Parcelable接口的使用

转自：Kenny技术博客

什么是Parcelable ？

Parcelable是一个接口、用来实现序列化。与此类似的还有一个接口Serializable，这是JavaSE本身支持的，而Parcelable是Android特有的。二者比较：

1. Parcelable使用起来稍复杂点，而后者使用起来非常简单。下面例子中会看到。
2. Parcelable效率比Serializable高，支持Intent数据传递，也支持进程间通信（IPC）。
3. Parcelable使用时要用到一个Parcel，可以简单将其看为一个容器，序列化时将数据写入Parcel，反序列化时从中取出。
4. 在使用内存的时候，Parcelable比Serializable性能高，所以推荐使用Parcelable。Serializable在序列化的时候会产生大量的临时变量，从而引起频繁的GC。Parcelable不能使用在要将数据存储在磁盘上的情况，因为Parcelable在外界有变化的情况下不能很好的保证数据的持续性。尽管Serializable效率低点，但此时还是建议使用Serializable 。

Parcelable 传递对象

Android序列化对象主要有两种方法：

1.实现Serializable接口通过intent来传递对象-Serializable方式;

2.实现Parcelable接口,Parcelable是Android特有的功能，效率比实现Serializable接口高，像用于Intent数据传递也都支持，而且还可以用在进程间通信(IPC)，除了基本类型外，只有实现了Parcelable接口的类才能被放入Parcel中。

源码

package android.os;

/**
 * Interface for classes whose instances can be written to
 * and restored from a {@link Parcel}.  Classes implementing the Parcelable
 * interface must also have a non-null static field called <code>CREATOR</code>
 * of a type that implements the {@link Parcelable.Creator} interface.
 *
 * <p>A typical implementation of Parcelable is:</p>
 * Parcelable的典型实现是:
 * <pre>
 * 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();
 *     }
 * }</pre>
 */
public interface Parcelable {
    /**
     * Flag for use with {@link #writeToParcel}: the object being written
     * is a return value, that is the result of a function such as
     * "<code>Parcelable someFunction()</code>",
     * "<code>void someFunction(out Parcelable)</code>", or
     * "<code>void someFunction(inout Parcelable)</code>".  Some implementations
     * may want to release resources at this point.
     */
    public static final int PARCELABLE_WRITE_RETURN_VALUE = 0x0001;

    /**
     * Bit masks for use with {@link #describeContents}: each bit represents a
     * kind of object considered to have potential special significance when
     * marshalled.
     */
    public static final int CONTENTS_FILE_DESCRIPTOR = 0x0001;

    /**
     * Describe the kinds of special objects contained in this Parcelable's
     * marshalled representation.
     * 内容描述接口，基本不用管,return 0;
     * @return a bitmask indicating the set of special object types marshalled
     * by the Parcelable.
     */
    public int describeContents();

    /**
     * Flatten this object in to a Parcel.
     * //写入接口函数，打包
     * @param dest The Parcel in which the object should be written.
     * @param flags Additional flags about how the object should be written.
     * May be 0 or {@link #PARCELABLE_WRITE_RETURN_VALUE}.
     */
    public void writeToParcel(Parcel dest, int flags);

    /**
     * Interface that must be implemented and provided as a public CREATOR
     * field that generates instances of your Parcelable class from a Parcel.
     */
    //读取接口，目的是要从Parcel中构造一个实现了Parcelable的类的实例处理。
    //因为实现类在这里还是不可知的，所以需要用到模板的方式，继承类名通过模板参数传入。
    //为了能够实现模板参数的传入，这里定义Creator嵌入接口,内含两个接口函数分别返回单个和多个继承类实例。
    public interface Creator<T> {
        /**
         * Create a new instance of the Parcelable class, instantiating it
         * from the given Parcel whose data had previously been written by
         * {@link Parcelable#writeToParcel Parcelable.writeToParcel()}.
         *
         * @param source The Parcel to read the object's data from.
         * @return Returns a new instance of the Parcelable class.
         */
        public T createFromParcel(Parcel source);

        /**
         * Create a new array of the Parcelable class.
         *
         * @param size Size of the array.
         * @return Returns an array of the Parcelable class, with every entry
         * initialized to null.
         */
        public T[] newArray(int size);
    }

    /**
     * Specialization of {@link Creator} that allows you to receive the
     * ClassLoader the object is being created in.
     */
    public interface ClassLoaderCreator<T> extends Creator<T> {
        /**
         * Create a new instance of the Parcelable class, instantiating it
         * from the given Parcel whose data had previously been written by
         * {@link Parcelable#writeToParcel Parcelable.writeToParcel()} and
         * using the given ClassLoader.
         *
         * @param source The Parcel to read the object's data from.
         * @param loader The ClassLoader that this object is being created in.
         * @return Returns a new instance of the Parcelable class.
         */
        public T createFromParcel(Parcel source, ClassLoader loader);
    }
}

怎么实现Parcelable接口？

从parcelable接口定义中，我们可以看到，实现parcelable接口，需要我们实现下面几个方法：

1. describeContents方法。内容接口描述，默认返回0就可以;

2. writeToParcel 方法。该方法将类的数据写入外部提供的Parcel中.即打包需要传递的数据到Parcel容器保存，以便从parcel容器获取数据，该方法声明如下：
   writeToParcel (Parcel dest, int flags)

3. 静态的Parcelable.Creator接口，本接口有两个方法：
   createFromParcel(Parcel in) 从Parcel容器中读取传递数据值，封装成Parcelable对象返回逻辑层。
   newArray(int size) 创建一个类型为T，长度为size的数组，仅一句话（return new T[size])即可。方法是供外部类反序列化本类数组使用。

代码实现

1.封装数据，把实现parcelable接口的Person对象传递到TwoActivity里;

public class DemoActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        // 封装数据
        Person p = new Person();
        p.setId(1);
        p.setName("xiaoming");
        // 用Intent传递Person对象
        Intent i = new Intent(this, TwoActivity.class);
        i.putExtra("Person", p);
        startActivity(i);
    }
}

2.TwoActivity获取数据，从DemoActivity传递的Person对象给解析

public class TwoActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);

        Person p = (Person)getIntent().getParcelableExtra("Person");

        System.out.println("p_id"+p.getId());
        System.out.println("p_name"+p.getName());
    }
}

3.parcelable接口的实现

public class Person implements Parcelable{

    // 成员变量
    private int id;
    private String name;

    // 1.必须实现Parcelable.Creator接口,否则在获取Person数据的时候，会报错，如下：
    // android.os.BadParcelableException:
    // Parcelable protocol requires a Parcelable.Creator object
    // called  CREATOR on class com.um.demo.Person
    // 2.这个接口实现了从Percel容器读取Person数据，并返回Person对象给逻辑层使用
    // 3.实现Parcelable.Creator接口对象名必须为CREATOR，不如同样会报错上面所提到的错；
    // 4.在读取Parcel容器里的数据事，必须按成员变量声明的顺序读取数据，不然会出现获取数据出错
    // 5.反序列化对象
    public static final Parcelable.Creator<Person> CREATOR = new Creator(){

        @Override
        public Person createFromParcel(Parcel source) {
            // TODO Auto-generated method stub
            // 必须按成员变量声明的顺序读取数据，不然会出现获取数据出错
            Person p = new Person();
            p.setId(source.readInt());
            p.setName(source.readString());
            return p;
        }

        @Override
        public Person[] newArray(int size) {
            // TODO Auto-generated method stub
            return new Person[size];
        }
    };

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public int describeContents() {
        // TODO Auto-generated method stub
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        // TODO Auto-generated method stub
        // 1.必须按成员变量声明的顺序封装数据，不然会出现获取数据出错
        // 2.序列化对象
        dest.writeInt(id);
        dest.writeString(name);
    }
}