Deep Dive Into Android State Restoration

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TWITTER @cyrilmottier ! WEBSITE cyrilmottier.com

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The story of a newbie Android developer

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Kevin has just developed his first Android app

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He discovers an annoying bug: Fields are cleared on rotate

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3 options

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3 options Don’t care

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3 options Don’t care Block orientation

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3 options Don’t care Block orientation Use configChanges Hint: all options are

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1 ! 4 ! 5 ! 6 ! 7

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Kevin’s satisfied

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Still having issues on…

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language changes Still having issues on…

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ANGRY! Angry Kevin is

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1 ! 4 ! 5 ! 6 ! 7

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The nightmare continues… Still having issues when moving the app to the background

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God save the STATE

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State restoration key components

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The container Parcel

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Parcelable The container Parcel The content Primitives types Primitives arrays

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The content Primitives types Primitives arrays 1 parcel.writeInt(1);! 2 parcel.writeLong(2L);! 3 parcel.writeFloat(3F);! 4 parcel.writeString("Hi!"); Parcelable

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1 parcel.writeIntArray(new int[]{1, 2, 3});! 2 parcel.writeLongArray(new long[]{1L, 2L, 3L});! 3 parcel.writeDoubleArray(new double[]{1, 2, 3});! 4 parcel.writeStringArray(new String[]{! 5 "Hi", "Droidcon", "guys!"! 6 }); The content Primitives types Primitives arrays Parcelable

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1 public final class Suggestion implements Parcelable {! 2 ! 3 public final String id;! 4 public final String name;! 5 public final int type;! 6 ! 7 public Suggestion(String id, String name, int type) {! 8 this.id = Objects.requireNonNull(id);! 9 this.name = Objects.requireNonNull(name);! 10 this.type = type;! 11 }! 12 ! 13 }

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1 @Override! 2 public int describeContents() {! 3 return 0;! 4 }! 5 ! 6 @Override! 7 public void writeToParcel(Parcel dest, int flags) {! 8 dest.writeString(id);! 9 dest.writeString(name);! 10 dest.writeInt(type);! 11 }! 12 13 public static final Parcelable.Creator CREATOR = ! 14 new Parcelable.Creator() {! 15 public Suggestion createFromParcel(Parcel in) {! 16 return new Suggestion(in.readString(), // ! 17 in.readString(), //! 18 in.readInt());! 19 }! 20 ! 21 public Suggestion[] newArray(int size) {! 22 return new Suggestion[size];! 23 }! 24 };

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Parcelable.Creator! The base creator interface ! Parcelable.ClassLoaderCreator! A creator with the ClassLoader passed on read. ! ParcelableCompat & ParcelableCompatCreatorCallbacks! Compatibility stuﬀ

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Bundle A key-value map & type-safe Parcelable

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Parcel! internally uses reflection (required to get the CREATOR instance) …i.e. beware Proguard

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Activity level state restoration

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onCreate(null)

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onCreate(null)

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: non-null Bundle onSaveInstanceState( ) onCreate(null)

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onSaveInstanceState( ) onCreate( ) onCreate(null) : non-null Bundle

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onSaveInstanceState( ) onCreate( ) onRestoreInstanceState( ) onCreate(null) : non-null Bundle

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onSaveInstanceState( ) onCreate( ) onRestoreInstanceState( ) onCreate(null) : non-null Bundle

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onSaveInstanceState( ) onCreate( ) onRestoreInstanceState( ) onCreate(null) : non-null Bundle

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What to save? Non persistent or non reconstructible info

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1 public class SearchActivity extends Activity {! 2 ! 3 private static final String STATE_OUTWARD = "state:outward";! 4 ! 5 private DateComponents mOutward;! 6 ! 7 @Override! 8 protected void onCreate(Bundle inState) {! 9 super.onCreate(inState);! 10 ! 11 if (inState != null) {! 12 DateComponents components = inState.getParcelable(STATE_OUTWARD);! 13 if (components != null) {! 14 setOutward(components);! 15 }! 16 }! 17 }! 18 ! 19 @Override! 20 protected void onSaveInstanceState(Bundle outState) {! 21 super.onSaveInstanceState(outState);! 22 outState.putParcelable(STATE_OUTWARD, mOutward);! 23 }! 24 }

