Lecture Mobile Application Development (119310)

Android Framework Basics

Agenda

Framework Components Part VII - Fragments
- Fragment Implementation
- Fragment Lifecycle
- Changing Fragments in Code

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

Fragment View Layout

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

Frament 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="de.hdmstuttgart.fragmentexampleapp.ExampleListFragment"
        android:layout_width="248dp"
        android:layout_height="match_parent" />
    <fragment
        android:id="@+id/fragment2"
        android:name="de.hdmstuttgart.fragmentexampleapp.ExampleDetailFragment"
        android:layout_width="wrap_content"
        android:layout_height="match_parent" />
</LinearLayout>

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

Fragment Implementation

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);
	}
}

Fragment Lifecycle

Fragments exist in three states when started:

Resumed: Fragment is visible and running
Paused: Another Activity is in theforeground and has focus but the Fragment is still visible
Stopped: Fragment is not visible but still alive. Either Activity has been stopped or Fragment removed

The lifecycle of the Activity directly affects the lifecycle of the Fragment

40%

Fragment Lifecycle

Fragment specific lifecycle callbacks are:

onAttach(): called when Fragment is added to Activity
onCreateView(): creates the view hierarchy of the Fragment
onActivityCreated(): called when Activity’s onCreate() method returned
onDestroyView(): called when view hierarchy of Fragement is removed
onDetach(): called when Fragment is disassociated with Activity

40%

Android Assignment - Fragments

Conduct Android Assignment on Fragments
Write an app that uses fragments to change the UI dependending on the screen-layout

Adding / Replacing Fragments dynamically

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

Adding / Replacing 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();

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

Recab Questions

When would you use a Fragment and when only an Activity?
When dynamically replacing a Fragement, 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?