尽管调用onSaveInstance（），捆绑的saveInstanceState在onCreateView（）中始终为null

Question

I have FragmentA hosted by ActivityA . When the user selects an item from the options menu, ActivityB is started which hosts FragmentB . For now, I want to retain a String and a boolean from FragmentA by overriding onSaveInstanceState() , so when the user returns to FragmentA , their information is preserved.

Code from FragmentA :

@Override
public void onSaveInstanceState(Bundle savedInstanceState)
{
    //LOGS SHOW THAT THIS IS ALWAYS CALLED WITH CORRECT VALUES
    super.onSaveInstanceState(savedInstanceState);
    savedInstanceState.putString("string", "example");
    savedInstanceState.putBoolean("boolean", bool);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle savedInstanceState)
{
    View v = inflater.inflate(R.layout.fragmentA, parent, false);

    //LOGS SHOW THAT THIS IS ALWAYS NULL
    if (savedInstanceState != null)
    {
        if (savedInstanceState.getString("text") != null)
        {
            mObject.setText(savedInstanceState.getString("string"));
        }

        bool = savedInstanceState.getBoolean("boolean");
    }

    ...
}

From reading previous problems similar to mine:

1) I decided to place the code to recover the information in onCreateView() because onCreate() will not always be called. (Although tests with the code in onCreate() also have the same problem.)

2) I also did not call setRetainInstance(true) , since this will cause Bundle savedInstanceState to always be null .

3) I made sure that the XML layout for FragmentA has an id. The various children elements of this layout also have ids.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/fragmentA"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical"> 

    ...
</LinearLayout>

Despite this, Bundle savedInstanceState is always null. What am I missing here?

Any help appreciated. Thanks!

Answer 1

The savedInstanceState is null when no data was been previously saved. To save data you must override the onSaveInstanceStateBundle(Bundle) method as described in the Android documentation:

you should use the onPause() method to write any persistent data (such as user edits) to storage. In addition, the method onSaveInstanceState(Bundle) is called before placing the activity in such a background state, allowing you to save away any dynamic instance state in your activity into the given Bundle, to be later received in onCreate(Bundle) if the activity needs to be re-created. See the Process Lifecycle section for more information on how the lifecycle of a process is tied to the activities it is hosting. Note that it is important to save persistent data in onPause() instead of onSaveInstanceState(Bundle) because the latter is not part of the lifecycle callbacks, so will not be called in every situation as described in its documentation.

More info here

Answer 2

Use the onPause() to save your persistent data, that's referring to data that you would like to keep permanently, and hence, you save it in your SharedPreferences or your Database . onSaveInstanceState on the other hand retrains the data in case an activity is destroyed and you'd like to get that data back, a good scenario would be a user filling in a form. You said in your example that you're navigating from Activity A to Activity B , then you go back to Activity A , when you first navigate, Activity A is not destroyed, it's only sent to the background, so when you return to it, it's already there and will be brought to the foreground, your values should actually be there unchanged, and onCreate and onCreateView will not be called as the Activity is still alive (although it might be killed in case of low memory on device).

Best and fastest way to test your onSaveInstanceState is the most often used scenario, Orientation Change , orientation change will cause the activity to get destroyed completely and re-created, so allow orientation change on Activity A , put some values in your saveStateBundle and rotate the device, now this will call all your methods from the start, onCreate , onCreateView , ... etc to create the activity with the appropriate layout, your savedInstanceState should not be null now.

Note this is assuming your application is staying alive, if you're going to close the app completely and still want to keep the data, then put your information in the SharedPreferences or Database and retrieve them when you start the app again.

// Edit 1

to show you how to store values in SharedPreferences anywhere in your application, these values are persistent even if your application is closed. (although onSaveInstanceState should be enough to what you're looking for but hope this helps)

// SharedPreference
SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());
// Storing
preferences.edit().putString("valueNameOrKey", StringValue).commit();
// Retrieving
preferences.getString("valueNameOrKey", defaultValueToReturnInCaseThatKeyIsNotFound);

// Edit 2

To remove any key/value pair from the SharedPreferences you can do this:

preferences.edit().remove("valueNameOrKey").commit();

But then pay attention to what happens when you retrieve the value, since the key will not be available, it's going to return the default value instead like this:

preferences.getString("valueNameoOrKey", ""); // "" Is my default value since I'm using a String

since you can use putString, putInt, putBoolean etc, same goes for the get functions, your default value must match the expected return type.