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

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 vi开发者_运维技巧ew, or do I need to just implement methods in the custom view which allow me to get and set its "state"?

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);
  }
}

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<SavedState> CREATOR =
        new Parcelable.Creator<SavedState>() {
          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

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
    }
}

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;
    }
}

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<E> 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 < 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 < 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);
    } 
}

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<Parcelable> container) {
    dispatchFreezeSelfOnly(container);
}

@Override
protected void dispatchRestoreInstanceState(SparseArray<Parcelable> container) {
    dispatchThawSelfOnly(container);
}

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.

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<Parcelable> 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<Parcelable> 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<SavedState> CREATOR =
            new ClassLoaderCreator<SavedState>() {
          
            // This was also missing
            @Override
            public SavedState createFromParcel(Parcel in, ClassLoader loader)
            {
                return Build.VERSION.SDK_INT >= 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];
            }
        };
    }
}

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

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->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<String, View>(
            "coutryIL" to countryInputLayout,
            "countryACTV" to countryAutoCompleteTextView,
            "streetIL" to streetInputLayout,
            "streetET" to streetEditText,
            "cityIL" to cityInputLayout,
            "cityET" to cityEditText,
            "postCodeIL" to postCodeInputLayout,
            "postCodeET" to postCodeEditText,
    )
    private var viewIds: HashMap<String, Int>? = 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 -> view.id = generateViewId() }
            viewIds = HashMap<String, Int>(childrenToSave.mapValues { (key, view) -> 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<String, Int>
        // For each id, assign to the view of same index
        if (viewIds != null) {
            viewIds!!.forEach { (key, id) -> childrenToSave[key]!!.id = id }
        }
        super.onRestoreInstanceState(bundle.getParcelable(SUPER_INSTANCE_STATE))
    }

    companion object {
        private const val SUPER_INSTANCE_STATE = "saved_instance_state_parcelable"
        private const val STATE_VIEW_IDS = "state_view_ids"
    }
}