android中Parcelable接口的使用

android中Parcelable接口的使用

1. 简介

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 。

二、源码解析

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 CREATOR, which 
 * is an object implementing the {@link Parcelable.Creator 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 CREATOR 
 *             = new Parcelable.Creator() { 
 *         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(); 
 *     } 
 * }
 
 */  
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 
     * "Parcelable someFunction()", 
     * "void someFunction(out Parcelable)", or 
     * "void someFunction(inout Parcelable)".  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 {  
        /** 
         * 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 extends Creator {  
        /** 
         * 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类:

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) {  
        // 序列化过程：必须按成员变量声明的顺序进行封装  
        dest.writeInt(id);  
        dest.writeString(name);  
    }  

    // 反序列过程：必须实现Parcelable.Creator接口，并且对象名必须为CREATOR  
    // 读取Parcel里面数据时必须按照成员变量声明的顺序，Parcel数据来源上面writeToParcel方法，读出来的数据供逻辑层使用  
    public static final Parcelable.Creator CREATOR = new Creator() {  

        @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, "李四"));  
                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());  
    }  
}