Question

我正在尝试使用Parcel进行编写，然后回读Parcelable。出于某种原因，当我从文件中读回对象时，它会以null的形式返回。

public void testFoo() {
    final Foo orig = new Foo("blah blah");

    // Wrote orig to a parcel and then byte array
    final Parcel p1 = Parcel.obtain();
    p1.writeValue(orig);
    final byte[] bytes = p1.marshall();


    // Check to make sure that the byte array seems to contain a Parcelable
    assertEquals(4, bytes[0]); // Parcel.VAL_PARCELABLE


    // Unmarshall a Foo from that byte array
    final Parcel p2 = Parcel.obtain();
    p2.unmarshall(bytes, 0, bytes.length);
    final Foo result = (Foo) p2.readValue(Foo.class.getClassLoader());


    assertNotNull(result); // FAIL
    assertEquals( orig.str, result.str );
}


protected static class Foo implements Parcelable {
    protected static final Parcelable.Creator<Foo> CREATOR = new Parcelable.Creator<Foo>() {
        public Foo createFromParcel(Parcel source) {
            final Foo f = new Foo();
            f.str = (String) source.readValue(Foo.class.getClassLoader());
            return f;
        }

        public Foo[] newArray(int size) {
            throw new UnsupportedOperationException();
        }

    };


    public String str;

    public Foo() {
    }

    public Foo( String s ) {
        str = s;
    }

    public int describeContents() {
        return 0;
    }

    public void writeToParcel(Parcel dest, int ignored) {
        dest.writeValue(str);
    }


}

我错过了什么？

更新：为了简化测试，我删除了原始示例中的文件读取和写入。

Answer 1

啊，我终于找到了问题。实际上有两个。

CREATOR必须是公开的，不受保护。但更重要的是，
取消编组数据后，您必须致电setDataPosition(0)。

以下是修订后的工作代码：

public void testFoo() {
    final Foo orig = new Foo("blah blah");
    final Parcel p1 = Parcel.obtain();
    final Parcel p2 = Parcel.obtain();
    final byte[] bytes;
    final Foo result;

    try {
        p1.writeValue(orig);
        bytes = p1.marshall();

        // Check to make sure that the byte stream seems to contain a Parcelable
        assertEquals(4, bytes[0]); // Parcel.VAL_PARCELABLE

        p2.unmarshall(bytes, 0, bytes.length);
        p2.setDataPosition(0);
        result = (Foo) p2.readValue(Foo.class.getClassLoader());

    } finally {
        p1.recycle();
        p2.recycle();
    }


    assertNotNull(result);
    assertEquals( orig.str, result.str );

}

protected static class Foo implements Parcelable {
    public static final Parcelable.Creator<Foo> CREATOR = new Parcelable.Creator<Foo>() {
        public Foo createFromParcel(Parcel source) {
            final Foo f = new Foo();
            f.str = (String) source.readValue(Foo.class.getClassLoader());
            return f;
        }

        public Foo[] newArray(int size) {
            throw new UnsupportedOperationException();
        }

    };


    public String str;

    public Foo() {
    }

    public Foo( String s ) {
        str = s;
    }

    public int describeContents() {
        return 0;
    }

    public void writeToParcel(Parcel dest, int ignored) {
        dest.writeValue(str);
    }


}

Answer 2

小心！不要使用Parcel序列化到文件

Parcel不是通用序列化机制。此类（以及用于将任意对象放入包中的相应Parcelable API）被设计为高性能IPC传输。因此，将任何Parcel数据放入持久存储中是不合适的：Parcel中任何数据的底层实现的更改都可能导致旧数据不可读。

来自http://developer.android.com/reference/android/os/Parcel.html

Answer 3

我发现Parcelable最常用于数据包中的Android，但更具体地说是在发送和接收消息的Handler中。例如，您可能需要在后台运行AsyncTask或Runnable，但会将结果数据发布到主线程或Activity。

这是一个简单的例子。如果我的Runnable看起来像这样：

package com.example;

import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.Log;

import com.example.data.ProductInfo;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import com.squareup.okhttp.OkHttpClient;

public class AsyncRunnableExample extends Thread {
    public static final String KEY = "AsyncRunnableExample_MSG_KEY";

    private static final String TAG = AsyncRunnableExample.class.getSimpleName();
    private static final TypeToken<ProductInfo> PRODUCTINFO =
              new TypeToken<ProductInfo>() {
              };
    private static final Gson GSON = new Gson();

    private String productCode;
    OkHttpClient client;
    Handler handler;

    public AsyncRunnableExample(Handler handler, String productCode)
    {
        this.handler = handler;
        this.productCode = productCode;
        client = new OkHttpClient();
    }

