| page.title=Pausing and Resuming an Activity |
| parent.title=Managing the Activity Lifecycle |
| parent.link=index.html |
| |
| trainingnavtop=true |
| previous.title=Starting an Activity |
| previous.link=starting.html |
| next.title=Stopping and Restarting an Activity |
| next.link=stopping.html |
| |
| @jd:body |
| |
| <div id="tb-wrapper"> |
| <div id="tb"> |
| |
| <h2>This lesson teaches you to</h2> |
| <ol> |
| <li><a href="#Pause">Pause Your Activity</a></li> |
| <li><a href="#Resume">Resume Your Activity</a></li> |
| </ol> |
| |
| <h2>You should also read</h2> |
| <ul> |
| <li><a href="{@docRoot}guide/components/activities.html">Activities</a> |
| </li> |
| </ul> |
| |
| <h2>Try it out</h2> |
| |
| <div class="download-box"> |
| <a href="http://developer.android.com/shareables/training/ActivityLifecycle.zip" |
| class="button">Download the demo</a> |
| <p class="filename">ActivityLifecycle.zip</p> |
| </div> |
| |
| </div> |
| </div> |
| |
| <p>During normal app use, the foreground activity is sometimes obstructed by other |
| visual components that cause the activity to <em>pause</em>. For example, when a semi-transparent |
| activity opens (such as one in the style of a dialog), the previous activity pauses. As long as the |
| activity is still partially visible but currently not the activity in focus, it remains paused.</p> |
| |
| <p>However, once the activity is fully-obstructed and not visible, it <em>stops</em> (which is |
| discussed in the next lesson).</p> |
| |
| <p>As your activity enters the paused state, the system calls the {@link |
| android.app.Activity#onPause onPause()} method on your {@link android.app.Activity}, which allows |
| you to stop ongoing actions that should not continue while paused (such as a video) or persist |
| any information that should be permanently saved in case the user continues to leave your app. If |
| the user returns to your activity from the paused state, the system resumes it and calls the |
| {@link android.app.Activity#onResume onResume()} method.</p> |
| |
| <p class="note"><strong>Note:</strong> When your activity receives a call to {@link |
| android.app.Activity#onPause()}, it may be an indication that the activity will be paused for a |
| moment and the user may return focus to your activity. However, it's usually the first indication |
| that the user is leaving your activity.</p> |
| |
| <img src="{@docRoot}images/training/basics/basic-lifecycle-paused.png" /> |
| <p class="img-caption"><strong>Figure 1.</strong> When a semi-transparent activity obscures |
| your activity, the system calls {@link android.app.Activity#onPause onPause()} and the activity |
| waits in the Paused state (1). If the user returns to the activity while it's still paused, the |
| system calls {@link android.app.Activity#onResume onResume()} (2).</p> |
| |
| |
| <h2 id="Pause">Pause Your Activity</h2> |
| |
| <p>When the system calls {@link android.app.Activity#onPause()} for your activity, it |
| technically means your activity is still partially visible, but most often is an indication that |
| the user is leaving the activity and it will soon enter the Stopped state. You should usually use |
| the {@link android.app.Activity#onPause()} callback to:</p> |
| |
| <ul> |
| <li>Stop animations or other ongoing actions that could consume CPU.</li> |
| <li>Commit unsaved changes, but only if users expect such changes to be permanently saved when |
| they leave (such as a draft email).</li> |
| <li>Release system resources, such as broadcast receivers, handles to sensors (like |
| GPS), or any resources that may affect battery life while your activity is paused and the user |
| does not need them.</li> |
| </ul> |
| |
| <p>For example, if your application uses the {@link android.hardware.Camera}, the |
| {@link android.app.Activity#onPause()} method is a good place to release it.</p> |
| |
| <pre> |
| @Override |
| public void onPause() { |
| super.onPause(); // Always call the superclass method first |
| |
| // Release the Camera because we don't need it when paused |
| // and other activities might need to use it. |
| if (mCamera != null) { |
| mCamera.release() |
| mCamera = null; |
| } |
| } |
| </pre> |
| |
| <p>Generally, you should <strong>not</strong> use {@link android.app.Activity#onPause()} to store |
| user changes (such as personal information entered into a form) to permanent storage. The only time |
| you should persist user changes to permanent storage within {@link android.app.Activity#onPause()} |
| is when you're certain users expect the changes to be auto-saved (such as when drafting an email). |
| However, you should avoid performing CPU-intensive work during {@link |
| android.app.Activity#onPause()}, such as writing to a database, because it can slow the visible |
| transition to the next activity (you should instead perform heavy-load shutdown operations during |
| {@link android.app.Activity#onStop onStop()}).</p> |
| |
| <p>You should keep the amount of operations done in the {@link android.app.Activity#onPause |
| onPause()} method relatively simple in order to allow for a speedy transition to the user's next |
| destination if your activity is actually being stopped.</p> |
| |
| <p class="note"><strong>Note:</strong> When your activity is paused, the {@link |
| android.app.Activity} instance is kept resident in memory and is recalled when the activity resumes. |
| You don’t need to re-initialize components that were created during any of the callback methods |
| leading up to the Resumed state.</p> |
| |
| |
| |
| <h2 id="Resume">Resume Your Activity</h2> |
| |
| <p>When the user resumes your activity from the Paused state, the system calls the {@link |
| android.app.Activity#onResume()} method.</p> |
| |
| <p>Be aware that the system calls this method every time your activity comes into the foreground, |
| including when it's created for the first time. As such, you should implement {@link |
| android.app.Activity#onResume()} to initialize components that you release during {@link |
| android.app.Activity#onPause()} and perform any other initializations that must occur each time the |
| activity enters the Resumed state (such as begin animations and initialize components only used |
| while the activity has user focus).</p> |
| |
| <p>The following example of {@link android.app.Activity#onResume()} is the counterpart to |
| the {@link android.app.Activity#onPause()} example above, so it initializes the camera that's |
| released when the activity pauses.</p> |
| |
| <pre> |
| @Override |
| public void onResume() { |
| super.onResume(); // Always call the superclass method first |
| |
| // Get the Camera instance as the activity achieves full user focus |
| if (mCamera == null) { |
| initializeCamera(); // Local method to handle camera init |
| } |
| } |
| </pre> |
| |
| |
| |
| |
| |
| |
| |