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How to correct add a listener to a Fragment

I got two Fragments (basic-setup) with a FragmentPagerAdapter for use in a TabLayout.
tabLayout.setupWithViewPager(myViewPagerWithAdapter);
My Adapter takes an array of Fragments and titles as argument (also the FragmentManger for the super call).
MyViewPagerAdapter adapter = new MyViewPagerAdapter(getSupportFragmentManager() ,fragments, titles);
My Fragments (indeed 2) got a WebView in their xml file. To use the WebViews outside the fragments (it's kinda odd to code the control of the WebViews inside fragments, because the MainActivity is mainly involved with my user interaction). To know when my WebViews in the fragments were inflated, I added a simple interface, called OnFragmentCreatedListener, which is triggered in public void onActivityCreated(Bundle savedInstanceState) of the fragments. I add the listener through following way:
private OnFragmentCreatedListener listener;
...
public void addOnFragmentCreatedListener(OnFragmentCreatedListener listener) {
this.listener = listener;
}
In my MainActivity I add this listener like this:
public class MainActivity extends AppCompatActivity implements OnFragmentCreatedListener {
#Override
protected void onCreate(Bundle savedInstanceState) {
...
MyFragment fragment = MyFragment.newInstance();
...
fragment.addOnFragmentCreatedListener(this);
...
}
#Override
public void onFragmentCreatedListener() {
// Do my stuff
}
Now when the screen rotates and the activity is restarted, the listener in the fragment stays null (wasn't initialized). Now I don't know what to do, because I don't understand this behavior. The code should do the same like in the first protected void onCreate(Bundle savedInstanceState) of the MainActivity. It's adding the listener but it stays null (like wtf?).
Thank you for spending your time!

Android declaring a textview in parent class xml and setting listener in child class

Actually, I have 2 activities, BaseActivity and ChildActivity. ChildActivity is extending BaseActivity. I am declaring a TextView in Base class XML and I wanted to set listener of that TextView in ChildActivity.
I can directly call the functions defined in BaseActivity from ChildActivity, but how can I set onClickListener in child class?

For that You can use
interface
create one interface
interface Textview_InterFace
{
Textview findTextview();
}
Second step is to implement to your BaseActivity and also implement method
And in Child Activity
public class childActivity extends BaseActivity
{
Textview_Interface my_textview;
Textview textview;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
my_textview = (Textview_Interface) this;
textview = my_textview.findTextview();
//perform here your onClickListner
}

Android Access Fragment Variable

What I am basicaly trying is to access a variable inside a Fragment and get rid of it in my activity.
It worked to get the variable of the activity in my fragment but not the other way around:
what I did:
// get method of MainActivity
final MainActivity activity = (MainActivity) getActivity();
is it even possible to make this "the other way around"?
(Access variable of Fragment in my Activity)?

You need to implement listeners.
You can read more about here:Communicating with Other Fragments
Here is a code example how to pass data (or null) from Activity to a Fragment:
public class FragmentA extends Fragment implements FragmentCommunicator{
#Override
public void onAttach(Activity activity) {
super.onAttach(activity);
((MainActivity)getActivity()).fragmentCommunicator = this;
}
#Override
public void passDataToFragment(String str) {
//str is the string variable you pass from the Activity, it can be null...
}
}}
Next the FragmentCommunicator Class:
public interface FragmentCommunicator{
public void passDataToFragment(String str);}
And the Activity:
public class MainActivity extends FragmentActivity{
public FragmentCommunicator fragmentCommunicator;
public void someMethod(String someString) {
fragmentCommunicator.passDataToFragment(someString);
}}
When you call passDataToFragment() from the Activity it will pass the string (or any other variable) to the fragment method passDataToFragment().

You can get your fragment by id/tag from fragment manager and do whatever you want with them.

that's super easy.
if you are adding your fragments in runtime (using FragmentManager)
you are creating objects of that fragment...just keep the reference with you and you are good to call any function or access any variable of the fragment.
e.g. you fragment is MyFragment
MyFragment mf = new MyFragment();
getSupportFragmentManger().beginTrans......you know the code to add a fragment
then simply call any method...for say... change() by mf.change();
or you can do something like
MyFragment mf = (MyFragment) findFragmentById(R.id.container);
and then again mf.change();

Java syntax explanation

I'm new to Java and I encountered the following code:
public class MainActivity extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button button = new Button(this);
button.setText("Touch That!");
button.setOnClickListener( new View.OnClickListener() {
#Override
public void onClick(View v) {
MainActivity.this.onButtonClick(v);
}
});
RelativeLayout relativeLayout = (RelativeLayout) findViewById(R.id.rootlayout);
relativeLayout.addView(button);
}
public void onButtonClick(View view){
//do something when button is clicked.
}
}
I didn't understand the syntax, View.OnClickListener() c'tor is called and it is followed by {} and overidding method.
what does this syntax stand for?
to which object this refers?
My guess is the button. but if I'm right why to use MainActivity.this instead of this? (the object that invoked the method)

This is an anonymous class declaration. It means that you will override some methods inside the class dynamically.
Take a look at this arcticle:
http://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html
About your second question, MainActivity.this refers to the instance of the Activity you are currently in. If you call only this, it would refer to the actual object. When you call MainActivity.this, you will get the instance of MainActivity you are in, even if there is more activities created. Take a look at Android's activity lifecycle.
What's the difference between this and Activity.this
Hope it helps.

By calling
new View.OnClickListener(){}
you are creating an object implementing interface OnClickListerner that requires you to implement the click method.
Someone can correct if I am wrong.

Set progressBar visibility from Fragments?

I'd like to know how I can set eg a progressBar visibility from within several Fragments?
class MyActivity extends FragmentActivity {
public setProgressBar(boolean toggle) {
progressBar.setVisibility(toggle);
}
}
Now I want to call setProgressBar(true/false) from within MyActivity, but also from any Fragment classes.

In your Fragment call getActivity(), cast it to MyActivity and call setProgressBar(toggle)
((MyActivity) getActivity()).setProgressBar(toggle);