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1page.title=Stopping and Restarting an Activity
2parent.title=Managing the Activity Lifecycle
3parent.link=index.html
4
5trainingnavtop=true
6previous.title=Pausing and Resuming an Activity
7previous.link=pausing.html
8next.title=Recreating an Activity
9next.link=recreating.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="#Stop">Stop Your Activity</a></li>
19      <li><a href="#Start">Start/Restart 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"
32class="button">Download the demo</a>
33 <p class="filename">ActivityLifecycle.zip</p>
34</div>
35
36  </div>
37</div>
38
39<p>Properly stopping and restarting your activity is an important process in the activity lifecycle
40that ensures your users perceive that your app is always alive and doesn't lose their progress.
41There are a few of key scenarios in which your activity is stopped and restarted:</p>
42
43<ul>
44  <li>The user opens the Recent Apps window and switches from your app to another app. The
45activity in your app that's currently in the foreground is stopped. If the user returns to your
46app from the Home screen launcher icon or the Recent Apps window, the activity restarts.</li>
47  <li>The user performs an action in your app that starts a new activity. The current activity
48is stopped when the second activity is created. If the user then presses the <em>Back</em>
49button, the first activity is restarted.</li>
50  <li>The user receives a phone call while using your app on his or her phone.</li>
51</ul>
52
53<p>The {@link android.app.Activity} class provides two lifecycle methods, {@link
54android.app.Activity#onStop()} and {@link android.app.Activity#onRestart()}, which allow you to
55specifically handle how your activity handles being stopped and restarted. Unlike the paused state,
56which identifies a partial UI obstruction, the stopped state guarantees that the UI is no longer
57visible and the user's focus is in a separate activity (or an entirely separate app).</p>
58
59<p class="note"><strong>Note:</strong> Because the system retains your {@link android.app.Activity}
60instance in system memory when it is stopped, it's possible that you don't need to implement the
61{@link android.app.Activity#onStop()} and {@link android.app.Activity#onRestart()} (or even {@link
62android.app.Activity#onStart()} methods at all. For most activities that are relatively simple, the
63activity will stop and restart just fine and you might only need to use {@link
64android.app.Activity#onPause()} to pause ongoing actions and disconnect from system resources.</p>
65
66<img src="{@docRoot}images/training/basics/basic-lifecycle-stopped.png" />
67<p class="img-caption"><strong>Figure 1.</strong> When the user leaves your activity, the system
68calls {@link android.app.Activity#onStop onStop()} to stop the activity (1). If the user returns
69while the activity is stopped, the system calls {@link android.app.Activity#onRestart onRestart()}
70(2), quickly followed by {@link android.app.Activity#onStart onStart()} (3) and {@link
71android.app.Activity#onResume()} (4). Notice that no matter what scenario causes the activity to
72stop, the system always calls {@link android.app.Activity#onPause onPause()} before calling {@link
73android.app.Activity#onStop onStop()}.</p>
74
75
76
77<h2 id="Stop">Stop Your Activity</h2>
78
79<p>When your activity receives a call to the {@link android.app.Activity#onStop()} method, it's no
80longer visible and should release almost all resources that aren't needed while the user is not
81using it. Once your activity is stopped, the system might destroy the instance if it needs to
82recover system memory. In extreme cases, the system might simply kill your app process without
83calling the activity's final {@link android.app.Activity#onDestroy()} callback, so it's important
84you use {@link android.app.Activity#onStop()} to release resources that might leak memory.</p>
85
86<p>Although the {@link android.app.Activity#onPause onPause()} method is called before
87{@link android.app.Activity#onStop()}, you should use {@link android.app.Activity#onStop onStop()}
88to perform larger, more CPU intensive shut-down operations, such as writing information to a
89database.</p>
90
91<p>For example, here's an implementation of {@link android.app.Activity#onStop onStop()} that
92saves the contents of a draft note to persistent storage:</p>
93
94<!-- TODO: Find a better example for onStop, because this kind of thing should probably use a
95separate thread but that's too complicated to show here. -->
96<pre>
97&#64;Override
98protected void onStop() {
99    super.onStop();  // Always call the superclass method first
100
101    // Save the note's current draft, because the activity is stopping
102    // and we want to be sure the current note progress isn't lost.
