Using the Android Parcel

Parcel 是一种轻量级高效率的Inter-process communication (IPC)通信机制.使用Parcels，android系统可以将实体对象分解为可以在进程之间传递的基元对象。

Parcels同样可以进程之内的数据传输，例如，一个android有多个activities，activity之间使用Intent传递数据User，例如

// inside CollectDataActivity, construct intent to pass along the next Activity, i.e. screen
Intent in = new Intent(this, ProcessDataActivity.class);
in.putExtra("userid", id); // (key,value) pairs
in.putExtra("age", age);
in.putExtra("phone", phone);
in.putExtra("is_registered", true);

// call next Activity --> next screen comes up
startActivity(in);

但是我们可以以一种更加直接的方式，使用Parcelable 类在组件之间传递实体对象，例如：

In the first Activity:

// in CollectDataActivity, populate the Parcelable User object using its setter methods
User usr = new User();
usr.setId(id); // collected from user input// etc..

// pass it to another component
Intent in = new Intent(this, ProcessDataActivity.class);
in.putExtra("user", usr);
startActivity(in);

In the second Activity:

// in ProcessDataActivity retrieve User 
Intent intent = getIntent();
User usr = (User) intent.getParcelableExtra("user");

要达到这样的目的，我们只需要User类实现Parcelable 接口

　　1 使User类实现Parcelable 接口

　　2 实现describeContents方法

　　3 实现抽象方法writeToParcel，该方法将User实体写入Parcel

　　4 创建静态CREATOR方法，实现Parcelable.Creator接口

　　5 添加一个构造函数，将Pracel转换为User对象，读取的顺序和writeToParcel方法中写入的顺序一致

import android.os.Parcel;
import android.os.Parcelable;

public class User implements Parcelable {
    private long id;
    private int age;
    private String phone;
    private boolean registered;

    // No-arg Ctor
    public User(){}

    // all getters and setters go here //...

    /** Used to give additional hints on how to process the received parcel.*/
    @Override
    public int describeContents() {
    // ignore for now
    return 0;
    }

    @Override
    public void writeToParcel(Parcel pc, int flags) {
    pc.writeLong(id);
    pc.writeInt(age);
    pc.writeString(phone);
    pc.writeInt( registered ? 1 :0 );
   }

   /** Static field used to regenerate object, individually or as arrays */
  public static final Parcelable.Creator<User> CREATOR = new Parcelable.Creator<User>() {
         public User createFromParcel(Parcel pc) {
             return new User(pc);
         }
         public User[] newArray(int size) {
             return new User[size];
         }
   };

   /**Ctor from Parcel, reads back fields IN THE ORDER they were written */
   public User(Parcel pc){
       id         = pc.readLong();
       age        =  pc.readInt();
       phone      = pc.readString();
       registered = ( pc.readInt() == 1 );
  }
}

不要使用Pcarcel传递图片信息，更加高效的方式传递图片的URI或者Resource ID。

另外Pcarcel不能用于序列化存储，它是一种高效的传输方式，但是不能取代传统的序列化存储机制