceState             for custom class restore         if  savedInstanceState    null                val json   savedInstanceState getString  CUSTOM CLASS               if   json   isEmpty                      val gson   Gson                   val customClass  CustomClass   gson fromJson json  CustomClass  class java                                    for single value restore       val myBoolean  Boolean   savedInstanceState getBoolean  MyBoolean         val myDouble  Double   savedInstanceState getDouble  myDouble         val myInt  Int   savedInstanceState getInt  MyInt         val myString  String   savedInstanceState getString  MyString

User · Answer

Instead of that  you should use ViewModel  which will retain the data until the activity life cycle

User · Answer

Although the accepted answer is correct  there is a faster and easier method to save the Activity state on Android using a library called Icepick  Icepick is an annotation processor that takes care of all the boilerplate code used in saving and restoring state for you    Doing something like this with Icepick   class MainActivity extends Activity      State String username     These will be automatically saved and restored    State String password     State int age      Override public void onCreate Bundle savedInstanceState        super onCreate savedInstanceState       Icepick restoreInstanceState this  savedInstanceState           Override public void onSaveInstanceState Bundle outState        super onSaveInstanceState outState       Icepick saveInstanceState this  outState           Is the same as doing this   class MainActivity extends Activity     String username    String password    int age      Override   public void onSaveInstanceState Bundle savedInstanceState        super onSaveInstanceState savedInstanceState       savedInstanceState putString  MyString   username       savedInstanceState putString  MyPassword   password       savedInstanceState putInt  MyAge   age           remember you would need to actually initialize these variables before putting it in the     Bundle            Override   public void onRestoreInstanceState Bundle savedInstanceState        super onRestoreInstanceState savedInstanceState       username   savedInstanceState getString  MyString        password   savedInstanceState getString  MyPassword        age   savedInstanceState getInt  MyAge            Icepick will work with any object that saves its state with a Bundle

User · Answer

Kotlin  You must override onSaveInstanceState and onRestoreInstanceState to store and retrieve your variables you want to be persistent  Life cycle graph    Store variables  public override fun onSaveInstanceState savedInstanceState  Bundle        super onSaveInstanceState savedInstanceState          prepare variables here     savedInstanceState putInt  kInt   10      savedInstanceState putBoolean  kBool   true      savedInstanceState putDouble  kDouble   4 5      savedInstanceState putString  kString    Hello Kotlin       Retrieve variables  public override fun onRestoreInstanceState savedInstanceState  Bundle        super onRestoreInstanceState savedInstanceState       val myInt   savedInstanceState getInt  kInt       val myBoolean   savedInstanceState getBoolean  kBool       val myDouble   savedInstanceState getDouble  kDouble       val myString   savedInstanceState getString  kString          use variables here

User · Answer

Both methods are useful and valid and both are best suited for different scenarios    The user terminates the application and re-opens it at a later date  but the application needs to reload data from the last session     this requires a persistent storage approach such as using SQLite  The user switches application and then comes back to the original and wants to pick up where they left off -  save and restore bundle data  such as application state data  in onSaveInstanceState   and onRestoreInstanceState   is usually adequate    If you save the state data in a persistent manner  it can be reloaded in an onResume   or onCreate    or actually on any lifecycle call   This may or may not be desired behaviour  If you store it in a bundle in an InstanceState  then it is transient and is only suitable for storing data for use in the same user    session     I use the term session loosely  but not between    sessions      It is not that one approach is better than the other  like everything  it is just important to understand what behaviour you require and to select the most appropriate approach

User · Answer

When an activity is created it s  onCreate   method is called       Override     protected void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           savedInstanceState is an object of Bundle class which is null for the first time  but it contains values when it is recreated  To save Activity s state you have to override onSaveInstanceState         Override     protected void onSaveInstanceState Bundle outState          outState putString  key   Welcome Back           super onSaveInstanceState outState           save state         put your values in  outState  Bundle object like outState putString  key   Welcome Back   and save by calling super  When activity will be destroyed it s state get saved in Bundle object and can be restored after recreation in onCreate   or onRestoreInstanceState    Bundle received in onCreate   and onRestoreInstanceState   are same       Override     protected void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           setContentView R layout activity main                restore activity s state          if savedInstanceState  null             String reStoredString savedInstanceState getString  key                          or      restores activity s saved state   Override     protected void onRestoreInstanceState Bundle savedInstanceState          String restoredMessage savedInstanceState getString  key

