Views and Fragments - android-fragments

Question

I have been reading about fragments lately, and almost everyone says that we should use it. I still can't understand the concept very well. I have read this, but I still have some questions.
First: A fragment must be related ( if its the right word) to an activity, let say MainActivity, the fragment has its own layout, the MainActivity has its own two. So what will be displayed on the screen? the fragment layout or the MainActivity or Both??
Second: If I want to convert an existing code to use fragments, what are the main changes?
Third: If I want to have more than one fragment, do I have to add a class that extends Fragment for each fragment I want to create??
Forth: onCreateView of the class that extends Fragment returns a view, is it correct to create a view inside it and return it for the main activity to add it to its layout??
Any help is appreciated.

Answer 1

Here are some tips about fragments what i understand so far , it might help you to understand Fragments :
1: About your first Question, yes both(Activity and Fragment) has their own layouts but Activity layout is act as base layout for fragments but this also depends on the layout you are working. If i state a simple example of HelloWorld app(which automatically created when you first create your Project in eclipse in updated adt), then you saw there Activity act as base and fragment layout show over it.
2: If you want to change the existing code to use fragments,firstly it depends upon the complexity of your code, and after that you have to change various things like if you are supporting api level 10 and below than you have to use Extra Libraries.there are lots of changes to be made but these all depends on your requirements.
3: Yes you have to create a Class which extends Fragment or any other Sub Class of Fragment. This Class is just like your Activity Class in which you have a xml layout to work with.
4: Yes you have to define a view inside OnCreateView() to return it to the activity to add to its layout or to show the UI.
Fragments are just like Activities , the pain comes when you working with Nested Fragments. and the life cycle of fragments are little different than Activity .
Note :please tell me if you have other queries or in case of any doubt about above written statements.

Answer 2

First:
In the layout of MainActivity you can embed multiple fragments layouts. You can even reuse these fragment layouts in any other activity. Ah, Good feature!
Second:
If I want to convert an existing code to use fragments, what are the
main changes?
To use fragments in your existing code you just need to,
The fragments will be added to the activity using the <fragment> element in the layout or can be added dynamically.
To check if the fragment is already part of your layout you can use the FragmentManager class -
DetailFragment fragment = (DetailFragment) getFragmentManager().
findFragmentById(R.id.detail_frag);
if (fragment==null || ! fragment.isInLayout()) {
// start new Activity
}
else {
fragment.update(...);
}
If a fragment is defined in an XML layout file, the android:name attribute points to the corresponding class.
To dynamically add fragments to an existing layout you typically define a container in the XML layout file in which you add a Fragment.
For this you can use, for example, a FrameLayout element.
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.replace(R.id.your_placehodler, new YourFragment());
ft.commit();
A new Fragment will replace an existing Fragment that was previously added to the container.
If you want to add the transaction to the backstack of Android, you use the addToBackStack() method.
This will add the action to the history stack of the activity, i.e., this will allow to revert the Fragment changes via the back button.
Third:
If I want to have more than one fragment, do I have to add a class
that extends Fragment for each fragment I want to create??
To define a new fragment you either extend the android.app.Fragment class or one of its subclasses,
for example, ListFragment, DialogFragment, PreferenceFragment or WebViewFragment.
Forth:
onCreateView of the class that extends Fragment returns a view, is it
correct to create a view inside it and return it for the main activity
to add it to its layout??
No need to return it to the main activity to add it to its layout. Just use FragmentTransaction's replace inside Main Activity followed by commit to be done.