103    ContentValues values = new ContentValues();
104    values.put(NotePad.Notes.COLUMN_NAME_NOTE, getCurrentNoteText());
105    values.put(NotePad.Notes.COLUMN_NAME_TITLE, getCurrentNoteTitle());
106
107    getContentResolver().update(
108            mUri,    // The URI for the note to update.
109            values,  // The map of column names and new values to apply to them.
110            null,    // No SELECT criteria are used.
111            null     // No WHERE columns are used.
112            );
113}
114</pre>
115
116<p>When your activity is stopped, the {@link android.app.Activity} object is kept resident in memory
117and is recalled when the activity resumes. You don’t need to re-initialize components that were
118created during any of the callback methods leading up to the Resumed state. The system also
119keeps track of the current state for each {@link android.view.View} in the layout, so if the user
120entered text into an {@link android.widget.EditText} widget, that content is retained so you don't
121need to save and restore it.</p>
122
123<p class="note"><strong>Note:</strong> Even if the system destroys your activity while it's stopped,
124it still retains the state of the {@link android.view.View} objects (such as text in an {@link
125android.widget.EditText}) in a {@link android.os.Bundle} (a blob of key-value pairs) and restores
126them if the user navigates back to the same instance of the activity (the <a
127href="recreating.html">next lesson</a> talks more about using a {@link android.os.Bundle} to save
128other state data in case your activity is destroyed and recreated).</p>
129
130
131
132<h2 id="Start">Start/Restart Your Activity</h2>
133
134<p>When your activity comes back to the foreground from the stopped state, it receives a call to
135{@link android.app.Activity#onRestart()}. The system also calls the {@link
136android.app.Activity#onStart()} method, which happens every time your activity becomes visible
137(whether being restarted or created for the first time). The {@link
138android.app.Activity#onRestart()} method, however, is called only when the activity resumes from the
139stopped state, so you can use it to perform special restoration work that might be necessary only if
140the activity was previously stopped, but not destroyed.</p>
141
142<p>It's uncommon that an app needs to use {@link android.app.Activity#onRestart()} to restore
143the activity's state, so there aren't any guidelines for this method that apply to
144the general population of apps. However, because your {@link android.app.Activity#onStop()}
145method should essentially clean up all your activity's resources, you'll need to re-instantiate them
146when the activity restarts. Yet, you also need to instantiate them when your activity is created
147for the first time (when there's no existing instance of the activity). For this reason, you
148should usually use the {@link android.app.Activity#onStart()} callback method as the counterpart
149to the {@link android.app.Activity#onStop()} method, because the system calls {@link
150android.app.Activity#onStart()} both when it creates your activity and when it restarts the
151activity from the stopped state.</p>
152
153<p>For example, because the user might have been away from your app for a long time before
154coming back it, the {@link android.app.Activity#onStart()} method is a good place to verify that
155required system features are enabled:</p>
156
157<pre>
158&#64;Override
159protected void onStart() {
160    super.onStart();  // Always call the superclass method first
161
162    // The activity is either being restarted or started for the first time
163    // so this is where we should make sure that GPS is enabled
164    LocationManager locationManager =
165            (LocationManager) getSystemService(Context.LOCATION_SERVICE);
166    boolean gpsEnabled = locationManager.isProviderEnabled(LocationManager.GPS_PROVIDER);
167
168    if (!gpsEnabled) {
169        // Create a dialog here that requests the user to enable GPS, and use an intent
170        // with the android.provider.Settings.ACTION_LOCATION_SOURCE_SETTINGS action
171        // to take the user to the Settings screen to enable GPS when they click "OK"
172    }
173}
174
175&#64;Override
176protected void onRestart() {
177    super.onRestart();  // Always call the superclass method first
178
179    // Activity being restarted from stopped state
180}
181</pre>
182
183
184
185
186<p>When the system destroys your activity, it calls the {@link android.app.Activity#onDestroy()}
187method for your {@link android.app.Activity}. Because you should generally have released most of
188your resources with {@link android.app.Activity#onStop()}, by the time you receive a call to {@link
189android.app.Activity#onDestroy()}, there's not much that most apps need to do. This method is your
190last chance to clean out resources that could lead to a memory leak, so you should be sure that
191additional threads are destroyed and other long-running actions like method tracing are also
192stopped.</p>
193
194