diff options
Diffstat (limited to 'docs/html/resources/faq/framework.jd')
| -rw-r--r-- | docs/html/resources/faq/framework.jd | 197 |
1 files changed, 197 insertions, 0 deletions
diff --git a/docs/html/resources/faq/framework.jd b/docs/html/resources/faq/framework.jd new file mode 100644 index 0000000..f4b8db0 --- /dev/null +++ b/docs/html/resources/faq/framework.jd @@ -0,0 +1,197 @@ +page.title=Android Application Framework FAQ +parent.title=FAQs, Tips, and How-to +parent.link=index.html +@jd:body + +<ul> + <li><a href="#1">Do all the Activities and Services of an + application run in a single process?</a></li> + <li><a href="#2">Do all Activities run in the main thread of + an application process?</a></li> + <li><a href="#3">How do I pass complicated data structures + from one Activity/Service to another?</a></li> + <li><a href="#4">How can I check if an Activity is already + running before starting it?</a></li> + <li><a href="#5">If an Activity starts a remote service, is + there any way for the Service to pass a message back to the Activity?</a></li> + <li><a href="#6">How to avoid getting the Application not + responding dialog?</a></li> + <li><a href="#7">How does an application know if a package is + added or removed?</a></li> +</ul> + + +<a name="1" id="1"></a> + +<h2>Do all the Activities and Services of an application run in a +single process?</h2> + +<p>All Activities and Services in an application run in a single process by +default. If needed, you can declare an <code>android:process</code> attribute +in your manifest file, to explicitly place a component (Activity/Service) in +another process.</p> + + + +<a name="2" id="2"></a> + +<h2>Do all Activities run in the main thread of an application +process?</h2> + +<p>By default, all of the application code in a single process runs +in the main UI thread. This is the same thread +that also handles UI events. The only exception is the code that handles +IPC calls coming in from other processes. The system maintains a +separate pool of transaction threads in each process to dispatch all +incoming IPC calls. The developer should create separate threads for any +long-running code, to avoid blocking the main UI thread.</p> + + + +<a name="3" id="3"></a> + +<h2>How do I pass data between Activities/Services within a single +application?</h2> + +<p>It depends on the type of data that you want to share:</p> + +<h3>Primitive Data Types</h3> + +<p>To share primitive data between Activities/Services in an +application, use Intent.putExtras(). For passing primitive data that +needs to persist use the +<a href="{@docRoot}guide/topics/data/data-storage.html#preferences"> +Preferences</a> storage mechanism.</p> + +<h3>Non-Persistent Objects</h3> + +<p>For sharing complex non-persistent user-defined objects for short +duration, the following approaches are recommended: +</p> + <h4>The android.app.Application class</h4> + <p>The android.app.Application is a base class for those who need to +maintain global application state. It can be accessed via +getApplication() from any Activity or Service. It has a couple of +life-cycle methods and will be instantiated by Android automatically if +your register it in AndroidManifest.xml.</p> + + <h4>A public static field/method</h4> + <p>An alternate way to make data accessible across Activities/Services is to use <em>public static</em> +fields and/or methods. You can access these static fields from any other +class in your application. To share an object, the activity which creates your object sets a +static field to point to this object and any other activity that wants to use +this object just accesses this static field.</p> + + <h4>A HashMap of WeakReferences to Objects</h4> + <p>You can also use a HashMap of WeakReferences to Objects with Long +keys. When an activity wants to pass an object to another activity, it +simply puts the object in the map and sends the key (which is a unique +Long based on a counter or time stamp) to the recipient activity via +intent extras. The recipient activity retrieves the object using this +key.</p> + + <h4>A Singleton class</h4> + <p>There are advantages to using a static Singleton, such as you can +refer to them without casting getApplication() to an +application-specific class, or going to the trouble of hanging an +interface on all your Application subclasses so that your various +modules can refer to that interface instead. </p> +<p>But, the life cycle of a static is not well under your control; so +to abide by the life-cycle model, the application class should initiate and +tear down these static objects in the onCreate() and onTerminate() methods +of the Application Class</p> +</p> + +<h3>Persistent Objects</h3> + +<p>Even while an application appears to continue running, the system +may choose to kill its process and restart it later. If you have data +that you need to persist from one activity invocation to the next, you +need to represent that data as state that gets saved by an activity when +it is informed that it might go away.</p> + +<p>For sharing complex persistent user-defined objects, the +following approaches are recommended: +<ul> + <li>Application Preferences</li> + <li>Files</li> + <li>contentProviders</li> + <li>SQLite DB</li> +</ul> +</p> + +<p>If the shared data needs to be retained across points where the application +process can be killed, then place that data in persistent storage like +Application Preferences, SQLite DB, Files or ContentProviders. Please refer to +the <a href="{@docRoot}guide/topics/data/data-storage.html">Data Storage</a> +for further details on how to use these components.</p> + + + + +<a name="4" id="4"></a> + +<h2>How can I check if an Activity is already running before starting +it?</h2> + +<p>The general mechanism to start a new activity if its not running— +or to bring the activity stack to the front if is already running in the +background— is the to use the NEW_TASK_LAUNCH flag in the startActivity() +call.</p> + + + +<a name="5" id="5"></a> + +<h2>If an Activity starts a remote service, is there any way for the +Service to pass a message back to the Activity?</h2> + +<p>The remote service can define a callback interface and register it with the +clients to callback into the clients. The +{@link android.os.RemoteCallbackList RemoteCallbackList} class provides methods to +register and unregister clients with the service, and send and receive +messages.</p> + +<p>The sample code for remote service callbacks is given in <a +href="{@docRoot}resources/samples/ApiDemos/src/com/example/android/apis/app/RemoteService.html">ApiDemos/RemoteService</a></p> + + + +<a name="6" id="6"></a> + +<h2>How to avoid getting the Application not responding dialog?</h2> + +<p>Please read the <a href="{@docRoot}guide/practices/design/responsiveness.html">Designing for Responsiveness</a> +document.</p> + + + + +<a name="7" id="7"></a> + +<h2>How does an application know if a package is added or removed? +</h2> + +<p>Whenever a package is added, an intent with PACKAGE_ADDED action +is broadcast by the system. Similarly when a package is removed, an +intent with PACKAGE_REMOVED action is broadcast. To receive these +intents, you should write something like this: +<pre> + <receiver android:name ="com.android.samples.app.PackageReceiver"> + <intent-filter> + <action android:name="android.intent.action.PACKAGE_ADDED"/> + <action android:name="android.intent.action.PACKAGE_REMOVED"/> + + <data android:scheme="package" /> + </intent-filter> + </receiver> + </pre> + <br> +Here PackageReceiver is a BroadcastReceiver class.Its onReceive() +method is invoked, every time an application package is installed or +removed. + +</p> + + + |