when is onRestoreInstanceState called?

问题

Sorry for my incomprehension, but I am new in the android development.

I have an application with activity A and activity B in it, and I go from activity A to activity B. When I left activity A, the onSaveInstanceState method was called, but when I went back to activity A (from activity B in the same application), the bundle in the onCreate method was null.

What can I do, to save the activity A's previous state? I only want to store the data for the application lifetime.

Can someone help me with this?

Here is my code for Activity A:

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

    if (savedInstanceState != null)
    {
        Log.v("Main", savedInstanceState.getString("test"));
    }
    else
    {
        Log.v("Main", "old instance");
    }
}  

@Override
public void onSaveInstanceState(Bundle savedInstanceState) 
{
    Log.v("Main", "save instance");

    savedInstanceState.putString("test", "my test string");

    super.onSaveInstanceState(savedInstanceState);
}


public void buttonClick(View view)
{
    Intent intent = new Intent(this, Activity2.class);
    startActivity(intent);
}

Here is my code for Activity B, when I press a button to go back to activity A:

public void onBack(View view)
{
    NavUtils.navigateUpFromSameTask(this);
}

回答1:

Saving and restoring state is meant to save the current temporary data that is obsolete when user exits the application. When you minimize or leave the Activity by opening next one it might be killed by the system due to lack of resources and restarted with savedInstanceState when you get back to it. So use onSaveInstanceState() only for saving minimize-restore session data.

So if you start a new Activity in front and get back to the previous one (what you are trying to do), the Activity A might not be killed (just stopped) and restarted without going being destroyed. You can force killing it and restoring by checking Don't keep activities in developer options menu.

If you call finish() or remove the Activity from recent task list the savedInstanceState will not be passed to onCreate() since the task was cleared.

If the value must be persistent consider using SharedPreferences.

回答2:

To answer your question, have a look at the android doc: https://developer.android.com/reference/android/app/Activity.html#onRestoreInstanceState(android.os.Bundle)

It says that onRestoreInstanceState is called after onStart() method in the activity lifecycle.

回答3:

This happens because you are navigating the hard way. If you used the phone's default back button navigation, your onCreate would get the bundle passed in.

To circumvent this issue, I suggest you save your state to shared preferences as well as a back up. When the bundle is null, restore it from the shared preferences.