User · Answer

To help reduce boilerplate I use the following interface and class to read write to a Bundle for saving instance state     First  create an interface that will be used to annotate your instance variables   import java lang annotation Documented  import java lang annotation ElementType  import java lang annotation Retention  import java lang annotation RetentionPolicy  import java lang annotation Target    Documented  Retention RetentionPolicy RUNTIME   Target           ElementType FIELD    public  interface SaveInstance        Then  create a class where reflection will be used to save values to the bundle   import android app Activity  import android app Fragment  import android os Bundle  import android os Parcelable  import android util Log   import java io Serializable  import java lang reflect Field          Save and load fields to from a   link Bundle   All fields should be annotated with   link    SaveInstance   lt  p gt      public class Icicle        private static final String TAG    Icicle                   Find all fields with the   link SaveInstance  annotation and add them to the   link Bundle                  param outState                The bundle from   link Activity onSaveInstanceState Bundle   or   link                Fragment onSaveInstanceState Bundle           param classInstance                The object to access the fields which have the   link SaveInstance  annotation          see  load Bundle  Object              public static void save Bundle outState  Object classInstance            save outState  classInstance  classInstance getClass                           Find all fields with the   link SaveInstance  annotation and add them to the   link Bundle                  param outState                The bundle from   link Activity onSaveInstanceState Bundle   or   link                Fragment onSaveInstanceState Bundle           param classInstance                The object to access the fields which have the   link SaveInstance  annotation          param baseClass                Base class  used to get all superclasses of the instance          see  load Bundle  Object  Class              public static void save Bundle outState  Object classInstance  Class lt   gt  baseClass            if  outState    null                return                    Class lt   gt  clazz   classInstance getClass            while  baseClass isAssignableFrom clazz                 String className   clazz getName                for  Field field   clazz getDeclaredFields                      if  field isAnnotationPresent SaveInstance class                         field setAccessible true                       String key   className         field getName                        try                           Object value   field get classInstance                           if  value instanceof Parcelable                                outState putParcelable key   Parcelable  value                             else if  value instanceof Serializable                                outState putSerializable key   Serializable  value                                                   catch  Throwable t                            Log d TAG   The field      key      was not added to the bundle                                                                      clazz   clazz getSuperclass                                    Load all saved fields that have the   link SaveInstance  annotation                 param savedInstanceState                The saved-instance   link Bundle  from an   link Activity  or   link Fragment           param classInstance                The object to access the fields which have the   link SaveInstance  annotation          see  save Bundle  Object              public static void load Bundle savedInstanceState  Object classInstance            load savedInstanceState  classInstance  classInstance getClass                           Load all saved fields that have the   link SaveInstance  annotation                 param savedInstanceState                The saved-instance   link Bundle  from an   link Activity  or   link Fragment           param classInstance                The object to access the fields which have the   link SaveInstance  annotation          param baseClass                Base class  used to get all superclasses of the instance          see  save Bundle  Object  Class              public static void load Bundle savedInstanceState  Object classInstance  Class lt   gt  baseClass            if  savedInstanceState    null                return                    Class lt   gt  clazz   classInstance getClass            while  baseClass isAssignableFrom clazz                 String className   clazz getName                for  Field field   clazz getDeclaredFields                      if  field isAnnotationPresent SaveInstance class                         String key   className         field getName                        field setAccessible true                       try                           Object fieldVal   savedInstanceState get key                           if  fieldVal    null                                field set classInstance  fieldVal                                                   catch  Throwable t                            Log d TAG   The field      key      was not retrieved from the bundle                                                                      clazz   clazz getSuperclass                           Example usage   public class MainActivity extends Activity         SaveInstance     private String foo        SaveInstance     private int bar        SaveInstance     private Intent baz        SaveInstance     private boolean qux        Override     public void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           Icicle load savedInstanceState  this               Override     public void onSaveInstanceState Bundle outState            super onSaveInstanceState outState           Icicle save outState  this                Note  This code was adapted from a library project named AndroidAutowire which is licensed under the MIT license

User · Answer

In 2020 we have some changes   If you want your Activity to restore its state after the process is killed and started again  you may want to use the    saved state    functionality  Previously  you needed to override two methods in the Activity  onSaveInstanceState and onRestoreInstanceState  You can also access the restored state in the onCreate method  Similarly  in Fragment  you have onSaveInstanceState method available  and the restored state is available in the onCreate  onCreateView  and onActivityCreated methods   Starting with AndroidX SavedState 1 0 0  which is the dependency of the AndroidX Activity and the AndroidX Fragment  you get access to the SavedStateRegistry  You can obtain the SavedStateRegistry from the Activity Fragment and then register your SavedStateProvider   class MyActivity   AppCompatActivity        companion object       private const val MY SAVED STATE KEY    quot MY SAVED STATE KEY  quot      private const val SOME VALUE KEY    quot SOME VALUE KEY  quot             private lateinit var someValue  String   private val savedStateProvider   SavedStateRegistry SavedStateProvider           Bundle   apply         putString SOME VALUE KEY  someValue                 override fun onCreate savedInstanceState  Bundle             super onCreate savedInstanceState      savedStateRegistry registerSavedStateProvider MY SAVED STATE KEY  savedStateProvider      someValue   savedStateRegistry consumeRestoredStateForKey MY SAVED STATE KEY   getString SOME VALUE KEY      quot  quot            As you can see  SavedStateRegistry enforces you to use the key for your data  This can prevent your data from being corrupted by another SavedStateProvider attached to the same Activity Fragment Also you can extract your SavedStateProvider to another class to make it work with your data by using whatever abstraction you want and in such a way achieve the clean saved state behavior in your application

