Android Parcelable Example

Few days back I had a requirement to send a ArrayList of my Custom Class Objects through Android Intent, I guess most of you also find this requirement now and then. I never thought it can be that tricky.

There are built in functions in Intent to send ArrayList of primitive objects e.g. String, Integer, but when it comes to Custom Data Handling Objects, BOOM… you need to take that extra pain!

Android has defined a new light weight IPC (Inter Process Communication) data structure called Parcel, where you can flatten your objects in byte stream, same as J2SDK’s Serialization concept.

So let’s come back to my original requirement, I had a Data Handling class, which groups together a set of information-

public class ParcelData {
      int id;
      String name;
      String desc;
      String[] cities = {"suwon", "delhi"};
}

I want an ArrayList<ParcelData> of Data Handling objects to be able to pass through Intent. To do that, I can’t use the ParcelData as it is; you need to implement an Interface android.os.Parcelable which will make Objects of this class Parcelable. So you need to define your data handling class as-

public class ParcelData implements Parcelable {
      int id;
      String name;
      String desc;
      String[] cities = {"suwon", "delhi"};
}

You need to overwrite 2 methods of android.os.Parcelable Interface-

• describeContents()- define the kind of object you are going to Parcel, you can use the hashCode() here.
• writeToParcel(Parcel dest, int flags)- actual object serialization/flattening happens here. You need to individually Parcel each element of the object.
  

public void writeToParcel(Parcel dest, int flags) {
      Log.v(TAG, "writeToParcel..."+ flags);
      dest.writeInt(id);
      dest.writeString(name);
      dest.writeString(desc);
      dest.writeStringArray(cities);
}

Note: Don’t try things like, dest.writeValue(this) to flatten the complete Object at a time (I don’t think all people have some weird imaginations like me, so you’ll never try this, right ?…) that will end up in recursive call of writeToParcel().

Up to this point you are done with the required steps to flatten/serialize your custom object.

Next, you need to add steps to un-marshal/un-flatten/de-serialize (whatever you call it) your custom data objects from Parcel. You need to define one weird variable called CREATOR of type Parcelable.Creator. If you don’t do that, Android framework will throw exception-

Parcelable protocol requires a Parcelable.Creator object called CREATOR

Following is a sample implementation of Parcelable.Creator<ParcelData> interface for my class ParcelData.java

/**
 * It will be required during un-marshaling data stored in a Parcel
 * @author prasanta
 */
public class MyCreator implements Parcelable.Creator<ParcelData> {
      public ParcelData createFromParcel(Parcel source) {
            return new ParcelData(source);
      }
      public ParcelData[] newArray(int size) {
            return new ParcelData[size];
      }
}

You need to define a Constructor in ParcelData.java which puts together all parceled data back

/**
* This will be used only by the MyCreator
* @param source
*/
public ParcelData(Parcel source){
    /*
     * Reconstruct from the Parcel
     */
    Log.v(TAG, "ParcelData(Parcel source): time to put back parcel data");
    id = source.readInt();
    name = source.readString();
    desc = source.readString();
    source.readStringArray(cities);
}

You can deserve some rest now…...as you are done with it. So now you can use, something like

Intent parcelIntent = new Intent(this, ParcelActivity.class);
ArrayList<ParcelData> dataList = new ArrayList<ParcelData>();
/*
 * Add elements in dataLists e.g.
 * ParcelData pd1 = new ParcelData();
 * ParcelData pd2 = new ParcelData();
 *
 * fill in data in pd1 and pd2
 *
 * dataLists.add(pd1);
 * dataLists.add(pd2);
 */
parcelIntent.putParcelableArrayListExtra("custom_data_list", data);

So, how hard is that? :-). Share your comments, I'm listening.