1page.title=Starting an Activity 2page.tags=activity lifecycle 3helpoutsWidget=true 4 5trainingnavtop=true 6 7@jd:body 8 9 10<div id="tb-wrapper"> 11 <div id="tb"> 12 13 <h2>This lesson teaches you to</h2> 14<ol> 15 <li><a href="#lifecycle-states">Understand the Lifecycle Callbacks</a></li> 16 <li><a href="#launching-activity">Specify Your App's Launcher Activity</a></li> 17 <li><a href="#Create">Create a New Instance</a></li> 18 <li><a href="#Destroy">Destroy the Activity</a></li> 19</ol> 20 21 <h2>You should also read</h2> 22 <ul> 23 <li><a href="{@docRoot}guide/components/activities.html">Activities</a></li> 24 </ul> 25 26<h2>Try it out</h2> 27 28<div class="download-box"> 29 <a href="http://developer.android.com/shareables/training/ActivityLifecycle.zip" 30class="button">Download the demo</a> 31 <p class="filename">ActivityLifecycle.zip</p> 32</div> 33 34 </div> 35</div> 36 37<p>Unlike other programming paradigms in which apps are launched with a {@code main()} method, the 38Android system initiates code in an {@link android.app.Activity} instance by invoking specific 39callback methods that correspond to specific stages of its 40lifecycle. There is a sequence of callback methods that start up an activity and a sequence of 41callback methods that tear down an activity.</p> 42 43<p>This lesson provides an overview of the most important lifecycle methods and shows you how to 44handle the first lifecycle callback that creates a new instance of your activity.</p> 45 46 47 48<h2 id="lifecycle-states">Understand the Lifecycle Callbacks</h2> 49 50<p>During the life of an activity, the system calls a core set of lifecycle methods in 51a sequence similar to a step pyramid. That is, each stage of the 52activity lifecycle is a separate step on the pyramid. As the system creates a new activity instance, 53each callback method moves the activity state one step toward the top. The top of the pyramid is the 54point at which the activity is running in the foreground and the user can interact with it.</p> 55 56<p>As the user begins to leave the activity, the system calls other methods that move the activity 57state back down the pyramid in order to dismantle the activity. In some cases, the activity will 58move only part way down the pyramid and wait (such as when the user switches to another app), from 59which point the activity can move back to the top (if the user returns to the activity) and 60resume where the user left off.</p> 61 62 63<img src="{@docRoot}images/training/basics/basic-lifecycle.png" /> 64<p class="img-caption"><strong>Figure 1.</strong> A simplified illustration of the Activity 65lifecycle, expressed as a step pyramid. This shows how, for every callback used to take 66the activity a step toward the Resumed state at the top, there's a callback method 67that takes the activity a step down. The activity can also return to the resumed state from the 68Paused and Stopped state.</p> 69 70 71<p>Depending on the complexity of your activity, you probably don't need to implement all the 72lifecycle methods. However, it's important that you understand each one and implement those that 73ensure your app behaves the way users expect. Implementing your activity lifecycle methods properly 74ensures your app behaves well in several ways, including that it:</p> 75<ul> 76 <li>Does not crash if the user receives a phone call or switches to another app 77while using your app.</li> 78 <li>Does not consume valuable system resources when the user is not actively using 79it.</li> 80 <li>Does not lose the user's progress if they leave your app and return to it at a 81later time.</li> 82 <li>Does not crash or lose the user's progress when the screen rotates between 83landscape and portrait orientation.</li> 84</ul> 85 86<!-- 87<p class="table-caption"><strong>Table 1.</strong> Activity lifecycle state pairs and callback 88methods.</p> 89<table> 90 <tr> 91 <th scope="col">Lifecycle State</th> 92 <th scope="col">Startup Method</th> 93 <th scope="col">Teardown Method</th> 94 </tr> 95 <tr> 96 <td>Created / Destroyed</td> 97 <td>{@link android.app.Activity#onCreate onCreate()}</td> 98 <td>{@link android.app.Activity#onDestroy()}</td> 99 </tr> 100 <tr> 101 <td>Started / Stopped</td> 102 <td>{@link android.app.Activity#onStart()}</td> 103 <td>{@link android.app.Activity#onStop()}</td> 104 </tr> 105 <tr> 106 <td>Resumed / Resumed</td> 107 <td>{@link android.app.Activity#onResume()}</td> 108 <td>{@link android.app.Activity#onPause()}</td> 109 </tr> 110</table> 111--> 112 113<p>As you'll learn in the following lessons, there are several situations in which an activity 114transitions between different states that are illustrated in figure 1. However, only three of 115these states can be static. That is, the activity can exist in one of only three states for an 116extended period of time:</p> 117<dl> 118 <dt>Resumed</dt> 119 <dd>In this state, the activity is in the foreground and the user can interact with it. 120(Also sometimes referred to as the "running" state.)