How to use Parcel in Android?

Question

I'm trying to use Parcel to write and then read back a Parcelable. For some reason, when I read the object back from the file, it's coming back as null.

public void testFoo() {
    final Foo orig = new Foo("blah blah");

    // Wrote orig to a parcel and then byte array
    final Parcel p1 = Parcel.obtain();
    p1.writeValue(orig);
    final byte[] bytes = p1.marshall();


    // Check to make sure that the byte array seems to contain a Parcelable
    assertEquals(4, bytes[0]); // Parcel.VAL_PARCELABLE


    // Unmarshall a Foo from that byte array
    final Parcel p2 = Parcel.obtain();
    p2.unmarshall(bytes, 0, bytes.length);
    final Foo result = (Foo) p2.readValue(Foo.class.getClassLoader());


    assertNotNull(result); // FAIL
    assertEquals( orig.str, result.str );
}


protected static class Foo implements Parcelable {
    protected static final Parcelable.Creator<Foo> CREATOR = new Parcelable.Creator<Foo>() {
        public Foo createFromParcel(Parcel source) {
            final Foo f = new Foo();
            f.str = (String) source.readValue(Foo.class.getClassLoader());
            return f;
        }

        public Foo[] newArray(int size) {
            throw new UnsupportedOperationException();
        }

    };


    public String str;

    public Foo() {
    }

    public Foo( String s ) {
        str = s;
    }

    public int describeContents() {
        return 0;
    }

    public void writeToParcel(Parcel dest, int ignored) {
        dest.writeValue(str);
    }


}

What am I missing?

UPDATE: To simplify the test I've removed the reading and writing of files in my original example.

Answer 1

If the object in question is already Serializable, why are you bothering with Parcel? Why not just persist the Serializable and be done with the matter?

Answer 2

Ah, I finally found the problem. There were two in fact.

1. CREATOR must be public, not protected. But more importantly,
2. You must call setDataPosition(0) after unmarshalling your data.

Here is the revised, working code:

public void testFoo() {
    final Foo orig = new Foo("blah blah");
    final Parcel p1 = Parcel.obtain();
    final Parcel p2 = Parcel.obtain();
    final byte[] bytes;
    final Foo result;

    try {
        p1.writeValue(orig);
        bytes = p1.marshall();

        // Check to make sure that the byte stream seems to contain a Parcelable
        assertEquals(4, bytes[0]); // Parcel.VAL_PARCELABLE

        p2.unmarshall(bytes, 0, bytes.length);
        p2.setDataPosition(0);
        result = (Foo) p2.readValue(Foo.class.getClassLoader());

    } finally {
        p1.recycle();
        p2.recycle();
    }


    assertNotNull(result);
    assertEquals( orig.str, result.str );

}

protected static class Foo implements Parcelable {
    public static final Parcelable.Creator<Foo> CREATOR = new Parcelable.Creator<Foo>() {
        public Foo createFromParcel(Parcel source) {
            final Foo f = new Foo();
            f.str = (String) source.readValue(Foo.class.getClassLoader());
            return f;
        }

        public Foo[] newArray(int size) {
            throw new UnsupportedOperationException();
        }

    };


    public String str;

    public Foo() {
    }

    public Foo( String s ) {
        str = s;
    }

    public int describeContents() {
        return 0;
    }

    public void writeToParcel(Parcel dest, int ignored) {
        dest.writeValue(str);
    }


}

Answer 3

Beware! Dont use Parcel for serialization to a file

http://developer.android.com/reference/android/os/Parcel.html