Activity lifecycle - onCreate called on every re-o

问题:

I have a simple activity that loads a bitmap in onCreate. I find that if I rotate the device I can see from the logs that onCreate called again. In fact, because all instance variables are set to default values again I know that the entire Activity has been re-instantiated.

After rotating 2 times I get an FC because not enough memory can be allocated for the bitmap. (Are all instances of the activty still alive somewhere? Or does the GC not clean up fast enough?)

@Override
public void onCreate(Bundle savedInstanceState) {
    File externalStorageDir = Environment.getExternalStorageDirectory();
    File picturesDir = new File(externalStorageDir, "DCIM/Camera");
    File[] files = picturesDir.listFiles(new FilenameFilter(){
        public boolean accept(File dir, String name) {
            return name.toLowerCase().endsWith(".jpg");
        }});
    if (files.length > 0) {
        Bitmap bm = BitmapFactory.decodeStream(new FileInputStream(files[0]));
        ImageView view = (ImageView) findViewById(R.id.photo);
        view.setImageBitmap(bm);
    }
}

From all that I read, onCreate should be called once during the lifetime of an application. Am I wrong about this? How can re-orienting the device cause the activity to be recreated?

回答1:

Activity is recreated after each rotation by default. You can override this behaviour with configChanges attribute of the activity tag in AndroidManifest. For further details and different options, see http://developer.android.com/guide/topics/resources/runtime-changes.html

回答2:

android:configChanges="keyboardHidden|orientation|screenSize"

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 decalare android: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).

http://developer.android.com/guide/topics/resources/runtime-changes.html

回答3:

What happen when orientation changed

Life Cycle of orientation

onPause();
onSaveInstanceState();
onStop();
onDestroy();

onCreate();
onStart();
onResume();

---- app recreated and now is running ---

If you do long operation in onCreate() and want prevent re-create your activity add configChanges attribute in your mainfest

<activity android:name=".MyActivity"
          android:configChanges="orientation|keyboardHidden|screenSize"
          android:label="@string/app_name">

screenSize if you targeting api >= 13

回答4:

If you want to prevent FC from not enough memory, you need to deallocate resources in onStop() or onPause(). this allows you to use fresh memory in onCreate().

This is an alternate solution to preventing the recreation of the activity by using

android:configChanges="keyboardHidden|orientation"

As sometimes your activity's layout is different in portrait and landscape (layout, layout-land). preventing recreate on orientation change will prevent your activity from using the other orientation's layout.

回答5:

On Create method will call everytime when you do orientation, to avoid this you have to use

//Define Below in you Manifest file.
           <activity
                  android:name="com.ecordia.activities.evidence.MediaAttachmentView"
                  android:configChanges="keyboardHidden|orientation|screenSize"
            </activity>

//Define Below in your activity. 

         @Override
            public void onConfigurationChanged(Configuration newConfig) {

              super.onConfigurationChanged(newConfig);

              if (newConfig.orientation == Configuration.ORIENTATION_PORTRAIT) {
                  //your code
              } else if (newConfig.orientation == Configuration.ORIENTATION_LANDSCAPE) {
            //your code

              }
            }

It will works like a charm!!

回答6:

Actvity Lifecycle when you rotate screen

onPause
onSaveInstanceState
onStop
onDestroy

onCreate
onStart
onRestoreInstanceState
onResume

回答7:

Yes, activity's onCreate() is called everytime when the orientation changes but you can avoid the re-creation of Activity by adding configChanges attribute of Activity in your AndroidManifest file in the activity tag.

android:configChanges="keyboardHidden|orientation"

回答8:

One of the most common and suggested “solutions” to dealing with orientation changes is to not deal with them. You can do this by setting the android:configChanges flag on your Activity in AndroidManifest.xml as shown below:

<activity
    android:name=".MyActivity"
    android:label="@string/title_my_activity"
    android:configChanges="orientation|screenSize|keyboardHidden" />

This is NOT the correct way to deal with orientation changes.

CORRECT way is to implement the onSaveInstanceState method (this could be in your Activity, Fragment or both) and place the values you need to save in the Bundle argument that gets passed to the method.

It is nicely described here: http://code.hootsuite.com/orientation-changes-on-android/

While it may seem a bit tedious to implement, handling orientation changes properly provides you with several benefits: you will be able to easily use alternate layouts in portrait and landscape orientations, and you will be able to handle many exceptional states such as low memory situations and interruptions from incoming phone calls without any extra code.

回答9:

Use

android:configChanges="keyboardHidden|orientation" in Manifest XML activity Tag

@Override
public void onConfigurationChanged(Configuration newConfig) {
    // TODO Auto-generated method stub
    super.onConfigurationChanged(newConfig);
}

Use the above code to perform changes related to orientation in your Activity Java Code

Cheers!!!

回答10:

Kindly see my way of doing it:-

http://animeshrivastava.blogspot.in/2017/08/activity-lifecycle-oncreate-beating_3.html

snippet is:-

@Override
    protected void onSaveInstanceState(Bundle b)
    {
            super.onSaveInstanceState(b);
        String str="Screen Change="+String.valueOf(screenChange)+"....";
            Toast.makeText(ctx,str+"You are changing orientation...",Toast.LENGTH_SHORT).show();
        screenChange=true;

    }

    @Override
    public void onCreate(Bundle b)
        {
        super.onCreate(b);
        ctx=getApplicationContext();
        if(!screenChange)
        {
             String str="Screen Change="+String.valueOf(screenChange);

回答11:

I had the same problem, in which my onCreate is called multiple times when the screen orientation is changed. My problem got solved when i add android:configChanges="orientation|keyboardHidden|screenSize" in the activity tag in manifest

回答12:

I had the same problem and I did some workaround

Define didLoad boolean variable with false value

private boolean didLoad = false;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity);

    if (!this.didLoad){
        // Your code...
        this.didLoad = true;
    }