How to prevent custom views from losing state across screen orientation changes

Question

I ve successfully implemented onRetainNonConfigurationInstance   for my main Activity to save and restore certain critical components across screen orientation changes    But it seems  my custom views are being re-created from scratch when the orientation changes  This makes sense  although in my case it s inconvenient because the custom view in question is an X Y plot and the plotted points are stored in the custom view    Is there a crafty way to implement something similar to onRetainNonConfigurationInstance   for a custom view  or do I need to just implement methods in the custom view which allow me to get and set its  state

User · Answer

To augment other answers - if you have multiple custom compound views with the same ID and they are all being restored with the state of the last view on a configuration change, all you need to do is tell the view to only dispatch save/restore events to itself by overriding a couple of methods.

class MyCompoundView : ViewGroup {

    ...

    override fun dispatchSaveInstanceState(container: SparseArray<Parcelable>) {
        dispatchFreezeSelfOnly(container)
    }

    override fun dispatchRestoreInstanceState(container: SparseArray<Parcelable>) {
        dispatchThawSelfOnly(container)
    }
}

For an explanation of what is happening and why this works, see this blog post. Basically your compound view's children's view IDs are shared by each compound view and state restoration gets confused. By only dispatching state for the compound view itself, we prevent their children from getting mixed messages from other compound views.

User · Answer

Easy with kotlin  Parcelize class MyState val superSavedState  Parcelable   val loading  Boolean    View BaseSavedState superSavedState   Parcelable   class MyView   View        var loading  Boolean   false      override fun onSaveInstanceState    Parcelable            val superState   super onSaveInstanceState           return MyState superState  loading             override fun onRestoreInstanceState state  Parcelable             val myState   state as  MyState         super onRestoreInstanceState myState  superSaveState    state           loading   myState  loading    false           redraw

User · Answer

I found that this answer was causing some crashes on Android versions 9 and 10  I think it s a good approach but when I was looking at some Android code I found out it was missing a constructor  The answer is quite old so at the time there probably was no need for it  When I added the missing constructor and called it from the creator the crash was fixed  So here is the edited code  public class CustomView extends LinearLayout        private int stateToSave                 Override     public Parcelable onSaveInstanceState             Parcelable superState   super onSaveInstanceState            SavedState ss   new SavedState superState               your custom state         ss stateToSave   this stateToSave           return ss              Override     protected void dispatchSaveInstanceState SparseArray lt Parcelable gt  container                dispatchFreezeSelfOnly container               Override     public void onRestoreInstanceState Parcelable state            SavedState ss    SavedState  state          super onRestoreInstanceState ss getSuperState                 your custom state         this stateToSave   ss stateToSave              Override     protected void dispatchRestoreInstanceState SparseArray lt Parcelable gt  container                dispatchThawSelfOnly container              static class SavedState extends BaseSavedState           int stateToSave           SavedState Parcelable superState                super superState                      private SavedState Parcel in                super in               this stateToSave   in readInt                          This was the missing constructor          RequiresApi Build VERSION CODES N          SavedState Parcel in  ClassLoader loader                        super in  loader               this stateToSave   in readInt                        Override         public void writeToParcel Parcel out  int flags                super writeToParcel out  flags               out writeInt this stateToSave                                  public static final Creator lt SavedState gt  CREATOR               new ClassLoaderCreator lt SavedState gt                                This was also missing              Override             public SavedState createFromParcel Parcel in  ClassLoader loader                                return Build VERSION SDK INT  gt   Build VERSION CODES N   new SavedState in  loader    new SavedState in                               Override             public SavedState createFromParcel Parcel in                    return new SavedState in  null                               Override             public SavedState   newArray int size                    return new SavedState size

