docs/html/training/volley/request.jd

page.title=Making a Standard Request

trainingnavtop=true

@jd:body

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<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#request-image">Request an Image</a></li>
  <li><a href="#request-json">Request JSON</a></li>
</ol>

</div>
</div>

<a class="notice-developers-video wide" href="https://www.youtube.com/watch?v=yhv8l9F44qo">
<div>
    <h3>Video</h3>
    <p>Volley: Easy, Fast Networking for Android</p>
</div>
</a>

<p>
This lesson describes how to use the common request types that Volley supports:</p>

<ul>
  <li>{@code StringRequest}. Specify a URL and receive a raw string in response. See
  <a href="requestqueue.html">Setting Up a Request Queue</a> for an example.</li>
  <li>{@code ImageRequest}. Specify a URL and receive an image in response.</li>
  <li>{@code JsonObjectRequest} and {@code JsonArrayRequest} (both subclasses of
  {@code JsonRequest}). Specify a URL and get a JSON object or array (respectively) in
  response.</li>
</ul>

<p>If your expected response is one of these types, you probably won't have to implement a
custom request. This lesson describes how to use these standard request types. For
information on how to implement your own custom request, see <a href="request-custom.html">
Implementing a Custom Request</a>.</p>


<h2 id="request-image">Request an Image</h2>

<p>Volley offers the following classes for requesting images. These classes layer on top
of each other to offer different levels of support for processing images:</p>

<ul>
  <li>{@code ImageRequest}&mdash;a canned request for getting an image at a given URL and
  calling back with a decoded bitmap. It also provides convenience features like specifying
  a size to resize to. Its main benefit is that Volley's thread scheduling ensures that
  expensive image operations (decoding, resizing) automatically happen on a worker thread.</li>

  <li>{@code ImageLoader}&mdash;a helper class that handles loading and caching images from
  remote URLs. {@code ImageLoader} is a an orchestrator for large numbers of {@code ImageRequest}s,
  for example when putting multiple thumbnails in a {@link android.widget.ListView}.
  {@code ImageLoader} provides an in-memory cache to sit in front of the normal Volley
  cache, which is important to prevent flickering. This makes it possible to achieve a
  cache hit without blocking or deferring off the main thread, which is impossible when
  using disk I/O. {@code ImageLoader} also does response coalescing, without which almost
  every response handler would set a bitmap on a view and cause a layout pass per image.
  Coalescing makes it possible to deliver multiple responses simultaneously, which improves
  performance.</li>
  <li>{@code NetworkImageView}&mdash;builds on {@code ImageLoader} and effectively replaces
  {@link android.widget.ImageView} for situations where your image is being fetched over
  the network via URL. {@code NetworkImageView} also manages canceling pending requests if
  the view is detached from the hierarchy.</li>
</ul>

<h3>Use ImageRequest</h3>

<p>Here is an example of using {@code ImageRequest}. It retrieves the image specified by
the URL and displays it in the app. Note that this snippet interacts with the
{@code RequestQueue} through a singleton class (see <a href="{@docRoot}
training/volley/requestqueue.html#singleton">Setting Up a RequestQueue</a> for more discussion of
this topic):</p>

<pre>
ImageView mImageView;
String url = "http://i.imgur.com/7spzG.png";
mImageView = (ImageView) findViewById(R.id.myImage);
...

// Retrieves an image specified by the URL, displays it in the UI.
ImageRequest request = new ImageRequest(url,
    new Response.Listener&lt;Bitmap&gt;() {
        &#64;Override
        public void onResponse(Bitmap bitmap) {
            mImageView.setImageBitmap(bitmap);
        }
    }, 0, 0, null,
    new Response.ErrorListener() {
        public void onErrorResponse(VolleyError error) {
            mImageView.setImageResource(R.drawable.image_load_error);
        }
    });
// Access the RequestQueue through your singleton class.
MySingleton.getInstance(this).addToRequestQueue(request);</pre>


<h3>Use ImageLoader and NetworkImageView</h3>

<p>You can use {@code ImageLoader} and {@code NetworkImageView} in concert to efficiently
manage the display of multiple images, such as in a {@link android.widget.ListView}. In your
layout XML file, you use {@code NetworkImageView} in much the same way you would use
{@link android.widget.ImageView}, for example:</p>

<pre>&lt;com.android.volley.toolbox.NetworkImageView
        android:id=&quot;&#64;+id/networkImageView&quot;
        android:layout_width=&quot;150dp&quot;
        android:layout_height=&quot;170dp&quot;
        android:layout_centerHorizontal=&quot;true&quot; /&gt;</pre>

<p>You can use {@code ImageLoader} by itself to display an image, for example:</p>

<pre>
ImageLoader mImageLoader;
ImageView mImageView;
// The URL for the image that is being loaded.
private static final String IMAGE_URL =
    "http://developer.android.com/images/training/system-ui.png";
...
mImageView = (ImageView) findViewById(R.id.regularImageView);

// Get the ImageLoader through your singleton class.
mImageLoader = MySingleton.getInstance(this).getImageLoader();
mImageLoader.get(IMAGE_URL, ImageLoader.getImageListener(mImageView,
         R.drawable.def_image, R.drawable.err_image));
</pre>

<p>However, {@code NetworkImageView} can do this for you if all you're doing is populating
an {@link android.widget.ImageView}. For example:</p>

<pre>
ImageLoader mImageLoader;
NetworkImageView mNetworkImageView;
private static final String IMAGE_URL =
    "http://developer.android.com/images/training/system-ui.png";
...

