1page.title=Stopping and Restarting an Activity 2page.tags=activity lifecycle 3helpoutsWidget=true 4 5trainingnavtop=true 6 7@jd:body 8 9<div id="tb-wrapper"> 10 <div id="tb"> 11 12 <h2>This lesson teaches you to</h2> 13 <ol> 14 <li><a href="#Stop">Stop Your Activity</a></li> 15 <li><a href="#Start">Start/Restart Your Activity</a></li> 16 </ol> 17 18 <h2>You should also read</h2> 19 <ul> 20 <li><a href="{@docRoot}guide/components/activities.html">Activities</a> 21 </li> 22 </ul> 23 24<h2>Try it out</h2> 25 26<div class="download-box"> 27 <a href="http://developer.android.com/shareables/training/ActivityLifecycle.zip" 28class="button">Download the demo</a> 29 <p class="filename">ActivityLifecycle.zip</p> 30</div> 31 32 </div> 33</div> 34 35<p>Properly stopping and restarting your activity is an important process in the activity lifecycle 36that ensures your users perceive that your app is always alive and doesn't lose their progress. 37There are a few of key scenarios in which your activity is stopped and restarted:</p> 38 39<ul> 40 <li>The user opens the Recent Apps window and switches from your app to another app. The 41activity in your app that's currently in the foreground is stopped. If the user returns to your 42app from the Home screen launcher icon or the Recent Apps window, the activity restarts.</li> 43 <li>The user performs an action in your app that starts a new activity. The current activity 44is stopped when the second activity is created. If the user then presses the <em>Back</em> 45button, the first activity is restarted.</li> 46 <li>The user receives a phone call while using your app on his or her phone.</li> 47</ul> 48 49<p>The {@link android.app.Activity} class provides two lifecycle methods, {@link 50android.app.Activity#onStop()} and {@link android.app.Activity#onRestart()}, which allow you to 51specifically handle how your activity handles being stopped and restarted. Unlike the paused state, 52which identifies a partial UI obstruction, the stopped state guarantees that the UI is no longer 53visible and the user's focus is in a separate activity (or an entirely separate app).</p> 54 55<p class="note"><strong>Note:</strong> Because the system retains your {@link android.app.Activity} 56instance in system memory when it is stopped, it's possible that you don't need to implement the 57{@link android.app.Activity#onStop()} and {@link android.app.Activity#onRestart()} (or even {@link 58android.app.Activity#onStart()} methods at all. For most activities that are relatively simple, the 59activity will stop and restart just fine and you might only need to use {@link 60android.app.Activity#onPause()} to pause ongoing actions and disconnect from system resources.</p> 61 62<img src="{@docRoot}images/training/basics/basic-lifecycle-stopped.png" /> 63<p class="img-caption"><strong>Figure 1.</strong> When the user leaves your activity, the system 64calls {@link android.app.Activity#onStop onStop()} to stop the activity (1). If the user returns 65while the activity is stopped, the system calls {@link android.app.Activity#onRestart onRestart()} 66(2), quickly followed by {@link android.app.Activity#onStart onStart()} (3) and {@link 67android.app.Activity#onResume()} (4). Notice that no matter what scenario causes the activity to 68stop, the system always calls {@link android.app.Activity#onPause onPause()} before calling {@link 69android.app.Activity#onStop onStop()}.</p> 70 71 72 73<h2 id="Stop">Stop Your Activity</h2> 74 75<p>When your activity receives a call to the {@link android.app.Activity#onStop()} method, it's no 76longer visible and should release almost all resources that aren't needed while the user is not 77using it. Once your activity is stopped, the system might destroy the instance if it needs to 78recover system memory. In extreme cases, the system might simply kill your app process without 79calling the activity's final {@link android.app.Activity#onDestroy()} callback, so it's important 80you use {@link android.app.Activity#onStop()} to release resources that might leak memory.</p> 81 82<p>Although the {@link android.app.Activity#onPause onPause()} method is called before 83{@link android.app.Activity#onStop()}, you should use {@link android.app.Activity#onStop onStop()} 84to perform larger, more CPU intensive shut-down operations, such as writing information to a 85database.</p> 86 87<p>For example, here's an implementation of {@link android.app.Activity#onStop onStop()} that 88saves the contents of a draft note to persistent storage:</p> 89 90<!-- TODO: Find a better example for onStop, because this kind of thing should probably use a 91separate thread but that's too complicated to show here. --> 92<pre> 93@Override 94protected void onStop() { 95 super.onStop(); // Always call the superclass method first 96 97 // Save the note's current draft, because the activity is stopping 98 // and we want to be sure the current note progress isn't lost. 99 ContentValues values = new ContentValues(); 100 values.put(NotePad.Notes.COLUMN_NAME_NOTE, getCurrentNoteText()); 101 values.put(NotePad.Notes.COLUMN_NAME_TITLE, getCurrentNoteTitle()); 102 103 getContentResolver().update( 104 mUri, // The URI for the note to update. 