Android App Development - 06 Fragments
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Fragments allow dividing the graphical interface of an activity into independent and reusable modules. A fragment has its own lifecycle and can be dynamically added or removed from an activity during runtime. Communication between a fragment and its activity can occur through interfaces or by accessing the activity from within the fragment. Fragments help create flexible and adaptable user interfaces for different screen sizes and configurations.
Android App Development - 06 Fragments
- 1. Fragments
- 2. Fragments A Fragment is a portion of the graphical interface of an Activity. It has its own life cycle and can be added or removed while the Activity is in the foreground. A Fragment is added to an Activity within a ViewGroup because the Fragment defines its graphical interface. Each Activity has a Back Stack to preserve the states of all the Fragments replaced in its graphical interface. Pattern: it is recommended to develop a Fragment which can be reused in any other Activity regardless of the type of terminal or configuration. Fragment
- 3. Fragments The introduction of the Fragment was dictated by the need to support small smartphone and tablet. The Dual Pane Pattern is the ideal way to develop an application that supports all of the Android devices. Dual Pane Pattern
- 4. Fragments To create a Fragment the class must extend the Fragment class. The life cycle of a Fragment is very similar to that of the Activity. The lifecycle differs from the Activity one for: ● onAttach(): the Fragment has been inserted in the GUI of Activity ● onCreateView(): it is used to define the View, which is the interface of the Fragment and that ends up in the hierarchy of Activity ● onActivityCreated(): the Activity.onCreate() is executed ● onDestroyView(): the view is destroyed ● OnDetach(): there is no longer any connection between the Fragment and Activity. Fragment Life Cycle
- 5. Fragments The direct comparison between Fragment and Activity LifeCycle highlights the management system of the Fragment added to the Activity. You can replace a Fragment with another because the Fragment Activity has a BackStack to save the data of all instances of added Fragment. Fragment Life Cycle
- 6. Fragments A Fragment can not have a graphical interface. If you want to assign a layout to a Fragment you must use the method Fragment.onCreateView(). @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { return super.onCreateView(inflater, container, savedInstanceState); } ● Inflater: useful to retrieve a view from an xml file in layout folder ● Container: the ViewGroup that contains the Fragment ● SavedInstanceState: the Bundle used to retrieve data saved before destroying as for Activity lifecycle User Interface
- 7. Fragments There are 2 ways to allocate and add a Fragment to an Activity: ● Layout file: when the Activity prepares its GUI and finds a <fragment /> tag, a Fragment is instantiated using the class defined in: android:name="com.example.fragment.ExampleFragment" ● FragmentTransaction: you can add a Fragment to the Activity at any time when it is in the foreground using the FragmentManager and FragmentTransaction objects. A Fragment must have a tag for its identification inside Activity backstack. In both cases, you can define a tag as a string. Fragment Activity adding
- 8. Fragments <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:orientation="horizontal" > <fragment android:id="@+id/list" android:name="com.example.news.ArticleListFragment" android:layout_width="0dp" android:layout_height="match_parent" android:layout_weight="1" /> <fragment android:id="@+id/viewer" android:name="com.example.news.ArticleReaderFragment" android:layout_width="0dp" android:layout_height="match_parent" android:layout_weight="2" /> </LinearLayout> Fragment Activity adding – Layout File
- 9. Fragments In order to add a Fragment to an Activity at runtime you need to do the following: FragmentManager fragmentManager = getFragmentManager() FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction(); ExampleFragment fragment = new ExampleFragment(); fragmentTransaction.add(R.id.fragment_container, fragment); fragmentTransaction.addToBackStack(tag); fragmentTransaction.commit(); You must call FragmentTransaction.commit() at the end. N.B. To support the integration of a Fragment from layouts, the Fragments have only one constructor and it's a zero-parameters one. Fragment Activity adding – FragmentTransaction
- 10. Fragments To create a new instance of the Fragment and pass parameters you can use the Static Factory Method: public class CustomFragment extends Fragment { private static final String BOOLEAN_KEY = "boolean"; private static final String PARCELABLE_KEY = "parcelable"; public static CustomFragment newInstance (boolean booleanValue, Parcelable parcelableValue) { CustomFragment customFragment = new CustomFragment(); Bundle bundle = new Bundle(); bundle.putBoolean(BOOLEAN_KEY, booleanValue); bundle.putParcelable(PARCELABLE_KEY, parcelableValue); customFragment.setArguments(bundle); return customFragment; } } Fragment Activity adding – Instance creation
- 11. Fragments If it helps, you can declare the zero-paramter constructor as private so it can not be instantiated without the parameters of Static Factory Method. private CustomFragment(){ } To retrieve the data contained in Bundle at any point in the life cycle of the Fragment you can invoke the method Fragment.getArguments () to retrieve the entire Bundle. getArguments().getBoolean(BOOLEAN_KEY); getArguments().getParcelable(PARCELABLE_KEY); Fragment Activity adding – Instance creation
- 12. Fragments ● To retrieve the reference to the Activity that contains the Fragment you can use the Fragment.getActivity() method: View listView = getActivity().findViewById(R.id.list); ● To retrieve the reference of the Fragment contained by the Activity you can use the method: ExampleFragment fragment = (ExampleFragment) getFragmentManager().findFragmentById(R.id.example_fragment); Pattern: never use the casting of Fragment.getActivity () to retrieve a reference to a particular Activity because this binds him to the Activity and the Fragment cannot be reused in another Activity. Communication with Activity
- 13. Fragments To adapt a Fragment to a variety of Activity, you can declare an interface within the class of Fragment and let the Activity implement it: public class CustomFragment extends Fragment { public interface OnSomethingHappenedListener{ public void onSomethingHappened(); } } public class MainActivity extends ActionBarActivity implements OnSomethingHappenedListener { @Override public void onSomethingHappened() { } } Communication with Activity
- 14. Fragments You need to override the Fragment.onAttach() method (which has the bound Activity as a parameter) to verify that the Activity is an instance of the interface. @Override public void onAttach(Activity activity) { super.onAttach(activity); try { mListener = (OnSomethingHappenedListener) activity; } catch (ClassCastException e) { throw new ClassCastException(activity.toString() + " must implement OnSomethingHappenedListener"); } } Communication with Activity
- 15. Fragments In order to notify the Activity that the Fragment needs to add one or more actions and change, therefore, the ActionBar, the Fragment must set Fragment.setHasOptionsMenu() as true in Fragment.onCreate(). The management of the Action Items is entrusted to methods with the same name as those of Activity: ● Fragment.onCreateOptionsMenu() ● Fragment.onPrepareOptionsMenu() ● Fragment.onOptionsItemSelected() When the user selects an Action, the first invoked callback is the Activity one and if this returns false the Fragment one is invoked. Contextual Action Items