// Get the NetworkImageView that will display the image.
mNetworkImageView = (NetworkImageView) findViewById(R.id.networkImageView);

// Get the ImageLoader through your singleton class.
mImageLoader = MySingleton.getInstance(this).getImageLoader();

// Set the URL of the image that should be loaded into this view, and
// specify the ImageLoader that will be used to make the request.
mNetworkImageView.setImageUrl(IMAGE_URL, mImageLoader);
</pre>

<p>The above snippets access the {@code RequestQueue} and the {@code ImageLoader}
through a singleton class, as described in <a href="{@docRoot}training/volley/requestqueue.html#singleton">
Setting Up a RequestQueue</a>. This approach ensures that your app creates single instances of
these classes that last the lifetime of your app. The reason that this is important for
{@code ImageLoader} (the helper class that handles loading and caching images) is that
the main function of the in-memory cache is to allow for flickerless rotation. Using a
singleton pattern allows the bitmap cache to outlive the activity. If instead you create the
{@code ImageLoader} in an activity, the {@code ImageLoader} would be recreated along with
the activity every time the user rotates the device. This would cause flickering.</p>

<h4 id="lru-cache">Example LRU cache</h4>

<p>The Volley toolbox provides a standard cache implementation via the
{@code DiskBasedCache} class. This class caches files directly onto the hard disk in the
specified directory. But to use {@code ImageLoader}, you should provide a custom
in-memory LRU bitmap cache that implements the {@code ImageLoader.ImageCache} interface.
You may want to set up your cache as a singleton; for more discussion of this topic, see
<a href="{@docRoot}training/volley/requestqueue.html#singleton">
Setting Up a RequestQueue</a>.</p>

<p>Here is a sample implementation for an in-memory {@code LruBitmapCache} class.
It extends the {@link android.support.v4.util.LruCache} class and implements the
{@code ImageLoader.ImageCache} interface:</p>

<pre>
import android.graphics.Bitmap;
import android.support.v4.util.LruCache;
import android.util.DisplayMetrics;
import com.android.volley.toolbox.ImageLoader.ImageCache;

public class LruBitmapCache extends LruCache&lt;String, Bitmap&gt;
        implements ImageCache {

    public LruBitmapCache(int maxSize) {
        super(maxSize);
    }

    public LruBitmapCache(Context ctx) {
        this(getCacheSize(ctx));
    }

    &#64;Override
    protected int sizeOf(String key, Bitmap value) {
        return value.getRowBytes() * value.getHeight();
    }

    &#64;Override
    public Bitmap getBitmap(String url) {
        return get(url);
    }

    &#64;Override
    public void putBitmap(String url, Bitmap bitmap) {
        put(url, bitmap);
    }

    // Returns a cache size equal to approximately three screens worth of images.
    public static int getCacheSize(Context ctx) {
        final DisplayMetrics displayMetrics = ctx.getResources().
                getDisplayMetrics();
        final int screenWidth = displayMetrics.widthPixels;
        final int screenHeight = displayMetrics.heightPixels;
        // 4 bytes per pixel
        final int screenBytes = screenWidth * screenHeight * 4;

        return screenBytes * 3;
    }
}
</pre>

<p>Here is an example of how to instantiate an {@code ImageLoader} to use this
cache:</p>

<pre>
RequestQueue mRequestQueue; // assume this exists.
ImageLoader mImageLoader = new ImageLoader(mRequestQueue, new LruBitmapCache(
            LruBitmapCache.getCacheSize()));
</pre>


<h2 id="request-json">Request JSON</h2>

<p>Volley provides the following classes for JSON requests:</p>

<ul>
  <li>{@code JsonArrayRequest}&mdash;A request for retrieving a
  {@link org.json.JSONArray}
  response body at a given URL.</li>
  <li>{@code JsonObjectRequest}&mdash;A request for retrieving a
  {@link org.json.JSONObject}
  response body at a given URL, allowing for an optional
  {@link org.json.JSONObject}
  to be passed in as part of the request body.</li>
</ul>

<p>Both classes are based on the common base class {@code JsonRequest}. You use them
following the same basic pattern you use for other types of requests. For example, this
snippet fetches a JSON feed and displays it as text in the UI:</p>

<pre>
TextView mTxtDisplay;
ImageView mImageView;
mTxtDisplay = (TextView) findViewById(R.id.txtDisplay);
String url = "http://my-json-feed";

JsonObjectRequest jsObjRequest = new JsonObjectRequest
        (Request.Method.GET, url, null, new Response.Listener&lt;JSONObject&gt;() {

    &#64;Override
    public void onResponse(JSONObject response) {
        mTxtDisplay.setText("Response: " + response.toString());
    }
}, new Response.ErrorListener() {

    &#64;Override
    public void onErrorResponse(VolleyError error) {
        // TODO Auto-generated method stub

    }
});

// Access the RequestQueue through your singleton class.
MySingleton.getInstance(this).addToRequestQueue(jsObjRequest);
</pre>

For an example of implementing a custom JSON request based on
<a href="http://code.google.com/p/google-gson/">Gson</a>, see the next lesson,
<a href="request-custom.html">Implementing a Custom Request</a>.