Fun with Parcelables

Imagine that you have a task to create wizard-like application on Android which consists of three pages and user must fill some information about himself on each page.

Basic implementation will contain Model and three Activities.

Model will look like this:

public class Model implements Serializable { // or Parcelable, it doesn't matter
  // Page #1 info
  public String nameFirst;
  public String nameLast;

  // Page #2 info
  public long birthday;

  // Page #3 info
  public String address;
}

And each activity will contain these lines:

public class Activity{N} extends Activity {
  
  private Model mModel;
  
  @Override
  public void onCreate(Bundle savedInstanceState) {
    mModel = (Model) getIntent().getSerializableExtra("model");
    // ...
  }

  private void goToNextScreen() {
    startActivity(new Intent(this, Activity{N+1}.class).putExtra("model", mModel));
  }

  // ...  
}

So far so good.

So whats wrong here?

Now imagine that user has filled form on screen #3 and went back to change something on screen #1. Everything seems OK until user goes forward again. All entered information on following screens will disappear. This is happening because Model is actually copied between activities and we're not updating previous Models with new information.

It can be solved, for example, by using startActivityForResult and passing newer Model as Activity's result in onBackKeyPressed method. This approche is good if we have simple data and few screens but in big projects it becames a nightmare to manage it.

We need some automated solution that is easy to use.

Lets play with Parcelables

A lot of people out there hate Parcelable. And they are not wrong - you have to write tons of code for each class for serialization and it is very easy, for example, to miss that one variable.

But Parcelable has one property that I always liked - Creator. With its help we can control not only object serialization but allocation as well.

Idea is following - after creating Model, we put it into a weak cache. During deserialization we will just take Model from that cache. If no Model is found there - just proceed with typical deserialiation. This way all Activities will share the same Model instance and if Activity was killed by Android - we just deserialize it.

For this we will need cache with weak references and base Parcelable class. For actual serialization I will use Jackson library but it can be anything (for example Serializable). For me Jackson library is a preferred way just because it is highly configurable.

Here is our base Parcelable class:

public class JsonParcelable implements Parcelable {

  private static final WeakCache<JsonParcelable> sCache = new WeakCache<>();

  @JsonIgnore
  private String mId;

  protected JsonParcelable() {
    mId = UUID.randomUUID().toString();
    sCache.put(getClass().getName() + "%" + mId, this);
  }

  @Override
  public int describeContents() {
    return 0;
  }

  @Override
  public void writeToParcel(Parcel parcel, int flags) {
    parcel.writeString(mId);
    parcel.writeString(getClass().getName());
    parcel.writeString(PackUtils.pack(this)); // serialize to JSON here
  }

  public static final Creator<JsonParcelable> CREATOR = new Creator<JsonParcelable>() {
    @Override
    public JsonParcelable createFromParcel(Parcel parcel) {
      try {
        String id = parcel.readString();
        String className = parcel.readString();
        String json = parcel.readString();

        synchronized (sCache) {
          JsonParcelable object = sCache.get(className + "%" + id);
          if (object != null) {
            return object;
          }
          return (JsonParcelable) PackUtils.unpack(Class.forName(className), json); // deserialize from JSON here

        }
      } catch (Exception ex) {
        throw new RuntimeException(ex);
      }
    }

    @Override
    public JsonParcelable[] newArray(int size) {
      return new JsonParcelable[size];
    }
  };
}

And weak cache class:

public class WeakCache<T> {

  private Map<String, Ref<T>> mObjects = new HashMap<>();
  private ReferenceQueue<T> mQueue = new ReferenceQueue<>();

  public T get(String id) {
    Ref ref;
    while ((ref = (Ref) mQueue.poll()) != null) {
      mObjects.remove(ref.id);
    }

    T object = null;

    Ref<T> refT = mObjects.get(id);
    if (refT == null || (object = refT.get()) == null) {
      mObjects.remove(id);
    }
    return object;
  }

  public T put(String id, T object) {
    mObjects.put(id, new Ref<>(id, object, mQueue));
    return object;
  }

  static class Ref<T> extends WeakReference<T> {
    public final String id;

    Ref(String id, T r, ReferenceQueue<? super T> q) {
      super(r, q);
      this.id = id;
    }
  }
}

How to use it?

It is very easy to use it. All you need to do is just to extend JsonParcelable and pass that object as Parcelable through Intent. Everything else is done automatically.

So our model will look like this:

public class Model extends JsonParcelable {
  // ...
}

To start Activity just use:

startActivity(new Intent(...).putExtra(mModel));

And to get Model inside Activity:

mModel = getIntent().getParcelableExtra("model");

Thats it!!!