android_frameworks_base/docs/html/training/volley/simple.jd

page.title=Sending a Simple Request

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@jd:body

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<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#manifest">Add the INTERNET Permission</a></li>
  <li><a href="#simple">Use newRequestQueue</a></li>
  <li><a href="#send">Send a Request</a></li>
  <li><a href="#cancel">Cancel a Request</a></li>
</ol>

</div>
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<a class="notice-developers-video wide" href="https://developers.google.com/events/io/sessions/325304728">
<div>
    <h3>Video</h3>
    <p>Volley: Easy, Fast Networking for Android</p>
</div>
</a>

<p>At a high level, you use Volley by creating a {@code RequestQueue} and passing it
{@code Request} objects. The {@code RequestQueue} manages worker threads for running the
network operations, reading from and writing to the cache, and parsing responses. Requests
do the parsing of raw responses and Volley takes care of dispatching the parsed response
back to the main thread for delivery.</p>

<p> This lesson describes how to send a request using the <code>Volley.newRequestQueue</code>
convenience method, which sets up a {@code RequestQueue} for you.
See the next lesson,
<a href="requestqueue.html">Setting Up a RequestQueue</a>, for information on how to set
up a {@code RequestQueue} yourself.</p>

<p>This lesson also describes how to add a request to a {@code RequestQueue} and cancel a
request.</p>

<h2 id="manifest">Add the INTERNET Permission</h2>

<p>To use Volley, you must add the
{@link android.Manifest.permission#INTERNET android.permission.INTERNET} permission
to your app's manifest. Without this, your app won't be able to connect to the network.</p>


<h2 id="simple">Use newRequestQueue</h2>

<p>Volley provides a convenience method <code>Volley.newRequestQueue</code> that sets up a
{@code RequestQueue} for you, using default values, and starts the queue. For example:</p>

<pre>
final TextView mTextView = (TextView) findViewById(R.id.text);
...

// Instantiate the RequestQueue.
RequestQueue queue = Volley.newRequestQueue(this);
String url ="http://www.google.com";

// Request a string response from the provided URL.
StringRequest stringRequest = new StringRequest(Request.Method.GET, url,
            new Response.Listener<String>() {
    &#64;Override
    public void onResponse(String response) {
        // Display the first 500 characters of the response string.
        mTextView.setText("Response is: "+ response.substring(0,500));
    }
}, new Response.ErrorListener() {
    &#64;Override
    public void onErrorResponse(VolleyError error) {
        mTextView.setText("That didn't work!");
    }
});
// Add the request to the RequestQueue.
queue.add(stringRequest);
</pre>

<p>Volley always delivers parsed responses on the main thread. Running on the main thread
is convenient for populating UI controls with received data, as you can freely modify UI
controls directly from your response handler, but it's especially critical to many of the
important semantics provided by the library, particularly related to canceling requests.
</p>

<p>See <a href="requestqueue.html">Setting Up a RequestQueue</a> for a
description of how to set up a {@code RequestQueue} yourself, instead of using the
<code>Volley.newRequestQueue</code> convenience method.</p>

<h2 id="send">Send a Request</h2>

<p>To send a request, you simply construct one and add it to the {@code RequestQueue} with
{@code add()}, as shown above. Once you add the request it moves through the pipeline,
gets serviced, and has its raw response parsed and delivered.</p>

<p>When you call {@code add()}, Volley runs one cache processing thread and a pool of
network dispatch threads. When you add a request to the queue, it is picked up by the cache
thread and triaged: if the request can be serviced from cache, the cached response is
parsed on the cache thread and the parsed response is delivered on the main thread. If the
request cannot be serviced from cache, it is placed on the network queue. The first
available network thread takes the request from the queue, performs the HTTP transaction,
parsse the response on the worker thread, writes the response to cache, and posts the parsed
response back to the main thread for delivery.</p>

<p>Note that expensive operations like blocking I/O and parsing/decoding are done on worker
threads. You can add a request from any thread, but responses are always delivered on the
main thread.</p>

<p>Figure 1 illustrates the life of a request:</p>

 <img src="{@docRoot}images/training/volley-request.png"
  alt="system bars">
<p class="img-caption"><strong>Figure 1.</strong> Life of a request.</p>


<h2 id="cancel">Cancel a Request</h2>

<p>To cancel a request, call {@code cancel()} on your {@code Request} object. Once cancelled,
Volley guarantees that your response handler will never be called. What this means in
practice is that you can cancel all of your pending requests in your activity's
{@link android.app.Activity#onStop onStop()} method and you don't have to litter your
response handlers with checks for {@code getActivity() == null},
whether {@code onSaveInstanceState()} has been called already, or other defensive
boilerplate.</p>

<p>To take advantage of this behavior, you would typically have to
track all in-flight requests in order to be able to cancel them at the
appropriate time. There is an easier way: you can associate a tag object with each
request. You can then use this tag to provide a scope of requests to cancel. For
example, you can tag all of your requests with the {@link android.app.Activity} they
are being made on behalf of, and call {@code requestQueue.cancelAll(this)} from
{@link android.app.Activity#onStop onStop()}.
Similarly, you could tag all thumbnail image requests in a
{@link android.support.v4.view.ViewPager} tab with their respective tabs and cancel on swipe
to make sure that the new tab isn't being held up by requests from another one.</p>

<p>Here is an example that uses a string value for the tag:</p>

<ol>
<li>Define your tag and add it to your requests.
<pre>
public static final String TAG = "MyTag";
StringRequest stringRequest; // Assume this exists.
RequestQueue mRequestQueue;  // Assume this exists.

// Set the tag on the request.
stringRequest.setTag(TAG);

// Add the request to the RequestQueue.
mRequestQueue.add(stringRequest);</pre>
</li>

<li>In your activity's {@link android.app.Activity#onStop onStop()} method, cancel all requests that have this tag.
<pre>
&#64;Override
protected void onStop () {
    super.onStop();
    if (mRequestQueue != null) {
        mRequestQueue.cancelAll(TAG);
    }
}
</pre></li></ol>

<p>Take care when canceling requests. If you are depending on your response handler to
advance a state or kick off another process, you need to account for this. Again, the
response handler will not be called.
</p>