page.title=Designing for Responsiveness
@jd:body

<p>It's possible to write code that wins every performance test in the world, but still sends users in a fiery rage when they try to use the application. These are the applications that aren't <em>responsive</em> enough &mdash; the ones that feel
sluggish, hang or freeze for significant periods, or take too long to process
input. </p>

<p>In Android, the system guards against applications that are insufficiently responsive for a period of time by displaying a dialog to the user, called the Application Not Responding (ANR) dialog. The user can choose to let the application continue, but the user won't appreciate having to act on this dialog every time he or she uses your application. So it's important to design responsiveness into your application, so that the system never has cause to display an ANR to the user. </p>

<p>Generally, the system displays an ANR if an application cannot respond to user input. For example, if an  application blocks on some I/O operation (frequently a network access), then the main application thread won't be able to process incoming user input events. After a time, the system concludes that the application has hung, and displays the ANR to give the user the option to kill it. 

<p>Similarly, if your application spends too much time building an elaborate in-memory
structure, or perhaps computing the next move in a game, the system will 
conclude that your application has hung. It's always important to make
sure these computations are efficient using the techniques above, but even the
most efficient code still takes time to run.</p>

<p>In both of these cases, the fix is usually to create a child thread, and do
most of your work there. This keeps the main thread (which drives the user
interface event loop) running, and prevents the system from concluding your code
has frozen. Since such threading usually is accomplished at the class
level, you can think of responsiveness as a <em>class</em> problem. (Compare
this with basic performance, which was described above as a <em>method</em>-level
concern.)</p>

<div class="sidebox-wrapper">
<div class="sidebox">
<img src="{@docRoot}images/anr.png" width="240" height="320" alt="Screenshot of ANR dialog box">
<p style="margin-top:.5em;padding:.5em;">An ANR dialog displayed to the user.</p>
</div>
</div>

<p>This document discusses how the Android system determines whether an application is
not responding and provides guidelines for
ensuring that your application is responsive. </p>

<p>This document covers these topics: </p>
<ul>
    <li><a href="#anr">What Triggers ANR?</a></li>
    <li><a href="#avoiding">How to Avoid ANR</a></li>
    <li><a href="#reinforcing">Reinforcing Responsiveness</a></li>
</ul>

<h2 id="anr">What Triggers ANR?</h2>

<p>In Android, application responsiveness is monitored by the Activity Manager
and Window Manager system services. Android will display the ANR dialog
for a particular application when it detects one of the following
conditions:</p>
<ul>  
    <li>No response to an input event (e.g. key press, screen touch) within 5 seconds</li>
    <li>A {@link android.content.BroadcastReceiver BroadcastReceiver} hasn't finished executing within 10 seconds</li>
</ul>

<h2 id="avoiding">How to Avoid ANR</h2>

<p>Given the above definition for ANR, let's examine why this can occur in
Android applications and how best to structure your application to avoid ANR.</p>

<p>Android applications normally run entirely on a single (i.e. main) thread.
This means that anything your application is doing in the main thread that
takes a long time to complete can trigger the ANR dialog because your
application is not giving itself a chance to handle the input event or Intent
broadcast.</p>

<p>Therefore any method that runs in the main thread should do as little work
as possible. In particular, Activities should do as little as possible to set
up in key life-cycle methods such as <code>onCreate()</code> and
<code>onResume()</code>. Potentially long running operations such as network
or database operations, or computationally expensive calculations such as
resizing bitmaps should be done in a child thread (or in the case of databases
operations, via an asynchronous request). However, this does not mean that
your main thread should block while waiting for the child thread to
complete &mdash; nor should you call <code>Thread.wait()</code> or
<code>Thread.sleep()</code>. Instead of blocking while waiting for a child
thread to complete, your main thread should provide a {@link
android.os.Handler Handler} for child threads to post back to upon completion.
Designing your application in this way will allow your main thread to remain
responsive to input and thus avoid ANR dialogs caused by the 5 second input
event timeout. These same practices should be followed for any other threads
that display UI, as they are also subject to the same timeouts.</p>

<p>The specific constraint on IntentReceiver execution time emphasizes what
they were meant to do: small, discrete amounts of work in the background such
as saving a setting or registering a Notification. So as with other methods
called in the main thread, applications should avoid potentially long-running
operations or calculations in BroadcastReceivers. But instead of doing intensive
tasks via child threads (as the life of a BroadcastReceiver is short), your
application should start a {@link android.app.Service Service} if a
potentially long running action needs to be taken in response to an Intent
broadcast. As a side note, you should also avoid starting an Activity from an
Intent Receiver, as it will spawn a new screen that will steal focus from
whatever application the user is currently has running. If your application
has something to show the user in response to an Intent broadcast, it should
do so using the {@link android.app.NotificationManager Notification
Manager}.</p>

<h2 id="reinforcing">Reinforcing Responsiveness</h2>

<p>Generally, 100 to 200ms is the threshold beyond which users will perceive
lag (or lack of "snappiness," if you will) in an application. As such, here
are some additional tips beyond what you should do to avoid ANR that will help
make your application seem responsive to users.</p>

<ul>
    <li>If your application is doing work in the background in response to
    user input, show that progress is being made ({@link
    android.widget.ProgressBar ProgressBar} and {@link
    android.app.ProgressDialog ProgressDialog} are useful for this).</li>
    <li>For games specifically, do calculations for moves in a child
    thread.</li>
    <li>If your application has a time-consuming initial setup phase, consider
    showing a splash screen or rendering the main view as quickly as possible
    and filling in the information asynchronously. In either case, you should
    indicate somehow that progress is being made, lest the user perceive that
    the application is frozen.</li>
</ul>