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onSaveInstanceState saves Window

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Window Fragments onSaveInstanceState saves

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Window Fragments Dialogs onSaveInstanceState saves

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Always call the SUPER METHODS Android has no guards on save-related methods

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android:stateNotNeeded For restart-on-crash apps only (i.e. launcher app)

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Developer options Don’t keep activities

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Developer options Don’t keep activities

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View level state restoration

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Android saves UI state AUTOMAGICALLY

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Android saves UI state AUTOMAGICALLY (aka “It just works!™”)

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Android saves UI state AUTOMAGICALLY (aka “It just works!™”) …except in some cases

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Works out-of-the-box if Views 1. Have an ID 2. Are “save” enabled 3. Come from the framework

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saveHierarchyState() It always begins with a call to

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EditText! @id/text RelativeLayout! @id/container CheckBox! @id/check_box TextView

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EditText! @id/text RelativeLayout! @id/container CheckBox! @id/check_box TextView SparseArray

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RelativeLayout! @id/container CheckBox! @id/check_box TextView SparseArray S1 EditText! @id/text

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@id/container SparseArray RelativeLayout! @id/container CheckBox! @id/check_box TextView S1 EditText! @id/text

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EditText! @id/text RelativeLayout! @id/container CheckBox! @id/check_box TextView @id/container S1 S2 SparseArray

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@id/text @id/container S1 S2 SparseArray RelativeLayout! @id/container CheckBox! @id/check_box TextView EditText! @id/text

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RelativeLayout! @id/container CheckBox! @id/check_box TextView @id/text @id/container S1 S2 SparseArray S3 EditText! @id/text

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@id/text @id/container @id/check_box S1 S2 S3 SparseArray RelativeLayout! @id/container CheckBox! @id/check_box TextView EditText! @id/text

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Controlling save setSaveEnabled(boolean) setSaveFromParentEnabled(boolean)

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restoreHierarchyState() It always ends with a call to

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@id/container S1 @id/text S2 @id/check_box S3 SparseArray RelativeLayout! @id/container CheckBox! @id/check_box TextView EditText! @id/text

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@id/container S1 S2 S3 @id/text @id/check_box SparseArray RelativeLayout! @id/container CheckBox! @id/check_box TextView EditText! @id/text

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RelativeLayout! @id/container CheckBox! @id/check_box TextView @id/container S1 S2 @id/check_box S3 S1 @id/text SparseArray EditText! @id/text

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@id/container S1 @id/text S2 @id/check_box S3 SparseArray RelativeLayout! @id/container CheckBox! @id/check_box TextView EditText! @id/text

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EditText! @id/text RelativeLayout! @id/container CheckBox! @id/check_box TextView S2 @id/container S1 @id/text @id/check_box S3 S2 SparseArray

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@id/container S1 @id/text S2 @id/check_box S3 SparseArray RelativeLayout! @id/container CheckBox! @id/check_box TextView EditText! @id/text

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RelativeLayout! @id/container CheckBox! @id/check_box TextView @id/container S1 @id/text S2 @id/check_box S3 S3 SparseArray EditText! @id/text

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Ensure your Views’ IDs are unique & constant

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1 static class SavedState extends BaseSavedState {! 2 int checked;! 3 ! 4 SavedState(Parcelable superState) { super(superState); }! 5 ! 6 private SavedState(Parcel in) {! 7 super(in);! 8 checked = in.readInt();! 9 }! 10 ! 11 @Override! 12 public void writeToParcel(Parcel out, int flags) {! 13 super.writeToParcel(out, flags);! 14 out.writeInt(checked);! 15 }! 16 ! 17 public static final Parcelable.Creator CREATOR = ! 18 new Parcelable.Creator() {! 19 public SavedState createFromParcel(Parcel in) {! 20 return new SavedState(in);! 21 }! 22 public SavedState[] newArray(int size) {! 23 return new SavedState[size];! 24 }! 25 };! 26 }

