How to use Parcel in Android?

Mike_82156 0
Viewed 88244

I'm trying to use Parcel to write and then read back a Parcelable. For some reason, when I read the object back from the file, it's coming back as null.

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);
    }


}

What am I missing?

UPDATE: To simplify the test I've removed the reading and writing of files in my original example.

parcel android
5 Answers

I find that Parcelable is most often used in Android within data Bundles, but more specifically within a Handler that is sending and receiving messages. As an example, you might have an AsyncTask or a Runnable that needs to run in the background but post resulting data to the Main thread or Activity.

Here's a simple example. If I have a Runnable that looks like this:

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());
        }

    }
}

As you can see, this runnable takes a Handler in its constructor. This is called from some Activity like this:

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();
    }

Now, all that is left is the heart of this question, how you define a class as Parcelable. I've chosen a fairly complex class to show because there are some things you would not see with a simple one. Here is the ProductInfo class, which Parcels and unParcels cleanly:

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;
    }
}

The CREATOR is critical, as is the resulting constructor taking a Parcel. I included the more complex data types so you could see how to Parcel and unParcel Arrays of Parcelable objects. This is a common thing when using Gson to convert JSON into objects with children as in this example.

user_29697190
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