User · Answer

The answers here already are great  but don t necessarily work for custom ViewGroups  To get all custom Views to retain their state  you must override onSaveInstanceState   and onRestoreInstanceState Parcelable state  in each class  You also need to ensure they all have unique ids  whether they re inflated from xml or added programmatically   What I came up with was remarkably like Kobor42 s answer  but the error remained because I was adding the Views to a custom ViewGroup programmatically and not assigning unique ids   The link shared by mato will work  but it means none of the individual Views manage their own state - the entire state is saved in the ViewGroup methods   The problem is that when multiple of these ViewGroups are added to a layout  the ids of their elements from the xml are no longer unique  if its defined in xml   At runtime  you can call the static method View generateViewId   to get a unique id for a View  This is only available from API 17   Here is my code from the ViewGroup  it is abstract  and mOriginalValue is a type variable    public abstract class DetailRow lt E gt  extends LinearLayout        private static final String SUPER INSTANCE STATE    saved instance state parcelable       private static final String STATE VIEW IDS    state view ids       private static final String STATE ORIGINAL VALUE    state original value        private E mOriginalValue      private int   mViewIds                Override     protected Parcelable onSaveInstanceState                 Create a bundle to put super parcelable in         Bundle bundle   new Bundle            bundle putParcelable SUPER INSTANCE STATE  super onSaveInstanceState                Use abstract method to put mOriginalValue in the bundle          putValueInTheBundle mOriginalValue  bundle  STATE ORIGINAL VALUE              Store mViewIds in the bundle - initialize if necessary          if  mViewIds    null                   We need as many ids as child views             mViewIds   new int getChildCount                 for  int i   0  i  lt  mViewIds length  i                         generate a unique id for each view                 mViewIds i    View generateViewId                       assign the id to the view at the same index                 getChildAt i  setId mViewIds i                                    bundle putIntArray STATE VIEW IDS  mViewIds              return the bundle         return bundle              Override     protected void onRestoreInstanceState Parcelable state                We know state is a Bundle          Bundle bundle    Bundle  state             Get mViewIds out of the bundle         mViewIds   bundle getIntArray STATE VIEW IDS              For each id  assign to the view of same index         if  mViewIds    null                for  int i   0  i  lt  mViewIds length  i                      getChildAt i  setId mViewIds i                                       Get mOriginalValue out of the bundle         mOriginalValue   getValueBackOutOfTheBundle bundle  STATE ORIGINAL VALUE              get super parcelable back out of the bundle and pass it to            super onRestoreInstanceState Parcelable          state   bundle getParcelable SUPER INSTANCE STATE           super onRestoreInstanceState state

User · Answer

You do this by implementing View onSaveInstanceState and View onRestoreInstanceState and extending the View BaseSavedState class   public class CustomView extends View      private int stateToSave             Override   public Parcelable onSaveInstanceState           begin boilerplate code that allows parent classes to save state     Parcelable superState   super onSaveInstanceState         SavedState ss   new SavedState superState         end      ss stateToSave   this stateToSave       return ss          Override   public void onRestoreInstanceState Parcelable state          begin boilerplate code so parent classes can restore state     if   state instanceof SavedState           super onRestoreInstanceState state         return             SavedState ss    SavedState state      super onRestoreInstanceState ss getSuperState           end      this stateToSave   ss stateToSave         static class SavedState extends BaseSavedState       int stateToSave       SavedState Parcelable superState          super superState              private SavedState Parcel in          super in         this stateToSave   in readInt                Override     public void writeToParcel Parcel out  int flags          super writeToParcel out  flags         out writeInt this stateToSave                required field that makes Parcelables from a Parcel     public static final Parcelable Creator lt SavedState gt  CREATOR           new Parcelable Creator lt SavedState gt                public SavedState createFromParcel Parcel in                return new SavedState in                         public SavedState   newArray int size                return new SavedState size                              The work is split between the View and the View s SavedState class  You should do all the work of reading and writing to and from the Parcel in the SavedState class  Then your View class can do the work of extracting the state members and doing the work necessary to get the class back to a valid state   Notes  View onSavedInstanceState and View onRestoreInstanceState are called automatically for you if View getId returns a value    0  This happens when you give it an id in xml or call setId manually  Otherwise you have to call View onSaveInstanceState and write the Parcelable returned to the parcel you get in Activity onSaveInstanceState to save the state and subsequently read it and pass it to View onRestoreInstanceState from Activity onRestoreInstanceState   Another simple example of this is the CompoundButton

User · Answer

Here is another variant that uses a mix of the two above methods  Combining the speed and correctness of Parcelable with the simplicity of a Bundle    Override public Parcelable onSaveInstanceState         Bundle bundle   new Bundle           The vars you want to save - in this instance a string and a boolean     String someString    something       boolean someBoolean   true      State state   new State super onSaveInstanceState    someString  someBoolean       bundle putParcelable State STATE  state       return bundle      Override public void onRestoreInstanceState Parcelable state        if  state instanceof Bundle            Bundle bundle    Bundle  state          State customViewState    State  bundle getParcelable State STATE              The vars you saved - do whatever you want with them         String someString   customViewState getText            boolean someBoolean   customViewState isSomethingShowing             super onRestoreInstanceState customViewState getSuperState             return               Stops a bug with the wrong state being passed to the super     super onRestoreInstanceState BaseSavedState EMPTY STATE       protected static class State extends BaseSavedState       protected static final String STATE    YourCustomView STATE        private final String someText      private final boolean somethingShowing       public State Parcelable superState  String someText  boolean somethingShowing            super superState           this someText   someText          this somethingShowing   somethingShowing             public String getText            return this someText             public boolean isSomethingShowing            return this somethingShowing

User · Answer

I had the problem that onRestoreInstanceState restored all my custom views with the state of the last view  I solved it by adding these two methods to my custom view    Override protected void dispatchSaveInstanceState SparseArray lt Parcelable gt  container        dispatchFreezeSelfOnly container       Override protected void dispatchRestoreInstanceState SparseArray lt Parcelable gt  container        dispatchThawSelfOnly container

User · Answer

I think this is a much simpler version  Bundle is a built-in type which implements Parcelable  public class CustomView extends View     private int stuff     stuff     Override   public Parcelable onSaveInstanceState           Bundle bundle   new Bundle        bundle putParcelable  superState   super onSaveInstanceState         bundle putInt  stuff   this stuff          save stuff      return bundle          Override   public void onRestoreInstanceState Parcelable state          if  state instanceof Bundle     implicit null check             Bundle bundle    Bundle  state        this stuff   bundle getInt  stuff           load stuff       state   bundle getParcelable  superState              super onRestoreInstanceState state

User · Answer

Instead of using onSaveInstanceState and onRestoreInstanceState  you can also use a ViewModel  Make your data model extend ViewModel  and then you can use ViewModelProviders to get the same instance of your model every time the Activity is recreated   class MyData extends ViewModel          have all your properties with getters and setters here    public class MyActivity extends FragmentActivity        Override     public void onCreate Bundle savedInstanceState             super onCreate savedInstanceState           setContentView R layout activity main               the first time  ViewModelProvider will create a new MyData            object  When the Activity is recreated  e g  because the screen            is rotated   ViewModelProvider will give you the initial MyData            object back  without creating a new one  so all your property            values are retained from the previous view          myData   ViewModelProviders of this  get MyData class                          To use ViewModelProviders  add the following to dependencies in app build gradle   implementation  android arch lifecycle extensions 1 1 1  implementation  android arch lifecycle viewmodel 1 1 1    Note that your MyActivity extends FragmentActivity instead of just extending Activity    You can read more about ViewModels here    Android Developer Guide  Handle configuration changes Android Developer Guide  Saving UI States  Use ViewModel to handle configuration changes Tutorial ViewModels   A Simple Example

User · Answer

Based on  Fletcher Johns answer I came up with   custom layout can inflate from XML is able to save restore direct and indirect children  I improved  Fletcher Johns  answer to save the ids in String- gt Id map instead of IntArray  the only small drawback is that you must declare your saveable child views beforehand    open class AddressView  JvmOverloads constructor          context  Context          attrs  AttributeSet    null          defStyleAttr  Int   0          defStyleRes  Int   0     LinearLayout context  attrs  defStyleAttr  defStyleRes         protected lateinit var countryInputLayout  TextInputLayout     protected lateinit var countryAutoCompleteTextView  CountryAutoCompleteTextView     protected lateinit var cityInputLayout  TextInputLayout     protected lateinit var cityEditText  CityEditText     protected lateinit var postCodeInputLayout  TextInputLayout     protected lateinit var postCodeEditText  PostCodeEditText     protected lateinit var streetInputLayout  TextInputLayout     protected lateinit var streetEditText  StreetEditText          init           initView              private fun initView             val view   inflate context  R layout view address  this           orientation   VERTICAL          countryInputLayout   view findViewById R id countryInputLayout          countryAutoCompleteTextView   view findViewById R id countryAutoCompleteTextView           streetInputLayout   view findViewById R id streetInputLayout          streetEditText   view findViewById R id streetEditText           cityInputLayout   view findViewById R id cityInputLayout          cityEditText   view findViewById R id cityEditText           postCodeInputLayout   view findViewById R id postCodeInputLayout          postCodeEditText   view findViewById R id postCodeEditText                Declare your direct and indirect child views that need to be saved     private val childrenToSave get     mapOf lt String  View gt                quot coutryIL quot  to countryInputLayout               quot countryACTV quot  to countryAutoCompleteTextView               quot streetIL quot  to streetInputLayout               quot streetET quot  to streetEditText               quot cityIL quot  to cityInputLayout               quot cityET quot  to cityEditText               quot postCodeIL quot  to postCodeInputLayout               quot postCodeET quot  to postCodeEditText            private var viewIds  HashMap lt String  Int gt     null      override fun onSaveInstanceState    Parcelable               Create a bundle to put super parcelable in         val bundle   Bundle           bundle putParcelable SUPER INSTANCE STATE  super onSaveInstanceState               Store viewIds in the bundle - initialize if necessary          if  viewIds    null                childrenToSave values forEach   view - gt  view id   generateViewId                 viewIds   HashMap lt String  Int gt  childrenToSave mapValues    key  view  - gt  view id                       bundle putSerializable STATE VIEW IDS  viewIds           return bundle            override fun onRestoreInstanceState state  Parcelable                We know state is a Bundle          val bundle   state as Bundle            Get mViewIds out of the bundle         viewIds   bundle getSerializable STATE VIEW IDS  as HashMap lt String  Int gt             For each id  assign to the view of same index         if  viewIds    null                viewIds   forEach    key  id  - gt  childrenToSave key    id   id                     super onRestoreInstanceState bundle getParcelable SUPER INSTANCE STATE              companion object           private const val SUPER INSTANCE STATE    quot saved instance state parcelable quot          private const val STATE VIEW IDS    quot state view ids quot

[android] How to prevent custom views from losing state across screen orientation changes

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