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page.title=Designing for Responsiveness

@jd:body

<div id="qv-wrapper">

<div id="qv">

<h2>In this document</h2>

<ol>

  <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>

</ol>

</div>

</div>

<div class="figure">

<img src="{@docRoot}images/anr.png" alt="Screenshot of ANR dialog box" width="240" height="320"/>

<p><strong>Figure 1.</strong> An ANR dialog displayed to the user.</p>

</div>

<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, shown at right in Figure 1. 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. It's critical

to design responsiveness into your application, so that the system never has

cause to display an ANR dialog 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 is frozen, and displays the ANR to give the user the option to kill

it. </p>

<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 recommended approach is 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 that 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>

<p>This document describes how the Android system determines whether an

application is not responding and provides guidelines for ensuring that your

application stays responsive. </p>

<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>You can use {@link android.os.StrictMode} to help find potentially

long running operations such as network or database operations that

you might accidentally be doing your main thread.</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>