Android API之android.os.Parcelable

android.os.Parcelable

Interface for classes whose instances can be written to and restored from a Parcel. Classes implementing the Parcelable interface must also have a static field called CREATOR, which is an object implementing the Parcelable.Creator interface.

A typical implementation of Parcelable is:

public class MyParcelable implements Parcelable {
     private int mData;

     public int describeContents() {
         return 0;
     }

     public void writeToParcel(Parcel out, int flags) {
         out.writeInt(mData);
     }

     public static final Parcelable.Creator<MyParcelable> CREATOR
             = new Parcelable.Creator<MyParcelable>() {
         public MyParcelable createFromParcel(Parcel in) {
             return new MyParcelable(in);
         }

         public MyParcelable[] newArray(int size) {
             return new MyParcelable[size];
         }
     };
    
     private MyParcelable(Parcel in) {
         mData = in.readInt();
     }
}

android.os.Parcel

要点：它是能够通过IBinder发送的message容器。Parcel可能包含：1. flattened数据(在IPC的另一端unflattened)。2. live IBinder对象的引用。

Parcel并非通用的serialization机制。

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 Parcelable interface), and references to live IBinder 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 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.

Primitives
The most basic data functions are for writing and reading primitive data types: writeByte, readByte, writeDouble, readDouble, writeFloat, readFloat, writeInt, readInt, writeLong, readLong, writeString, 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.

Primitive Arrays
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:

writeBooleanArray(boolean[]), readBooleanArray(boolean[]), createBooleanArray()
writeByteArray(byte[]), writeByteArray(byte[], int, int), readByteArray(byte[]), createByteArray()
writeCharArray(char[]), readCharArray(char[]), createCharArray()
writeDoubleArray(double[]), readDoubleArray(double[]), createDoubleArray()
writeFloatArray(float[]), readFloatArray(float[]), createFloatArray()
writeIntArray(int[]), readIntArray(int[]), createIntArray()
writeLongArray(long[]), readLongArray(long[]), createLongArray()
writeStringArray(String[]), readStringArray(String[]), createStringArray().
writeSparseBooleanArray(SparseBooleanArray), readSparseBooleanArray().
Parcelables
The 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 writeParcelable(Parcelable, int) and readParcelable(ClassLoader) or writeParcelableArray and readParcelableArray(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: writeTypedArray, writeTypedList(List), readTypedArray and readTypedList. 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 Parcelable.Creator 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 and Parcelable.Creator.createFromParcel yourself.)

Bundles
A special type-safe container, called 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 writeBundle(Bundle), readBundle(), and readBundle(ClassLoader).

Active Objects
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.

Binder objects are a core facility of Android's general cross-process communication system. The IBinder 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 writeStrongBinder(IBinder), writeStrongInterface(IInterface), readStrongBinder(), writeBinderArray(IBinder[]), readBinderArray(IBinder[]), createBinderArray(), writeBinderList(List), readBinderList(List), createBinderArrayList().

FileDescriptor objects, representing raw Linux file descriptor identifiers, can be written and 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 writeFileDescriptor(FileDescriptor), readFileDescriptor().

Untyped Containers
A final class of methods are for writing and reading standard Java containers of arbitrary types. These all revolve around the writeValue(Object) and readValue(ClassLoader) methods which define the types of objects allowed. The container methods are writeArray(Object[]), readArray(ClassLoader), writeList(List), readList(List, ClassLoader), readArrayList(ClassLoader), writeMap(Map), readMap(Map, ClassLoader), writeSparseArray(SparseArray), readSparseArray(ClassLoader).