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Activity restart on rotation Android
安卓旋转屏幕时Activity重新启动

In my Android application, when i rotate the device (slide out the keyboard) then my Activity is restarted ( onCreate is called).Now, this is probably how it's supposed to be, but i do a lot of initial setting up in the onCreate method, so i need either :

  1. Put all the initial setting up in another function so it's not all lost on device rotation or
  2. Make it so onCreate is not called again and the layout just adjusts or
  3. Limit the app to just portrait so that onCreate is not called.
时间:

using the Application Class

Depending on what you're doing in your initialization you could consider creating a new class that extends Application and moving your initialization code into an overridden onCreate method within that class.


public class MyApplicationClass extends Application {
 @Override
 public void onCreate() {
 super.onCreate();
//TODO Put your application initialization code here.
 }
}

The onCreate in the application class is only called when the entire application is created, so the Activity restarts on orientation or keyboard visibility changes won't trigger it.

It's good practice to expose the instance of this class as a singleton and exposing the application variables you're initializing using getters and setters.

NOTE : you'll need to specify the name of your new Application class in the manifest for it to be registered and used :


<application
 android:name="com.you.yourapp.MyApplicationClass"

Reacting to Configuration changes

As a further alternative, you can have your application listen for events that would cause a restart – like orientation and keyboard visibility changes – and handle them within your Activity.

Start by adding the android:configChanges node to your Activity's manifest node


android:configChanges="keyboardHidden|orientation"

or for Android 3.2 (API level 13 ) and newer :


android:configChanges="keyboardHidden|orientation|screenSize"

Then within the Activity override the onConfigurationChanged method and call setContentView to force the GUI layout to be re-done in the new orientation.


@Override
public void onConfigurationChanged(Configuration newConfig) {
 super.onConfigurationChanged(newConfig);
 setContentView(R.layout.myLayout);
}

Update for Android 3.2 and higher :

Caution : Beginning with Android 3.2 (API level 13 ), the"screen size"also changes when the device switches between portrait and landscape orientation.Thus, if you want to prevent runtime restarts due to orientation change when developing for API level 13 or higher (as declared by the minSdkVersion and targetSdkVersion attributes), you must include the "screenSize" value in addition to the "orientation" value.that is, you must declareandroid:configChanges="orientation|screenSize". However, if your application targets API level 12 or lower, then your activity always handles this configuration change itself (this configuration change does not restart your activity, even when running on an Android 3.2 or higher device).

what i did...

in the manifest, to the activity section, added :


android:configChanges="keyboardHidden|orientation"

in the code for the activity, implemented :


//used in onCreate() and onConfigurationChanged() to set up the UI elements
public void InitializeUI()
{
//get views from ID's
 this.textViewHeaderMainMessage = (TextView) this.findViewById(R.id.TextViewHeaderMainMessage);

//etc... hook up click listeners, whatever you need from the Views
}

//Called when the activity is first created.
@Override
public void onCreate(Bundle savedInstanceState)
{
 super.onCreate(savedInstanceState);
 setContentView(R.layout.main);

 InitializeUI();
}

//this is called when the screen rotates.
//(onCreate is no longer called when screen rotates due to manifest, see: android:configChanges)
@Override
public void onConfigurationChanged(Configuration newConfig)
{
 super.onConfigurationChanged(newConfig);
 setContentView(R.layout.main);

 InitializeUI();
}

Instead of trying to stop the onCreate() from being fired altogether, maybe try checking the BundlesavedInstanceState being passed into the event to see if it is null or not.

For instance, if i have some logic that should be run when the Activity is truly created, not on every orientation change, i only run that logic in the onCreate() only if the savedInstanceState is null.

Otherwise, i still want the layout to redraw properly for the orientation.


public void onCreate(Bundle savedInstanceState) {

 super.onCreate(savedInstanceState);

 if(savedInstanceState == null){
 setupCloudMessaging();
 }

 setContentView(R.layout.activity_game_list);
}

not sure if this is the ultimate answer, but it works for me.

What you describe is the default behavior.you have to detect and handle these events yourself by adding :


android:configChanges

to your manifest and then the changes that you want to handle.so for orientation, you would use :


android:configChanges="orientation"

and for the keyboard being opened or closed you would use :


android:configChanges="keyboardHidden"

If you want to handle both you can just separate them with the pipe command like :


android:configChanges="keyboardHidden|orientation"

This will trigger the onConfigurationChanged method in whatever Activity you call.if you override the method you can pass in the new values.

Hope this helps.

I just discovered this lore :

For keeping the Activity alive through an orientation change, and handling it through onConfigurationChanged, the documentation and the code sample above suggest this in the Manifest file:


android:configChanges="keyboardHidden|orientation"

which has the extra benefit that it always works.

The bonus lore is that omitting the keyboardHidden may seem logical, but it causes failures in the emulator (for Android 2.1 at least) :specifying only orientation will make the emulator call both OnCreate and onConfigurationChanged sometimes, and only OnCreate other times.

I haven't seen the failure on a device, but i have heard about the emulator failing for others.so it's worth documenting.

You might also consider using the Android platform's way of persisting data across orientation changes :onRetainNonConfigurationInstance() and getLastNonConfigurationInstance().

This allows you to persist data across configuration changes, such as information you may have gotten from a server fetch or something else that's been computed in onCreate or since, while also allowing Android to re-layout your Activity using the xml file for the orientation now in use.

See here or here.

It should be noted that these methods are now deprecated (although still more flexible than handling orientation change yourself as most of the above solutions suggest) with the recommendation that everyone switch to Fragments and instead use setRetainInstance(true) on each Fragment you want to retain.

I just simply added


 android:configChanges="keyboard|keyboardHidden|orientation"

in the manifest file and did not add any onConfigurationChanged method in my activity.

so every time the keyboard slides out or in nothing happens.

...