Using Parcelable with circular references

Question

It appears that Parcelable doesn\'t gracefully handle circular references like Serializable does. In the following example, the Serialization of Bar works just fine, but writin

Answer 1

Perhaps the answer lies in a more intelligent set of writeToParcel and createFromParcel methods?

Off the top of my head, you could keep a list of objects you had already fully written to a given Parcel and identify them only by a tag (their local identityHashCode(), perhaps). (Note that this is not a global list, it is explicitly per-Parcel; perhaps itself stored via a semi-global Map > ? You'd need to be sure the set was forgotten once the parcel was fully written.)

The relevant bit of writeToParcel() would look something like this:

HashSet set = getWrittenSetFor(dest);
final int tag = identityHashCode();
if (set.contains(tag)) {
    // Already sent
    dest.writeInt(tag);
} else {
    set.put(tag);
    dest.writeInt(tag);
    dest.writeValue(this);
}

The corresponding createFromParcel() would be slightly more complex.

I expect there are lurking problems with this method, but it's where I'd start. As I've put it here, it depends on identityHashCode() being guaranteed to be different for different objects - it usually is on 32-bit JVMs (being the value of the underlying C++ pointer). Plain hashCode() might be worthwhile (perhaps with the addition of typing information?), or perhaps some sort of serial number.

Another option might be to just plain serialize your objects to a byte[] and write that into the Parcel, but it strikes me as a bit inefficient...