Unit Test Your Custom Parcelable

Your Android application needs to pass some custom data between processes in an Intent. You will be calling Intent.putExtra(String, Parcelable). It would be easier to test if the Parcel class where not final with only private constructors. That keeps us from testing the methods individually. We are forced to perform only round trip testing -- write our Parcelable to a Parcel, then call the creator with the same Parcel instance.

There is one important step right in the middle of the round trip. The Parcel needs to be reset to be ready for read. Think of this just like working with java.nio.ByteBuffer. With ByteBuffer, when you are done writing, you call flip. With Parcel, when you want to read -- call setDataPosition(0). Here is a sample test.

I'll decoreate a Person object as a ParcelablePerson that implements the Parcelable interface and has the requisite public static final CREATOR. Following TDD I've only created enough of the implementation code below to make the test compile.

    public void testPersonTakesRoundTripThroughParcel() throws Exception {
        Person testPerson = new Person();
        ParcelablePerson testObject = new ParcelablePerson(testPerson);
        Parcel parcel = Parcel.obtain();
        testObject.writeToParcel(parcel, 0);
        //done writing, now reset parcel for reading
        parcel.setDataPosition(0);
        //finish round trip
        ParcelablePerson createFromParcel = ParcelablePerson.CREATOR.createFromParcel(parcel);
       
        assertEquals(testPerson, createFromParcel.getPerson());
    }
    public class Person {
       ...
    }
    public class ParcelablePerson implements Parcelable {
        private Person person;
        public ParcelablePerson(Person person) {
            this.person = person;
        }
       
        public Person getPerson() {
            return person;
        }

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

        @Override
        public void writeToParcel(Parcel dest, int flags) {
            //call dest.write... methods
        }

        public static final Creator CREATOR = new Creator() {
            @Override
            public ParcelablePerson createFromParcel(Parcel source) {
                return null;
            }

            @Override
            public ParcelablePerson[] newArray(int size) {
                return null;
            }};
    }