savedInstanceState is always null, yet onSaveInsta

问题:

First of all I'm new to Android - this question could seem stupid :)

I've created an main Activity that contains a ViewPager. The ViewPager is linked to my ActionBar tabs. Currently my main Activity contains 3 tabs in this way.

My issue is the following: when I'm on the 3rd tab and create a new Activity and afterwards return to my main Activity I always return to my 1st tab.

I've tried the following approach in my main Activity:

// debugger breakpoints tell me this method is always called when other activity is shown ...
@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);

    int pos = getActionBar().getSelectedNavigationIndex();
    outState.putInt("tab", pos);
}

// however, in onCreate() my savedInstanceState is always 'null' ...
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    this.invalidateOptionsMenu();

    setContentView(R.layout.activity_main);

    final ActionBar actionBar = getActionBar();
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

    mSectionsPagerAdapter = new SectionsPagerAdapter(
            getSupportFragmentManager());

    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    mViewPager
            .setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
                @Override
                public void onPageSelected(int position) {
                    actionBar.setSelectedNavigationItem(position);
                }
            });

    for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
        actionBar.addTab(actionBar.newTab()
                .setText(mSectionsPagerAdapter.getPageTitle(i))
                .setTabListener(this));
    }

    // trying to restore state here, yet savedInstanceState is _always_ null ...
    if (savedInstanceState != null) {
        actionBar.setSelectedNavigationItem(savedInstanceState.getInt("tab", 0));
    }
}

I've read that the savedInstanceState will always be null when there's no id defined. My XML layout has an id defined though, so this should not be an issue. I've also read that the 'onSaveInstanceState()` method should always call the method on the super class first, so I've implemented the method accordingly.

Anyone have any idea how to fix my issue? How to get the ViewPager to show the right tab / fragment when the main Activity is recreated?

回答1:

Thanks She Smile, your suggestions helped me solve my issue, because it helped me find the source of my problem.

Apparently onCreate() could only have a savedInstanceState parameter when the Activity was completely destroyed and recreated. In my case the Activity still existed though, therefore I will not have a savedInstanceState parameter. My "fix" was to create a static int that contains the selected tab index. I realise this might not be the best solution, since this variable will be shared across all instances of the Activity, but since in my situation there will only even be one instance of my activity it should be fine.

public class MainActivity extends Activity implements ActionBar.TabListener {
    private static int tabIdx;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        if (savedInstanceState != null) {
            tabIdx = savedInstanceState.getInt("tab", 0);
        }
        getActionBar().setSelectedNavigationItem(tabIdx);
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);

        tabIdx = getActionBar().getSelectedNavigationIndex();
        outState.putInt("tab", tabIdx);
    }
}

回答2:

so if the onSaveInstance was Called

you can declare a global variable for your

 int pos=0; 

after onSaveInstance() gets called, method below is called:

@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
super.onRestoreInstanceState(savedInstanceState);
pos=savedInstanceState.getInt("tab", 0);    

}

so in your onCreate() function replace the condition from this:

if (savedInstanceState != null) {
    actionBar.setSelectedNavigationItem(savedInstanceState.getInt("tab", 0));
}

to this:

if(pos>0){
actionBar.setSelectedNavigationItem(pos);
}

回答3:

The onRestoreInstanceState() method is only called when there is a Bundle object to restore from, i.e. it is NOT NULL. Otherwise, the restore method will not be called.