    @Override
    public void run() {
        String url = "http://someserver/api/" + productCode;

        try
        {
            HttpURLConnection connection = client.open(new URL(url));
            InputStream is = connection.getInputStream();
            InputStreamReader isr = new InputStreamReader(is);

            // Deserialize HTTP response to concrete type.
            ProductInfo info = GSON.fromJson(isr, PRODUCTINFO.getType());

            Message msg = new Message();
            Bundle b = new Bundle();
            b.putParcelable(KEY, info);
            msg.setData(b);
            handler.sendMessage(msg);

        }
        catch (Exception err)
        {
            Log.e(TAG, err.toString());
        }

    }
}

正如您所看到的，此runnable在其构造函数中使用了Handler。这是从某些Activity调用的，如下所示：

static class MyInnerHandler extends Handler{
        WeakReference<MainActivity> mActivity;

        MyInnerHandler(MainActivity activity) {
            mActivity = new WeakReference<MainActivity>(activity);
        }

        @Override
        public void handleMessage(Message msg) {
            MainActivity theActivity = mActivity.get();
            ProductInfo info = (ProductInfo) msg.getData().getParcelable(AsyncRunnableExample.KEY);

            // use the data from the Parcelable 'ProductInfo' class here

            }
        }
    }
    private MyInnerHandler myHandler = new MyInnerHandler(this);

    @Override
    public void onClick(View v) {
        AsyncRunnableExample thread = new AsyncRunnableExample(myHandler, barcode.getText().toString());
        thread.start();
    }

现在，剩下的就是这个问题的核心，你如何将一个类定义为Parcelable。我选择了一个相当复杂的课程来展示，因为有些东西你不会用简单的东西看到。这是ProductInfo类，Parcels和unParcels干净利落：

public class ProductInfo implements Parcelable {

    private String brand;
    private Long id;
    private String name;
    private String description;
    private String slug;
    private String layout; 
    private String large_image_url;
    private String render_image_url;
    private String small_image_url;
    private Double price;
    private String public_url;
    private ArrayList<ImageGroup> images;
    private ArrayList<ProductInfo> related;
    private Double saleprice;
    private String sizes;
    private String colours;
    private String header;
    private String footer;
    private Long productcode;

    // getters and setters omitted here

    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeLong(id);
        dest.writeString(name);
        dest.writeString(description);
        dest.writeString(slug);
        dest.writeString(layout);
        dest.writeString(large_image_url);
        dest.writeString(render_image_url);
        dest.writeString(small_image_url);
        dest.writeDouble(price);
        dest.writeString(public_url);
        dest.writeParcelableArray((ImageGroup[])images.toArray(), flags);
        dest.writeParcelableArray((ProductInfo[])related.toArray(), flags);
        dest.writeDouble(saleprice);
        dest.writeString(sizes);
        dest.writeString(colours);
        dest.writeString(header);
        dest.writeString(footer);
        dest.writeLong(productcode);
    }

    public ProductInfo(Parcel in)
    {
        id = in.readLong();
        name = in.readString();
        description = in.readString();
        slug = in.readString();
        layout = in.readString();
        large_image_url = in.readString();
        render_image_url = in.readString();
        small_image_url = in.readString();
        price = in.readDouble();
        public_url = in.readString();
        images = in.readArrayList(ImageGroup.class.getClassLoader());
        related = in.readArrayList(ProductInfo.class.getClassLoader());
        saleprice = in.readDouble();
        sizes = in.readString();
        colours = in.readString();
        header = in.readString();
        footer = in.readString();
        productcode = in.readLong();
    }

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

        public ProductInfo[] newArray(int size) {
            return new ProductInfo[size];
        }
    };

    @Override
    public int describeContents() {
        return 0;
    }
}

CREATOR是至关重要的，因为生成的构造函数采用了包裹。我包含了更复杂的数据类型，因此您可以看到如何Parcel和UnParcel Parrays对象的数组。当使用Gson将JSON转换为带子节点的对象时，这是常见的事情。

Answer 4

要更好地了解包裹概念，请尝试以下链接

http://prasanta-paul.blogspot.com/2010/06/android-parcelable-example.html

希望这有助于：）

Answer 5

我也有类似的问题。只有 emmby 和this的以下片段帮助了我。

    public static final Parcelable.Creator<Foo> CREATOR = new Parcelable.Creator<Foo>() {
        public Foo createFromParcel(Parcel source) {
            final Foo f = new Foo();
            f.str = (String) source.readValue(Foo.class.getClassLoader());
            return f;
        }

        public Foo[] newArray(int size) {
            throw new UnsupportedOperationException();
        }

应该保存在每个实现Parcelable

的类中

如何在Android中使用Parcel？

5 个答案: