android中Parcelable接口的使用

一、理解

Parcelable是一個接口、用來實現序列化。與此類似的還有一個接口Serializable，這是JavaSE本身支持的，而Parcelable是android特有的。二者比較：

1、Parcelable使用起來稍復雜點，而后者使用起來非常簡單。下面例子中會看到。

2、Parcelable效率比Serializable高，支持Intent數據傳遞，也支持進程間通信（IPC）。

3、Parcelable使用時要用到一個Parcel，可以簡單將其看為一個容器，序列化時將數據寫入Parcel，反序列化時從中取出。

4、在使用內存的時候，Parcelable比Serializable性能高，所以推薦使用Parcelable。Serializable在序列化的時候會產生大量的臨時變量，從而引起頻繁的GC。Parcelable不能使用在要將數據存儲在磁盤上的情況，因為Parcelable在外界有變化的情況下不能很好的保證數據的持續性。盡管Serializable效率低點，但此時還是建議使用Serializable 。

二、源碼

結合源碼及注釋進行理解：

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 static field called <code>CREATOR</code>, which
 * is an object implementing the {@link Parcelable.Creator Parcelable.Creator}
 * interface.
 *
 * <p>A typical implementation of Parcelable is:</p>
 *
 * <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.
     *
     * @return a bitmask indicating the set of special object types marshalled
     * by the Parcelable.
     */
    public int describeContents();

    /**
     * Flatten this object in to a Parcel.
     *
     * @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.
     */
    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);
    }
}

三、示例

Student：

package com.home.testparcelable;

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

public class Student implements Parcelable {
    private int id;
    private String name;

    public Student() {
        super();
    }

    public Student(int id, String name) {
        super();
        this.id = id;
        this.name = name;
    }

    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() {
        return 0;
    }

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        // 序列化過程：必須按成員變量聲明的順序進行封裝
        dest.writeInt(id);
        dest.writeString(name);
    }

    // 反序列過程：必須實現Parcelable.Creator接口，並且對象名必須為CREATOR
    // 讀取Parcel里面數據時必須按照成員變量聲明的順序，Parcel數據來源上面writeToParcel方法，讀出來的數據供邏輯層使用
    public static final Parcelable.Creator<Student> CREATOR = new Creator<Student>() {

        @Override
        public Student createFromParcel(Parcel source) {
            return new Student(source.readInt(), source.readString());
        }

        @Override
        public Student[] newArray(int size) {
            return new Student[size];
        }
    };

}

MainActivity：

package com.home.testparcelable;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        Button btn = (Button) findViewById(R.id.main_btn);
        btn.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View arg0) {
                Intent in = new Intent(MainActivity.this, SecondActivity.class);
                in.putExtra("stu", new Student(1, "張三"));
                startActivity(in);
            }
        });
    }
}

SecondActivity：

package com.home.testparcelable;

import android.app.Activity;
import android.os.Bundle;

public class SecondActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Student stu = getIntent().getParcelableExtra("stu");
        System.out.println("id:" + stu.getId());
        System.out.println("name:" + stu.getName());
    }
}