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