</dd> 121 <dt>Paused</dt> 122 <dd>In this state, the activity is partially obscured by another activity—the 123other activity that's in the foreground is semi-transparent or doesn't cover the entire screen. The 124paused activity does not receive user input and cannot execute any code. 125 <dt>Stopped</dt> 126 <dd>In this state, the activity is completely hidden and not visible to the user; it is 127considered to be in the background. While stopped, the activity instance and all its state 128information such as member variables is retained, but it cannot execute any code.</dd> 129</dl> 130 131<p>The other states (Created and Started) are transient and the system quickly moves from them to 132the next state by calling the next lifecycle callback method. That is, after the system calls 133{@link android.app.Activity#onCreate onCreate()}, it quickly calls {@link 134android.app.Activity#onStart()}, which is quickly followed by {@link 135android.app.Activity#onResume()}.</p> 136 137<p>That's it for the basic activity lifecycle. Now you'll start learning about some of the 138specific lifecycle behaviors.</p> 139 140 141 142<h2 id="launching-activity">Specify Your App's Launcher Activity</h2> 143 144<p>When the user selects your app icon from the Home screen, the system calls the {@link 145android.app.Activity#onCreate onCreate()} method for the {@link android.app.Activity} in your app 146that you've declared to be the "launcher" (or "main") activity. This is the activity that serves as 147the main entry point to your app's user interface.</p> 148 149<p>You can define which activity to use as the main activity in the Android manifest file, <a 150href="{@docRoot}guide/topics/manifest/manifest-intro.html">{@code AndroidManifest.xml}</a>, which is 151at the root of your project directory.</p> 152 153<p>The main activity for your app must be declared in the manifest with an <a 154href="{@docRoot}guide/topics/manifest/intent-filter-element.html">{@code 155<intent-filter>}</a> that includes the {@link 156android.content.Intent#ACTION_MAIN MAIN} action and 157{@link android.content.Intent#CATEGORY_LAUNCHER LAUNCHER} category. For example:</p> 158 159<pre> 160<activity android:name=".MainActivity" android:label="@string/app_name"> 161 <intent-filter> 162 <action android:name="android.intent.action.MAIN" /> 163 <category android:name="android.intent.category.LAUNCHER" /> 164 </intent-filter> 165</activity> 166</pre> 167 168<p class="note"><strong>Note:</strong> When you create a new Android project with the Android SDK 169tools, the default project files include an {@link android.app.Activity} class that's declared in 170the manifest with this filter.</p> 171 172<p>If either the {@link android.content.Intent#ACTION_MAIN MAIN} action or 173{@link android.content.Intent#CATEGORY_LAUNCHER LAUNCHER} category are not declared for one of your 174activities, then your app icon will not appear in the Home screen's list of apps.</p> 175 176 177 178<h2 id="Create">Create a New Instance</h2> 179 180<p>Most apps include several different activities that allow the user to perform different actions. 181Whether an activity is the main activity that's created when the user clicks your app icon or a 182different activity that your app starts in response to a user action, the system creates 183every new instance of {@link android.app.Activity} by calling its {@link 184android.app.Activity#onCreate onCreate()} method.</p> 185 186<p>You must implement the {@link android.app.Activity#onCreate onCreate()} method to perform basic 187application startup logic that should happen only once for the entire life of the activity. For 188example, your implementation of {@link android.app.Activity#onCreate onCreate()} should define the 189user interface and possibly instantiate some class-scope variables.</p> 190 191<p>For example, the following example of the {@link android.app.Activity#onCreate onCreate()} 192method shows some code that performs some fundamental setup for the activity, such as 193declaring the user interface (defined in an XML layout file), defining member variables, 194and configuring some of the UI.