1page.title=Faster Screen Orientation Change 2parent.title=Articles 3parent.link=../browser.html?tag=article 4@jd:body 5 6 7<div id="qv-wrapper"> 8<div id="qv"> 9 10 <h2>See also</h2> 11 <ol> 12 <li><a href="{@docRoot}guide/topics/resources/runtime-changes.html">Handling Runtime 13Changes</a></li> 14 </ol> 15 16</div> 17</div> 18 19<p>Android is designed to run efficiently on a wide 20array of devices, with very different hardware configurations. Some 21devices, like the T-Mobile G1, can change their hardware configuration 22at runtime. For instance, when you open the keyboard, the screen change 23from the portrait orientation to the landscape orientation. 24 25<div class="sidebox-wrapper"> 26<div class="sidebox"> 27<h2>Using the alternate resources framework</h2> 28 29<p>The platform's support for loading orientation-specific 30resources at run time is based on the alternate resources framework.</p> 31 32<p>Providing orientation-specific resources is an important part of 33developing your app. If you are not familiar with resource directory qualifiers 34or how the platform uses them, please read 35<a href="{@docRoot}guide/topics/resources/resources-i18n.html#AlternateResources"> 36Alternate Resources</a>. 37</div> 38</div> 39 40<p>To make 41Android app development easier, the Android system automatically handles 42configuration change events and restarts the current activity with the new 43configuration. This is the default behavior that lets you declare 44resources like layouts and drawables based on the orientation, screen 45size, locale, etc. </p> 46 47<p>While this behavior is really powerful, since your application adapts 48automatically to the device's configuration at runtime, it is sometimes 49confusing for new Android developers, who wonder why their activity is 50destroyed and recreated. </p> 51 52<p>Facing this "issue," some developers choose to handle configuration changes 53themselves which is, in general, a short-term solution that will only complicate 54their lives later. On the other hand, the system's automatic resource handling 55is a very efficient and easy way to adapt an application's user interface to 56various devices and devices configurations. It sometimes comes at a price, 57though.</p> 58 59<p>When your application displays a lot of data, or data that is expensive to fetch, 60the automatic destruction/creation of the activities can be lead to a 61painful user experience. Take the example of <a href="http://code.google.com/p/apps-for-android/source/browse/trunk/Photostream/">Photostream</a>, 62a simple Flickr browsing application. After you launch the application and choose a Flickr account, the 63application downloads a set of 6 photos (on a T-Mobile G1) from the 64Flickr servers and displays them on screen. To improve the user 65experience, the application uses slightly different layouts and drawables in 66portrait and landscape modes and this is what the result looks like:</p> 67 68<p><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://1.bp.blogspot.com/_9l0GmPwgCzk/SZoGyJyg6-I/AAAAAAAAACU/ItuVwhegPb8/s1600-h/photostream_landscape.png"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 320px; height: 214px;" src="images/photostream_landscape.png" alt="" id="BLOGGER_PHOTO_ID_5303558969873198050" border="0"></a></p> 69 70<p><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://2.bp.blogspot.com/_9l0GmPwgCzk/SZoGx4I-QlI/AAAAAAAAACM/-GkZR5MUKhY/s1600-h/photostream_portrait.png"><img style="margin: 0px auto 10px; display: block; text-align: center; cursor: pointer; width: 214px; height: 320px;" src="images/photostream_portrait.png" alt="" id="BLOGGER_PHOTO_ID_5303558965135557202" border="0"></a></p> 71 72<p>Photostream lets Android take care of the configuration change when the 73screen is rotated. However, can you imagine how painful it would be for the user 74to see all the images being downloaded again? The obvious solution to this 75problem is to temporarily cache the images. They could be cached on the SD card 76(if there's one), in the Application object, in a static field, etc. None of 77these techniques is adapted to the current situation: why should we bother 78caching the images when the screen is not rotated? Fortunately for us, Android 79offers a great API exactly for that purpose.</p> 80 81<p>The Activity class has a special method called 82{@link android.app.Activity#onRetainNonConfigurationInstance()}. This method 83can be used to pass an arbitrary object <em>your future self</em> and Android 84is smart enough to call this method only when needed. In the case of Photostream, 85the application <a href="http://code.google.com/p/apps-for-android/source/browse/trunk/Photostream/src/com/google/android/photostream/PhotostreamActivity.java#226">used this method</a> 86to pass the downloaded images to the future activity on orientation change. 87The implementation can be summarized like so:</p> 88 89<pre class="prettyprint">@Override 90public Object onRetainNonConfigurationInstance() { 91 final LoadedPhoto[] list = new LoadedPhoto[numberOfPhotos]; 92 keepPhotos(list); 93 return list; 94} 95</pre> 96 97<p>In the new activity, in <code>onCreate()</code>, all you have to do to 98get your object back is to call {@link android.app.Activity#getLastNonConfigurationInstance()}. 99In Photostream, <a href="http://code.google.com/p/apps-for-android/source/browse/trunk/Photostream/src/com/google/android/photostream/PhotostreamActivity.java#251">this method is invoked</a> 100and if the returned value is not null, the grid is loaded with the list of 101photos from the previous activity:</p> 102 103<pre class="prettyprint">private void loadPhotos() { 104 final Object data = getLastNonConfigurationInstance(); 105 106 // The activity is starting for the first time, load the photos from Flickr 107 if (data == null) { 108 mTask = new GetPhotoListTask().execute(mCurrentPage); 109 } else { 110 // The activity was destroyed/created automatically, populate the grid 111 // of photos with the images loaded by the previous activity 112 final LoadedPhoto[] photos = (LoadedPhoto[]) data; 113 for (LoadedPhoto photo : photos) { 114 addPhoto(photo); 115 } 116 } 117} 118</pre> 119 120<p>Be very careful with the object you pass through 121<code>onRetainNonConfigurationChange()</code>, though. If the object you 122pass is for some reason tied to the Activity/Context, <a 123href="http://www.curious-creature.org/2008/12/18/avoid-memory-leaks-on-android/">you will leak</a> 124all the views and resources of the activity. This means you should 125never pass a View, a Drawable, an Adapter, etc. Photostream for 126instance extracts the bitmaps from the drawables and pass the bitmaps 127only, not the drawables. Finally, remember that 128<code>onRetainNonConfigurationChange()</code> should be used only to retain 129data that is expensive to load. Otherwise, keep it simple and let Android 130do everything.</p> 131
