Parcelable where/when is describeContents() used?

Question

Does anyone know where/when this method of a Parcelable is called?

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

It has to be overriden. But should I consider doing something useful with it?

Answer 1

There is a constant defined in Parcelable called CONTENTS_FILE_DESCRIPTOR which is meant to be used in describeContents() to create bitmask return value.

Description for CONTENTS_FILE_DESCRIPTOR in the API ref is:

Bit masks for use with describeContents(): each bit represents a kind of object considered to have potential special significance when marshalled.

Which really means: If you need to put FileDescriptor object into Parcelable you should/must specify CONTENTS_FILE_DESCRIPTOR as return value of describeContents(), i.e. by "special object" (in describeContents()'s description) they really mean: FileDescriptor.

This whole Parcelable functionality looks unfinished (read: has bad design). There is one other strange thing in the docs:

Classes implementing the Parcelable interface must also have a static field called CREATOR, which is an object implementing the Parcelable.Creator interface

Implementing multiple inheritance by rules defined in human readable form? :-)

It seems like C++ programmer designed Parceable and at some point he realized: Oh, damn, there is no multiple inheritance in Java... :-)

Answer 2

There is only two possible value, 0 or CONTENTS_FILE_DESCRIPTOR

if you are serializing POLO, this value should always be 0, the CONTENTS_FILE_DESCRIPTOR is reserved for ParcelFileDescriptor, which could serialize a File Descriptor(FD) in *unix system.

Answer 3

From android framework, the only usage occurs in ActivityManagerService.java:

//ActivityManagerService.java
public int startActivityIntentSender(IApplicationThread caller,
    IntentSender intent, Intent fillInIntent, String resolvedType,
    IBinder resultTo, String resultWho, int requestCode,
    int flagsMask, int flagsValues) {
   // Refuse possible leaked file descriptors
   if (fillInIntent != null && fillInIntent.hasFileDescriptors()) {
       throw new IllegalArgumentException("File descriptors passed in Intent");
   }
   //...
}

Intent.java hasFileDescriptors() comes from Bundle.java hasFileDescriptors(). And the bundle will iterator all data in mMap(hashMap) or mParcelledData(Parcel). We will figure out intent.hasFileDescriptors() just wraps Parcel/Parcelable describeContents().

While, maybe this is the only usage for describeContents() : it use to filter FileDescriptor from Intent pass...