1 page.title=Android Application Framework FAQ 2 excludeFromSuggestions=true 3 @jd:body 4 5 <ul> 6 <li><a href="#1">Do all the Activities and Services of an 7 application run in a single process?</a></li> 8 <li><a href="#2">Do all Activities run in the main thread of 9 an application process?</a></li> 10 <li><a href="#3">How do I pass complicated data structures 11 from one Activity/Service to another?</a></li> 12 <li><a href="#4">How can I check if an Activity is already 13 running before starting it?</a></li> 14 <li><a href="#5">If an Activity starts a remote service, is 15 there any way for the Service to pass a message back to the Activity?</a></li> 16 <li><a href="#6">How to avoid getting the Application not 17 responding dialog?</a></li> 18 <li><a href="#7">How does an application know if a package is 19 added or removed?</a></li> 20 </ul> 21 22 23 <a name="1" id="1"></a> 24 25 <h2>Do all the Activities and Services of an application run in a 26 single process?</h2> 27 28 <p>All Activities and Services in an application run in a single process by 29 default. If needed, you can declare an <code>android:process</code> attribute 30 in your manifest file, to explicitly place a component (Activity/Service) in 31 another process.</p> 32 33 34 35 <a name="2" id="2"></a> 36 37 <h2>Do all Activities run in the main thread of an application 38 process?</h2> 39 40 <p>By default, all of the application code in a single process runs 41 in the main UI thread. This is the same thread 42 that also handles UI events. The only exception is the code that handles 43 IPC calls coming in from other processes. The system maintains a 44 separate pool of transaction threads in each process to dispatch all 45 incoming IPC calls. The developer should create separate threads for any 46 long-running code, to avoid blocking the main UI thread.</p> 47 48 49 50 <a name="3" id="3"></a> 51 52 <h2>How do I pass data between Activities/Services within a single 53 application?</h2> 54 55 <p>It depends on the type of data that you want to share:</p> 56 57 <h3>Primitive Data Types</h3> 58 59 <p>To share primitive data between Activities/Services in an 60 application, use Intent.putExtras(). For passing primitive data that 61 needs to persist use the 62 <a href="{@docRoot}guide/topics/data/data-storage.html#preferences"> 63 Preferences</a> storage mechanism.</p> 64 65 <h3>Non-Persistent Objects</h3> 66 67 <p>For sharing complex non-persistent user-defined objects for short 68 duration, the following approaches are recommended: 69 </p> 70 <h4>Singleton class</h4> 71 <p>You can take advantage of the fact that your application 72 components run in the same process through the use of a singleton. 73 This is a class that is designed to have only one instance. It 74 has a static method with a name such as <code>getInstance()</code> 75 that returns the instance; the first time this method is called, 76 it creates the global instance. Because all callers get the same 77 instance, they can use this as a point of interaction. For 78 example activity A may retrieve the instance and call setValue(3); 79 later activity B may retrieve the instance and call getValue() to 80 retrieve the last set value.</p> 81 82 <h4>A public static field/method</h4> 83 <p>An alternate way to make data accessible across Activities/Services is to use <em>public static</em> 84 fields and/or methods. You can access these static fields from any other 85 class in your application. To share an object, the activity which creates your object sets a 86 static field to point to this object and any other activity that wants to use 87 this object just accesses this static field.</p> 88 89 <h4>A HashMap of WeakReferences to Objects</h4> 90 <p>You can also use a HashMap of WeakReferences to Objects with Long 91 keys. When an activity wants to pass an object to another activity, it 92 simply puts the object in the map and sends the key (which is a unique 93 Long based on a counter or time stamp) to the recipient activity via 94 intent extras. The recipient activity retrieves the object using this 95 key.</p> 96 97 <h3>Persistent Objects</h3> 98 99 <p>Even while an application appears to continue running, the system 100 may choose to kill its process and restart it later. If you have data 101 that you need to persist from one activity invocation to the next, you 102 need to represent that data as state that gets saved by an activity when 103 it is informed that it might go away.</p> 104 105 <p>For sharing complex persistent user-defined objects, the 106 following approaches are recommended: 107 <ul> 108 <li>Application Preferences</li> 109 <li>Files</li> 110 <li>contentProviders</li> 111 <li>SQLite DB</li> 112 </ul> 113 </p> 114 115 <p>If the shared data needs to be retained across points where the application 116 process can be killed, then place that data in persistent storage like 117 Application Preferences, SQLite DB, Files or ContentProviders. Please refer to 118 the <a href="{@docRoot}guide/topics/data/data-storage.html">Data Storage</a> 119 for further details on how to use these components.</p> 120 121 122 123 124 <a name="4" id="4"></a> 125 126 <h2>How can I check if an Activity is already running before starting 127 it?</h2> 128 129 <p>The general mechanism to start a new activity if its not running— 130 or to bring the activity stack to the front if is already running in the 131 background— is the to use the NEW_TASK_LAUNCH flag in the startActivity() 132 call.</p> 133 134 135 136 <a name="5" id="5"></a> 137 138 <h2>If an Activity starts a remote service, is there any way for the 139 Service to pass a message back to the Activity?</h2> 140 141 <p>See the {@link android.app.Service} documentation's for examples of 142 how clients can interact with a service. You can take advantage of the 143 fact that your components run in the same process to greatly simplify 144 service interaction from the generic remote case, as shown by the "Local 145 Service Sample". In some cases techniques like singletons may also make sense. 146 147 148 <a name="6" id="6"></a> 149 150 <h2>How to avoid getting the Application not responding dialog?</h2> 151 152 <p>Please read the <a href="{@docRoot}guide/practices/design/responsiveness.html">Designing for Responsiveness</a> 153 document.</p> 154 155 156 157 158 <a name="7" id="7"></a> 159 160 <h2>How does an application know if a package is added or removed? 161 </h2> 162 163 <p>Whenever a package is added, an intent with PACKAGE_ADDED action 164 is broadcast by the system. Similarly when a package is removed, an 165 intent with PACKAGE_REMOVED action is broadcast. To receive these 166 intents, you should write something like this: 167 <pre> 168 <receiver android:name ="com.android.samples.app.PackageReceiver"> 169 <intent-filter> 170 <action android:name="android.intent.action.PACKAGE_ADDED"/> 171 <action android:name="android.intent.action.PACKAGE_REMOVED"/> 172 173 <data android:scheme="package" /> 174 </intent-filter> 175 </receiver> 176 </pre> 177 <br> 178 Here PackageReceiver is a BroadcastReceiver class.Its onReceive() 179 method is invoked, every time an application package is installed or 180 removed. 181 182 </p> 183 184 185 186
