Android Fragments

1 Agenda

• Fragment Implementation
• Fragment Lifecycle
• Changing Fragments in Code

2 Fragment Basics

• Component introduced in Android 3.0 (API level 11)
• Multiple Fragments can be combined in one Activity
• Each Fragment defines a part of the UI in an Activity and must be embedded in an Activity
• The lifecycle of a Fragment is directly dependent on the Activity's lifecycle
• But: Fragments can be independently manipulated (e.g. added or removed)
• The Layout is defined in a layout.xml, the implementation in a class derived from the class Fragment

2.1 Fragment View Layout

• Two Fragments can be contained in one Activity (left) or in separate Activities (right)

  

2.2 Fragment View Layout

• Example Activity Layout with two Fragments

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/LinearLayout1"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="horizontal" >
    <fragment
        android:id="@+id/fragment1"
        android:name="com.example.fragmentexampleapp.ExampleListFragment"
        android:layout_width="248dp"
        android:layout_height="match_parent" />
    <fragment
        android:id="@+id/fragment2"
        android:name="com.example.fragmentexampleapp.ExampleDetailFragment"
        android:layout_width="wrap_content"
        android:layout_height="match_parent" />
</LinearLayout>

2.3 Fragment Implementation

• Create a subclass of Fragment

public class ExampleDetailFragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        
        return inflater.inflate(R.layout.detail_fragment,container, false);
    }
}

• And implement onCreateView() returning the View that will be embedded in the surrounding Activity
• LayoutInflater loads XML based layouts and creates View hierarchy

2.4 Fragment Implementation

ÜBERARBEITEN !!!! -> List ersetzen mit Fragment

• Create a subclass of one of Fragments subclasses, e.g. ListFragment

public class ExampleListFragment extends ListFragment {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        super.onCreate(savedInstanceState);
        List<String> songs = Arrays.asList("Blur","Metallica","Bosse");
        ArrayAdapter<String> bandArray = new ArrayAdapter<String>
            (getActivity(), android.R.layout.simple_list_item_1,songs);
        setListAdapter(bandArray);
    }
}

4 FragmentManager

• Instead of defining a fragment for an activity in the layout file, it can be added during runtime
• The FragmentManager class is used to create a FragmentTransaction
  • Provides APIs to add, remove and replace fragments
  • Call commit() when ready to make changes
• Fragments must have a container view in the layout

4.1 Loading Fragments dynamically

ArticleFragment newFragment = new ArticleFragment();
FragmentTransaction transaction = getFragmentManager().beginTransaction();

/* Replace whatever is in the fragment_container view with this fragment,
and add the transaction to the back stack so the user can navigate back */
transaction.replace(R.id.fragment_container, newFragment);
transaction.addToBackStack(null);

// Commit the transaction
transaction.commit();

5 Jetpack Navigation

Modern applications prefer the usage of Jetpack Navigation over manual fragment transactions.

6 Android Assignment 4

7 Summary

• Fragments are a good way to create flexible user interfaces
• They encapsulate a UI-Controller for an logical part of the UI. This simplifies component oriented UI programming on Android
• They can be statically added to layout files for different device types and also be used dynamically to change the UI corresponding to changes of the devices screen

8 Recap Questions

• When would you use a Fragment and when only an Activity?
• When dynamically replacing a Fragment, what is required in the layout? Try to replace a Fragment dynamically.
• Why do you have to call commit, when dynamically changing fragments?
• When accessing a view component that is added in one Fragment from another Fragment, what do you have to consider concerning the lifecycle?
• Which methods are "safe" to access view components from?