105 values, // The map of column names and new values to apply to them. 106 null, // No SELECT criteria are used. 107 null // No WHERE columns are used. 108 ); 109} 110</pre> 111 112<p>When your activity is stopped, the {@link android.app.Activity} object is kept resident in memory 113and is recalled when the activity resumes. You don’t need to re-initialize components that were 114created during any of the callback methods leading up to the Resumed state. The system also 115keeps track of the current state for each {@link android.view.View} in the layout, so if the user 116entered text into an {@link android.widget.EditText} widget, that content is retained so you don't 117need to save and restore it.</p> 118 119<p class="note"><strong>Note:</strong> Even if the system destroys your activity while it's stopped, 120it still retains the state of the {@link android.view.View} objects (such as text in an {@link 121android.widget.EditText}) in a {@link android.os.Bundle} (a blob of key-value pairs) and restores 122them if the user navigates back to the same instance of the activity (the <a 123href="recreating.html">next lesson</a> talks more about using a {@link android.os.Bundle} to save 124other state data in case your activity is destroyed and recreated).</p> 125 126 127 128<h2 id="Start">Start/Restart Your Activity</h2> 129 130<p>When your activity comes back to the foreground from the stopped state, it receives a call to 131{@link android.app.Activity#onRestart()}. The system also calls the {@link 132android.app.Activity#onStart()} method, which happens every time your activity becomes visible 133(whether being restarted or created for the first time). The {@link 134android.app.Activity#onRestart()} method, however, is called only when the activity resumes from the 135stopped state, so you can use it to perform special restoration work that might be necessary only if 136the activity was previously stopped, but not destroyed.</p> 137 138<p>It's uncommon that an app needs to use {@link android.app.Activity#onRestart()} to restore 139the activity's state, so there aren't any guidelines for this method that apply to 140the general population of apps. However, because your {@link android.app.Activity#onStop()} 141method should essentially clean up all your activity's resources, you'll need to re-instantiate them 142when the activity restarts. Yet, you also need to instantiate them when your activity is created 143for the first time (when there's no existing instance of the activity). For this reason, you 144should usually use the {@link android.app.Activity#onStart()} callback method as the counterpart 145to the {@link android.app.Activity#onStop()} method, because the system calls {@link 146android.app.Activity#onStart()} both when it creates your activity and when it restarts the 147activity from the stopped state.</p> 148 149<p>For example, because the user might have been away from your app for a long time before 150coming back it, the {@link android.app.Activity#onStart()} method is a good place to verify that 151required system features are enabled:</p> 152 153<pre> 154@Override 155protected void onStart() { 156 super.onStart(); // Always call the superclass method first 157 158 // The activity is either being restarted or started for the first time 159 // so this is where we should make sure that GPS is enabled 160 LocationManager locationManager = 161 (LocationManager) getSystemService(Context.LOCATION_SERVICE); 162 boolean gpsEnabled = locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER); 163 164 if (!gpsEnabled) { 165 // Create a dialog here that requests the user to enable GPS, and use an intent 166 // with the android.provider.Settings.ACTION_LOCATION_SOURCE_SETTINGS action 167 // to take the user to the Settings screen to enable GPS when they click "OK" 168 } 169} 170 171@Override 172protected void onRestart() { 173 super.onRestart(); // Always call the superclass method first 174 175 // Activity being restarted from stopped state 176} 177</pre> 178 179 180 181 182<p>When the system destroys your activity, it calls the {@link android.app.Activity#onDestroy()} 183method for your {@link android.app.Activity}. Because you should generally have released most of 184your resources with {@link android.app.Activity#onStop()}, by the time you receive a call to {@link 185android.app.Activity#onDestroy()}, there's not much that most apps need to do. This method is your 186last chance to clean out resources that could lead to a memory leak, so you should be sure that 187additional threads are destroyed and other long-running actions like method tracing are also 188stopped.</p> 189 190