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1 @Override! 2 public Parcelable onSaveInstanceState() {! 3 final Parcelable superState = super.onSaveInstanceState();! 4 SavedState ss = new SavedState(superState);! 5 ss.checked = isChecked() ? 1 : 0;! 6 return ss;! 7 }! 8 ! 9 @Override! 10 public void onRestoreInstanceState(Parcelable state) {! 11 SavedState ss = (SavedState) state;! 12 super.onRestoreInstanceState(ss.getSuperState());! 13 setChecked(ss.checked == 1);! 14 }

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FrameLayout! @id/root ImageBox! @id/container1 CheckBox! @id/check_box ImageView! @id/image ImageBox! @id/container2 CheckBox! @id/check_box ImageView! @id/image

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FrameLayout! @id/root ImageBox! @id/container1 ImageView! @id/image CheckBox! @id/check_box ImageBox! @id/container2 ImageView! @id/image CheckBox! @id/check_box Custom views with children with same IDs

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1 static class SavedState extends BaseSavedState {! 2 ! 3 SparseArray childrenStates;! 4 ! 5 SavedState(Parcelable superState) { super(superState); }! 6 ! 7 private SavedState(Parcel in, ClassLoader loader) {! 8 super(in);! 9 childrenStates = in.readSparseArray(loader);! 10 }! 11 ! 12 @Override! 13 public void writeToParcel(Parcel out, int flags) {! 14 super.writeToParcel(out, flags);! 15 out.writeSparseArray(childrenStates);! 16 }! 17 ! 18 public static final Creator CREATOR = ParcelableCompat.! 19 newCreator(new ParcelableCompatCreatorCallbacks() {! 20 @Override! 21 public SavedState createFromParcel(Parcel source, ClassLoader loader) {! 22 return new SavedState(source, loader);! 23 }! 24 @Override! 25 public SavedState[] newArray(int size) {! 26 return new SavedState[size];! 27 }! 28 });! 29 }

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1 @Override! 2 public Parcelable onSaveInstanceState() {! 3 final Parcelable superState = super.onSaveInstanceState();! 4 SavedState ss = new SavedState(superState);! 5 ss.childrenStates = new SparseArray();! 6 for (int i = 0; i < getChildCount(); i++) {! 7 getChildAt(i).saveHierarchyState(ss.childrenStates);! 8 }! 9 return ss;! 10 }! 11 ! 12 @Override! 13 public void onRestoreInstanceState(Parcelable state) {! 14 SavedState ss = (SavedState) state;! 15 super.onRestoreInstanceState(ss.getSuperState());! 16 for (int i = 0; i < getChildCount(); i++) {! 17 getChildAt(i).restoreHierarchyState(ss.childrenStates);! 18 }! 19 }

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That has solved nothing! Still need to block save/restore dispatch

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1 @Override! 2 protected void dispatchSaveInstanceState(SparseArray container) {! 3 dispatchFreezeSelfOnly(container);! 4 }! 5 ! 6 @Override! 7 protected void dispatchRestoreInstanceState(SparseArray container) {! 8 dispatchThawSelfOnly(container);! 9 }

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Fragment level state restoration

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Very similar to Activities state restoration lifecycle. (Fragments are tied to Activity after all)

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Fragment blocks Activity save mechanism with framework setSaveFromParentEnabled(false) with support library NoSaveStateFrameLayout

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2 distinct states Fragment + View common case View only detach, addToBackStack, etc.

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Can to be used to create smooth transitions between your Activities: ! - Save the state SA of A - Start B with no animations passing SA - Apply SA to B - Transition between A and B was smooth Leveraging save/restore

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Summarizing in three rules

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Always save the state An Android app must survive configuration changes & low memory conditions.

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Only save essential info Only save info that is non persistent or can not be reconstructed later.

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Use correct levels Save instance states at the appropriate component level: Activity, Fragment or View.

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Thank you! @cyrilmottier cyrilmottier.com

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