Parcelable where/when is describeContents() used?
Does anyone know where/when this method of a Parcelable is called?
@Override
public int describeCon开发者_如何学Ctents() {
return 0;
}
It has to be overriden. But should I consider doing something useful with it?
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... :-)
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.
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...
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