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Deep Dive Into Android State Restoration

Deep Dive Into Android State Restoration

Learn about how Android saves state in general in order to be able to restore an application in the exact same state the process was prior being killed because of a low memory condition or a configuration change. In this talk we mainly focus on the Parcelable and Parcel objects and how Android uses them to save/restore some important stateful information such as the complete UI state.

Cyril Mottier

September 22, 2014
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  1. 1 <activity! 2 android:name=".HomeActivity"! 3 android:configChanges="orientation! 4 ! 5 <!--

    Some annoying IntentFilters. -->! 6 ! 7 </activity> |locale">
  2. The content Primitives types Primitives arrays 1 parcel.writeInt(1);! 2 parcel.writeLong(2L);!

    3 parcel.writeFloat(3F);! 4 parcel.writeString("Hi!"); Parcelable
  3. 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
  4. 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 }
  5. 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<Suggestion> CREATOR = ! 14 new Parcelable.Creator<Suggestion>() {! 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 };
  6. Parcelable.Creator! The base creator interface ! Parcelable.ClassLoaderCreator! A creator with

    the ClassLoader passed on read. ! ParcelableCompat & ParcelableCompatCreatorCallbacks! Compatibility stuff
  7. 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 }
  8. Works out-of-the-box if Views 1. Have an ID 2. Are

    “save” enabled 3. Come from the framework
  9. 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<SavedState> CREATOR = ! 18 new Parcelable.Creator<SavedState>() {! 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 }
  10. 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 }
  11. 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
  12. 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<SavedState> CREATOR = ParcelableCompat.! 19 newCreator(new ParcelableCompatCreatorCallbacks<SavedState>() {! 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 }
  13. 1 @Override! 2 public Parcelable onSaveInstanceState() {! 3 final Parcelable

    superState = super.onSaveInstanceState();! 4 SavedState ss = new SavedState(superState);! 5 ss.childrenStates = new SparseArray<Parcelable>();! 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 }
  14. 1 @Override! 2 protected void dispatchSaveInstanceState(SparseArray<Parcelable> container) {! 3 dispatchFreezeSelfOnly(container);!

    4 }! 5 ! 6 @Override! 7 protected void dispatchRestoreInstanceState(SparseArray<Parcelable> container) {! 8 dispatchThawSelfOnly(container);! 9 }
  15. 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
  16. Only save essential info Only save info that is non

    persistent or can not be reconstructed later.
  17. Resources Dressed for Iceland • Cécile Bernard Moelwynion, Eryri, Cymru

    • Marc Poppleton Happy, Confused, Wink, Sad, Angry • Megan Sheehan Floppy-Disk • Alex Auda Samora Fonts Source Sans Pro Courier