See Also: Parcel Members
Container for a message (data and object references) that can be sent through an IBinder. A Parcel can contain both flattened data that will be unflattened on the other side of the IPC (using the various methods here for writing specific types, or the general Android.OS.Parcelable interface), and references to live Android.OS.BinderConsts objects that will result in the other side receiving a proxy IBinder connected with the original IBinder in the Parcel.
Parcel is not a general-purpose serialization mechanism. This class (and the corresponding Android.OS.Parcelable API for placing arbitrary objects into a Parcel) is designed as a high-performance IPC transport. As such, it is not appropriate to place any Parcel data in to persistent storage: changes in the underlying implementation of any of the data in the Parcel can render older data unreadable.
The bulk of the Parcel API revolves around reading and writing data of various types. There are six major classes of such functions available.
The most basic data functions are for writing and reading primitive data types: Parcel.WriteByte(sbyte), Parcel.ReadByte, Parcel.WriteDouble(double), Parcel.ReadDouble, Parcel.WriteFloat(float), Parcel.ReadFloat, Parcel.WriteInt(int), Parcel.ReadInt, Parcel.WriteLong(long), Parcel.ReadLong, Parcel.WriteString(string), Parcel.ReadString. Most other data operations are built on top of these. The given data is written and read using the endianess of the host CPU.
There are a variety of methods for reading and writing raw arrays of primitive objects, which generally result in writing a 4-byte length followed by the primitive data items. The methods for reading can either read the data into an existing array, or create and return a new array. These available types are:
The Android.OS.Parcelable protocol provides an extremely efficient (but low-level) protocol for objects to write and read themselves from Parcels. You can use the direct methods Parcel.WriteParcelable(IParcelable, Android.OS.ParcelableWriteFlags) and Parcel.ReadParcelable(Java.Lang.ClassLoader) or Parcel.writeParcelableArray(T[], int) and Parcel.ReadParcelableArray(Java.Lang.ClassLoader) to write or read. These methods write both the class type and its data to the Parcel, allowing that class to be reconstructed from the appropriate class loader when later reading.
There are also some methods that provide a more efficient way to work with Parcelables: Parcel.writeTypedArray(T[], int), Parcel.writeTypedList(java.util.List<T>), Parcel.readTypedArray(T[], android.os.Parcelable.Creator<T>) and Parcel.readTypedList(java.util.List<T>, android.os.Parcelable.Creator<T>). These methods do not write the class information of the original object: instead, the caller of the read function must know what type to expect and pass in the appropriate Android.OS.IParcelableCreator instead to properly construct the new object and read its data. (To more efficient write and read a single Parceable object, you can directly call Parcelable.writeToParcel(android.os.Parcel, int) and IParcelableCreator.CreateFromParcel(Parcel) yourself.)
A special type-safe container, called Android.OS.Bundle, is available for key/value maps of heterogeneous values. This has many optimizations for improved performance when reading and writing data, and its type-safe API avoids difficult to debug type errors when finally marshalling the data contents into a Parcel. The methods to use are Parcel.WriteBundle(Bundle), Parcel.ReadBundle, and Parcel.ReadBundle(Java.Lang.ClassLoader).
An unusual feature of Parcel is the ability to read and write active objects. For these objects the actual contents of the object is not written, rather a special token referencing the object is written. When reading the object back from the Parcel, you do not get a new instance of the object, but rather a handle that operates on the exact same object that was originally written. There are two forms of active objects available.
Android.OS.Binder objects are a core facility of Android's general cross-process communication system. The Android.OS.BinderConsts interface describes an abstract protocol with a Binder object. Any such interface can be written in to a Parcel, and upon reading you will receive either the original object implementing that interface or a special proxy implementation that communicates calls back to the original object. The methods to use are Parcel.WriteStrongBinder(IBinder), Parcel.WriteStrongInterface(IInterface), Parcel.ReadStrongBinder, Parcel.WriteBinderArray(IBinder[]), Parcel.ReadBinderArray(IBinder[]), Parcel.CreateBinderArray, Parcel.writeBinderList(java.util.List<android.os.IBinder>), Parcel.readBinderList(java.util.List<android.os.IBinder>), Parcel.CreateBinderArrayList.
FileDescriptor objects, representing raw Linux file descriptor identifiers, can be written and Android.OS.ParcelFileDescriptor objects returned to operate on the original file descriptor. The returned file descriptor is a dup of the original file descriptor: the object and fd is different, but operating on the same underlying file stream, with the same position, etc. The methods to use are Parcel.WriteFileDescriptor(Java.IO.FileDescriptor), Parcel.ReadFileDescriptor.
A final class of methods are for writing and reading standard Java containers of arbitrary types. These all revolve around the Parcel.WriteValue(Java.Lang.Object) and Parcel.ReadValue(Java.Lang.ClassLoader) methods which define the types of objects allowed. The container methods are Parcel.WriteArray(Java.Lang.Object[]), Parcel.ReadArray(Java.Lang.ClassLoader), Parcel.WriteList(IList), Parcel.ReadList(IList, Java.Lang.ClassLoader), Parcel.ReadArrayList(Java.Lang.ClassLoader), Parcel.WriteMap(IDictionary), Parcel.ReadMap(IDictionary, Java.Lang.ClassLoader), Parcel.writeSparseArray(android.util.SparseArray<java.lang.Object>), Parcel.ReadSparseArray(Java.Lang.ClassLoader).