</p> 195 196<pre> 197TextView mTextView; // Member variable for text view in the layout 198 199@Override 200public void onCreate(Bundle savedInstanceState) { 201 super.onCreate(savedInstanceState); 202 203 // Set the user interface layout for this Activity 204 // The layout file is defined in the project res/layout/main_activity.xml file 205 setContentView(R.layout.main_activity); 206 207 // Initialize member TextView so we can manipulate it later 208 mTextView = (TextView) findViewById(R.id.text_message); 209 210 // Make sure we're running on Honeycomb or higher to use ActionBar APIs 211 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) { 212 // For the main activity, make sure the app icon in the action bar 213 // does not behave as a button 214 ActionBar actionBar = getActionBar(); 215 actionBar.setHomeButtonEnabled(false); 216 } 217} 218</pre> 219 220<p class="caution"><strong>Caution:</strong> Using the {@link android.os.Build.VERSION#SDK_INT} to 221prevent older systems from executing new APIs works in this way on Android 2.0 (API level 2225) and higher only. Older versions will encounter a runtime exception.</p> 223 224<p>Once the {@link android.app.Activity#onCreate onCreate()} finishes execution, the system 225calls the {@link android.app.Activity#onStart()} and {@link android.app.Activity#onResume()} methods 226in quick succession. Your activity never resides in the Created or Started states. Technically, the 227activity becomes visible to the user when {@link android.app.Activity#onStart()} is called, but 228{@link android.app.Activity#onResume()} quickly follows and the activity remains in the Resumed 229state until something occurs to change that, such as when a phone call is received, the user 230navigates to another activity, or the device screen turns off.</p> 231 232<p>In the other lessons that follow, you'll see how the other start up methods, {@link 233android.app.Activity#onStart()} and {@link android.app.Activity#onResume()}, are useful during your 234activity's lifecycle when used to resume the activity from the Paused or Stopped states.</p> 235 236<p class="note"><strong>Note:</strong> The {@link android.app.Activity#onCreate onCreate()} 237method includes a parameter called <code>savedInstanceState</code> that's discussed in the 238latter lesson about <a href="recreating.html">Recreating an Activity</a>.</p> 239 240 241<img src="{@docRoot}images/training/basics/basic-lifecycle-create.png" /> 242<p class="img-caption"><strong>Figure 2.</strong> Another illustration of the activity lifecycle 243structure with an emphasis on the three main callbacks that the system calls in sequence when 244creating a new instance of the activity: {@link android.app.Activity#onCreate onCreate()}, {@link 245android.app.Activity#onStart()}, and {@link android.app.Activity#onResume()}. Once this sequence of 246callbacks complete, the activity reaches the Resumed state where users can interact with the 247activity until they switch to a different activity.</p> 248 249 250 251 252 253 254 255<h2 id="Destroy">Destroy the Activity</h2> 256 257<p>While the activity's first lifecycle callback is {@link android.app.Activity#onCreate 258onCreate()}, its very last callback is {@link android.app.Activity#onDestroy}. The system calls 259this method on your activity as the final 260signal that your activity instance is being completely removed from the system memory.</p> 261 262<p>Most apps don't need to implement this method because local class references are destroyed 263with the activity and your activity should perform most cleanup during {@link 264android.app.Activity#onPause} and {@link android.app.Activity#onStop}. However, if your 265activity includes background threads that you created during {@link 266android.app.Activity#onCreate onCreate()} or other long-running resources that could 267potentially leak memory if not properly closed, you should kill them during {@link 268android.app.Activity#onDestroy}.</p> 269 270<pre> 271@Override 272public void onDestroy() { 273 super.onDestroy(); // Always call the superclass 274 275 // Stop method tracing that the activity started during onCreate() 276 android.os.Debug.stopMethodTracing(); 277} 278</pre> 279 280<p class="note"><strong>Note:</strong> The system calls {@link android.app.Activity#onDestroy} 281after it has already called {@link android.app.Activity#onPause} and {@link 282android.app.Activity#onStop} in all situations except one: when you call {@link 283android.app.Activity#finish()} from within the {@link android.app.Activity#onCreate onCreate()} 284method. In some cases, such as when your activity operates as a temporary decision maker to 285launch another activity, you might call {@link android.app.Activity#finish()} from within {@link 286android.app.Activity#onCreate onCreate()} to destroy the activity. In this case, the system 287immediately calls {@link android.app.Activity#onDestroy} without calling any of the other 288lifecycle methods.</p> 289
