1 page.title=Pausing and Resuming an Activity 2 parent.title=Managing the Activity Lifecycle 3 parent.link=index.html 4 5 trainingnavtop=true 6 previous.title=Starting an Activity 7 previous.link=starting.html 8 next.title=Stopping and Restarting an Activity 9 next.link=stopping.html 10 11 @jd:body 12 13 <div id="tb-wrapper"> 14 <div id="tb"> 15 16 <h2>This lesson teaches you to</h2> 17 <ol> 18 <li><a href="#Pause">Pause Your Activity</a></li> 19 <li><a href="#Resume">Resume Your Activity</a></li> 20 </ol> 21 22 <h2>You should also read</h2> 23 <ul> 24 <li><a href="{@docRoot}guide/components/activities.html">Activities</a> 25 </li> 26 </ul> 27 28 <h2>Try it out</h2> 29 30 <div class="download-box"> 31 <a href="http://developer.android.com/shareables/training/ActivityLifecycle.zip" 32 class="button">Download the demo</a> 33 <p class="filename">ActivityLifecycle.zip</p> 34 </div> 35 36 </div> 37 </div> 38 39 <p>During normal app use, the foreground activity is sometimes obstructed by other 40 visual components that cause the activity to <em>pause</em>. For example, when a semi-transparent 41 activity opens (such as one in the style of a dialog), the previous activity pauses. As long as the 42 activity is still partially visible but currently not the activity in focus, it remains paused.</p> 43 44 <p>However, once the activity is fully-obstructed and not visible, it <em>stops</em> (which is 45 discussed in the next lesson).</p> 46 47 <p>As your activity enters the paused state, the system calls the {@link 48 android.app.Activity#onPause onPause()} method on your {@link android.app.Activity}, which allows 49 you to stop ongoing actions that should not continue while paused (such as a video) or persist 50 any information that should be permanently saved in case the user continues to leave your app. If 51 the user returns to your activity from the paused state, the system resumes it and calls the 52 {@link android.app.Activity#onResume onResume()} method.</p> 53 54 <p class="note"><strong>Note:</strong> When your activity receives a call to {@link 55 android.app.Activity#onPause()}, it may be an indication that the activity will be paused for a 56 moment and the user may return focus to your activity. However, it's usually the first indication 57 that the user is leaving your activity.</p> 58 59 <img src="{@docRoot}images/training/basics/basic-lifecycle-paused.png" /> 60 <p class="img-caption"><strong>Figure 1.</strong> When a semi-transparent activity obscures 61 your activity, the system calls {@link android.app.Activity#onPause onPause()} and the activity 62 waits in the Paused state (1). If the user returns to the activity while it's still paused, the 63 system calls {@link android.app.Activity#onResume onResume()} (2).</p> 64 65 66 <h2 id="Pause">Pause Your Activity</h2> 67 68 <p>When the system calls {@link android.app.Activity#onPause()} for your activity, it 69 technically means your activity is still partially visible, but most often is an indication that 70 the user is leaving the activity and it will soon enter the Stopped state. You should usually use 71 the {@link android.app.Activity#onPause()} callback to:</p> 72 73 <ul> 74 <li>Stop animations or other ongoing actions that could consume CPU.</li> 75 <li>Commit unsaved changes, but only if users expect such changes to be permanently saved when 76 they leave (such as a draft email).</li> 77 <li>Release system resources, such as broadcast receivers, handles to sensors (like 78 GPS), or any resources that may affect battery life while your activity is paused and the user 79 does not need them.</li> 80 </ul> 81 82 <p>For example, if your application uses the {@link android.hardware.Camera}, the 83 {@link android.app.Activity#onPause()} method is a good place to release it.</p> 84 85 <pre> 86 @Override 87 public void onPause() { 88 super.onPause(); // Always call the superclass method first 89 90 // Release the Camera because we don't need it when paused 91 // and other activities might need to use it. 92 if (mCamera != null) { 93 mCamera.release() 94 mCamera = null; 95 } 96 } 97 </pre> 98 99 <p>Generally, you should <strong>not</strong> use {@link android.app.Activity#onPause()} to store 100 user changes (such as personal information entered into a form) to permanent storage. The only time 101 you should persist user changes to permanent storage within {@link android.app.Activity#onPause()} 102 is when you're certain users expect the changes to be auto-saved (such as when drafting an email). 103 However, you should avoid performing CPU-intensive work during {@link 104 android.app.Activity#onPause()}, such as writing to a database, because it can slow the visible 105 transition to the next activity (you should instead perform heavy-load shutdown operations during 106 {@link android.app.Activity#onStop onStop()}).</p> 107 108 <p>You should keep the amount of operations done in the {@link android.app.Activity#onPause 109 onPause()} method relatively simple in order to allow for a speedy transition to the user's next 110 destination if your activity is actually being stopped.</p> 111 112 <p class="note"><strong>Note:</strong> When your activity is paused, the {@link 113 android.app.Activity} instance is kept resident in memory and is recalled when the activity resumes. 114 You dont need to re-initialize components that were created during any of the callback methods 115 leading up to the Resumed state.</p> 116 117 118 119 <h2 id="Resume">Resume Your Activity</h2> 120 121 <p>When the user resumes your activity from the Paused state, the system calls the {@link 122 android.app.Activity#onResume()} method.</p> 123 124 <p>Be aware that the system calls this method every time your activity comes into the foreground, 125 including when it's created for the first time. As such, you should implement {@link 126 android.app.Activity#onResume()} to initialize components that you release during {@link 127 android.app.Activity#onPause()} and perform any other initializations that must occur each time the 128 activity enters the Resumed state (such as begin animations and initialize components only used 129 while the activity has user focus).</p> 130 131 <p>The following example of {@link android.app.Activity#onResume()} is the counterpart to 132 the {@link android.app.Activity#onPause()} example above, so it initializes the camera that's 133 released when the activity pauses.</p> 134 135 <pre> 136 @Override 137 public void onResume() { 138 super.onResume(); // Always call the superclass method first 139 140 // Get the Camera instance as the activity achieves full user focus 141 if (mCamera == null) { 142 initializeCamera(); // Local method to handle camera init 143 } 144 } 145 </pre> 146 147 148 149 150 151 152 153
