1page.title=Designing for Responsiveness 2@jd:body 3 4<div id="qv-wrapper"> 5<div id="qv"> 6 7<h2>In this document</h2> 8<ol> 9 <li><a href="#anr">What Triggers ANR?</a></li> 10 <li><a href="#avoiding">How to Avoid ANR</a></li> 11 <li><a href="#reinforcing">Reinforcing Responsiveness</a></li> 12</ol> 13 14</div> 15</div> 16 17<div class="figure"> 18<img src="{@docRoot}images/anr.png" alt="Screenshot of ANR dialog box" width="240" height="320"/> 19<p><strong>Figure 1.</strong> An ANR dialog displayed to the user.</p> 20</div> 21 22<p>It's possible to write code that wins every performance test in the world, 23but still sends users in a fiery rage when they try to use the application. 24These are the applications that aren't <em>responsive</em> enough — the 25ones that feel sluggish, hang or freeze for significant periods, or take too 26long to process input. </p> 27 28<p>In Android, the system guards against applications that are insufficiently 29responsive for a period of time by displaying a dialog to the user, called the 30Application Not Responding (ANR) dialog, shown at right in Figure 1. The user 31can choose to let the application continue, but the user won't appreciate having 32to act on this dialog every time he or she uses your application. It's critical 33to design responsiveness into your application, so that the system never has 34cause to display an ANR dialog to the user. </p> 35 36<p>Generally, the system displays an ANR if an application cannot respond to 37user input. For example, if an application blocks on some I/O operation 38(frequently a network access), then the main application thread won't be able to 39process incoming user input events. After a time, the system concludes that the 40application is frozen, and displays the ANR to give the user the option to kill 41it. </p> 42 43<p>Similarly, if your application spends too much time building an elaborate in-memory 44structure, or perhaps computing the next move in a game, the system will 45conclude that your application has hung. It's always important to make 46sure these computations are efficient using the techniques above, but even the 47most efficient code still takes time to run.</p> 48 49<p>In both of these cases, the recommended approach is to create a child thread and do 50most of your work there. This keeps the main thread (which drives the user 51interface event loop) running and prevents the system from concluding that your code 52has frozen. Since such threading usually is accomplished at the class 53level, you can think of responsiveness as a <em>class</em> problem. (Compare 54this with basic performance, which was described above as a <em>method</em>-level 55concern.)</p> 56 57<p>This document describes how the Android system determines whether an 58application is not responding and provides guidelines for ensuring that your 59application stays responsive. </p> 60 61<h2 id="anr">What Triggers ANR?</h2> 62 63<p>In Android, application responsiveness is monitored by the Activity Manager 64and Window Manager system services. Android will display the ANR dialog 65for a particular application when it detects one of the following 66conditions:</p> 67<ul> 68 <li>No response to an input event (e.g. key press, screen touch) 69 within 5 seconds</li> 70 <li>A {@link android.content.BroadcastReceiver BroadcastReceiver} 71 hasn't finished executing within 10 seconds</li> 72</ul> 73 74<h2 id="avoiding">How to Avoid ANR</h2> 75 76<p>Given the above definition for ANR, let's examine why this can occur in 77Android applications and how best to structure your application to avoid ANR.</p> 78 79<p>Android applications normally run entirely on a single (i.e. main) thread. 80This means that anything your application is doing in the main thread that 81takes a long time to complete can trigger the ANR dialog because your 82application is not giving itself a chance to handle the input event or Intent 83broadcast.</p> 84 85<p>Therefore any method that runs in the main thread should do as little work 86as possible. In particular, Activities should do as little as possible to set 87up in key life-cycle methods such as <code>onCreate()</code> and 88<code>onResume()</code>. Potentially long running operations such as network 89or database operations, or computationally expensive calculations such as 90resizing bitmaps should be done in a child thread (or in the case of databases 91operations, via an asynchronous request). However, this does not mean that 92your main thread should block while waiting for the child thread to 93complete — nor should you call <code>Thread.wait()</code> or 94<code>Thread.sleep()</code>. Instead of blocking while waiting for a child 95thread to complete, your main thread should provide a {@link 96android.os.Handler Handler} for child threads to post back to upon completion. 97Designing your application in this way will allow your main thread to remain 98responsive to input and thus avoid ANR dialogs caused by the 5 second input 99event timeout. These same practices should be followed for any other threads 100that display UI, as they are also subject to the same timeouts.</p> 101 102<p>You can use {@link android.os.StrictMode} to help find potentially 103long running operations such as network or database operations that 104you might accidentally be doing your main thread.</p> 105 106<p>The specific constraint on IntentReceiver execution time emphasizes what 107they were meant to do: small, discrete amounts of work in the background such 108as saving a setting or registering a Notification. So as with other methods 109called in the main thread, applications should avoid potentially long-running 110operations or calculations in BroadcastReceivers. But instead of doing intensive 111tasks via child threads (as the life of a BroadcastReceiver is short), your 112application should start a {@link android.app.Service Service} if a 113potentially long running action needs to be taken in response to an Intent 114broadcast. As a side note, you should also avoid starting an Activity from an 115Intent Receiver, as it will spawn a new screen that will steal focus from 116whatever application the user is currently has running. If your application 117has something to show the user in response to an Intent broadcast, it should 118do so using the {@link android.app.NotificationManager Notification 119Manager}.</p> 120 121<h2 id="reinforcing">Reinforcing Responsiveness</h2> 122 123<p>Generally, 100 to 200ms is the threshold beyond which users will perceive 124lag (or lack of "snappiness," if you will) in an application. As such, here 125are some additional tips beyond what you should do to avoid ANR that will help 126make your application seem responsive to users.</p> 127 128<ul> 129 <li>If your application is doing work in the background in response to 130 user input, show that progress is being made ({@link 131 android.widget.ProgressBar ProgressBar} and {@link 132 android.app.ProgressDialog ProgressDialog} are useful for this).</li> 133 <li>For games specifically, do calculations for moves in a child 134 thread.</li> 135 <li>If your application has a time-consuming initial setup phase, consider 136 showing a splash screen or rendering the main view as quickly as possible 137 and filling in the information asynchronously. In either case, you should 138 indicate somehow that progress is being made, lest the user perceive that 139 the application is frozen.</li> 140</ul> 141