User · Answer

You need to override onSaveInstanceState Bundle savedInstanceState  and write the application state values you want to change to the Bundle parameter

User · Answer

onSaveInstanceState is called when the system needs memory and kills an application  It is not called when the user just closes the application  So I think application state should also be saved in onPause It should be saved to some persistent storage like Preferences or Sqlite

User · Answer

onSaveInstanceState   for transient data  restored in onCreate   onRestoreInstanceState     onPause   for persistent data  restored in onResume     From Android technical resources   onSaveInstanceState   is called by Android if the Activity is being stopped and may be killed before it is resumed  This means it should store any state necessary to re-initialize to the same condition when the Activity is restarted  It is the counterpart to the onCreate   method  and in fact the savedInstanceState Bundle passed in to onCreate   is the same Bundle that you construct as outState in the onSaveInstanceState   method  onPause   and onResume   are also complimentary methods  onPause   is always called when the Activity ends  even if we instigated that  with a finish   call for example   We will use this to save the current note back to the database  Good practice is to release any resources that can be released during an onPause   as well  to take up less resources when in the passive state

User · Answer

Note that it is NOT safe to use onSaveInstanceState and onRestoreInstanceState for persistent data  according to the documentation on Activity states in http   developer android com reference android app Activity html   The document states  in the  Activity Lifecycle  section       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 persistent data in onPause   and onResume     EDIT  For further clarification  here s the onSaveInstanceState   documentation      This method is called before an activity may be killed so that when it   comes back some time in the future it can restore its state  For   example  if activity B is launched in front of activity A  and at some   point activity A is killed to reclaim resources  activity A will have   a chance to save the current state of its user interface via this   method so that when the user returns to activity A  the state of the   user interface can be restored via onCreate Bundle  or   onRestoreInstanceState Bundle

User · Answer

using Android ViewModel  amp  SavedStateHandle to persist the serializable data public class MainActivity extends AppCompatActivity        Override     protected void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           ActivityMainBinding binding   ActivityMainBinding inflate getLayoutInflater             binding setViewModel new ViewModelProvider this  get ViewModel class            binding setLifecycleOwner this           setContentView binding getRoot                public static class ViewModel extends AndroidViewModel              This field SURVIVE the background process reclaim killing  amp  the configuration change         public final SavedStateHandle savedStateHandle             This field NOT SURVIVE the background process reclaim killing but SURVIVE the configuration change         public final MutableLiveData lt String gt  inputText2   new MutableLiveData lt  gt               public ViewModel  NonNull Application application  SavedStateHandle savedStateHandle                super application               this savedStateHandle   savedStateHandle                     in layout file  lt  xml version  quot 1 0 quot  encoding  quot utf-8 quot   gt   lt layout xmlns android  quot http   schemas android com apk res android quot  gt        lt data gt            lt variable             name  quot viewModel quot              type  quot com xxx viewmodelsavedstatetest MainActivity ViewModel quot    gt       lt  data gt        lt LinearLayout xmlns tools  quot http   schemas android com tools quot          android layout width  quot match parent quot          android layout height  quot match parent quot          android orientation  quot vertical quot          tools context  quot  MainActivity quot  gt             lt EditText             android layout width  quot match parent quot              android layout height  quot wrap content quot              android autofillHints  quot  quot              android hint  quot This field SURVIVE the background process reclaim killing  amp amp  the configuration change quot              android text      String viewModel savedStateHandle getLiveData  quot activity main inputText quot    quot  quot       gt            lt SeekBar             android layout width  quot match parent quot              android layout height  quot wrap content quot              android max  quot 100 quot              android progress      Integer viewModel savedStateHandle getLiveData  quot activity main progress quot   50      gt            lt EditText             android layout width  quot match parent quot              android layout height  quot wrap content quot              android hint  quot This field SURVIVE the background process reclaim killing  amp amp  the configuration change quot              android text      String viewModel savedStateHandle getLiveData  quot activity main inputText quot    quot  quot       gt            lt SeekBar             android layout width  quot match parent quot              android layout height  quot wrap content quot              android max  quot 100 quot              android progress      Integer viewModel savedStateHandle getLiveData  quot activity main progress quot   50      gt            lt EditText             android layout width  quot match parent quot              android layout height  quot wrap content quot              android hint  quot This field NOT SURVIVE the background process reclaim killing but SURVIVE the configuration change quot              android text     viewModel inputText2     gt        lt  LinearLayout gt   lt  layout gt   Test  1  start the test activity 2  press home key to go home 3  adb shell kill  lt the test activity process gt  4  open recent app list and restart the test activity

User · Answer

Now Android provides ViewModels for saving state  you should try to use that instead of saveInstanceState

[android] How to save an activity state using save instance state?

Examples related to android

Examples related to android-activity

Examples related to application-state