Creating Dynamic UI with Android Fragments

For the list of book titles, we have the option to define the fragment to contain either the ScrollView element that's nearest to the top (has an id value of scrollTitles) or just the RadioGroup element within that ScrollView element. When creating a fragment, we want to structure it such that the fragment is most easily reused. Although the RadioGroup element is all we need to display the list of titles, it seems likely that we'll always want the user to be able to scroll the list of titles if necessary. With this being the case, it makes sense to include the ScrollView element in this fragment.

To create a fragment for the book list, we define a new layout resource file called fragment_book_list.xml. We copy the top ScrollView element and its contents from the activity_main.xml resource file to the fragment_book_list.xml resource file. The resulting fragment_book_list.xml resource file is as follows:

<!-- List of Book Titles -->
<ScrollView
    android:layout_width="match_parent"
    android:layout_height="0dp"
    android:id="@+id/scrollTitles"
    android:layout_weight="1">
  <RadioGroup
      android:id="@+id/bookSelectGroup "
      android:layout_height="wrap_content"
      android:layout_width="wrap_content" >
    <RadioButton
        android:id="@+id/dynamicUiBook"
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:text="@string/dynamicUiTitle"
        android:checked="true"   />
    <RadioButton
        android:id="@+id/android4NewBook"
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:text="@string/android4NewTitle"    />

    <!-- Other RadioButtons elided for clarify -->

  </RadioGroup>
</ScrollView>

This gives us a layout resource consistent with the book title portion of the user interface as it appeared in the activity layout resource. This is a good start.

An effective fragment-oriented user interface is constructed with layout resources that minimize assumptions about where and how the fragment is used. The fewer assumptions we make about a fragment's use, the more reusable the fragment becomes.

The layout in the fragment_book_list.xml resource file as we now have it is very limiting because it includes significant assumptions. For example, the root ScrollView element includes a layout_height attribute with a value of 0. This assumes that the fragment will be placed within a layout that calculates the height for the fragment.

A layout_height attribute value of 0 prevents the ScrollView element from properly rendering when we use the fragment within any of the many layouts that require the ScrollView element to specify a meaningful height. A layout_height attribute value of 0 prevents the fragment from properly rendering even when doing something as simple as placing the fragment within a horizontally oriented LinearLayout element. The layout_weight attribute has similar issues.

In general, a good practice is to design the fragment to fully occupy whatever space it is placed within. This gives the layout in which the fragment is used the most control over the placement and sizing of the fragment.

To do this, we'll remove the layout_weight attribute from the ScrollView element and change the layout_height attribute value to match_parent. Because the ScrollView element is now the root node of the layout resource, we also need to add the android namespace prefix declaration.

The following code snippet shows the updated ScrollView element:

<ScrollView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/scrollTitles">

  <!—RadioGroup and RadioButton elements elided for clarity -->

</ScrollView>

With the updated ScrollView element, the fragment layout can now adapt to almost any layout it's referenced within.

For the book description, we'll define a layout resource file called fragment_book_desc.xml. The fragment layout includes the contents of the activity layout resource's bottom ScrollView element (has an id value of scrollDescription). Just as in the book list fragment, we'll remove the layout_weight attribute, set the layout_height attribute to match_parent, and add the android namespace prefix declaration.

The fragment_book_desc.xml layout resource file appears as follows:

<!-- Description of selected book -->
<ScrollView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/scrollDescription">
  <TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:textAppearance="?android:attr/textAppearanceMedium"
      android:text="@string/dynamicUiDescription"
      android:id="@+id/textView"
      android:paddingLeft="@dimen/activity_horizontal_margin"
      android:paddingRight="@dimen/activity_horizontal_margin"
      android:gravity="fill_horizontal"/>
</ScrollView>

All fragment classes must extend the android.app.Fragment class either directly or indirectly.

We'll call the class for the fragment that manages the book list, BookListFragment. The class will directly extend the Fragment class as follows:

Import android.app.Ftragment;
public class BookListFragment extends Fragment { … }

During the creation of a fragment, the Android framework calls a number of methods on that fragment. One of the most important of these is the onCreateView method. The onCreateView method is responsible for returning the view hierarchy represented by the fragment. The Android framework attaches that returned view hierarchy for the fragment to the appropriate place in the activity's overall view hierarchy.

In a case like the BookListFragment class where the Fragment class inherits directly from the Fragment class, we must override the onCreateView method and perform the work necessary to construct the view hierarchy.

The onCreateView method receives three parameters. We'll focus on just the first two for now:

The LayoutInflater class provides a method called inflate that handles the details of converting a layout resource into the corresponding view hierarchy and returns a reference to the root view of that hierarchy. Using the LayoutInflater.inflate method, we can implement our BookListFragment class' onCreateView method to construct and return the view hierarchy corresponding to the R.layout.fragment_book_list layout resource as shown in the following code:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View viewHierarchy = 
    inflater.inflate(R.layout.fragment_book_list, 
    container, false);
    return viewHierarchy;
}

You'll notice in the preceding code we include the container reference and a Boolean value of false in the call to the inflate method. The container reference provides the necessary layout parameters for the inflate method to properly format the new view hierarchy. The parameter value of false indicates that container is to be used only for the layout parameters. If this value were true, the inflate method would also attach the new view hierarchy to the container view group. We do not want to attach the new view hierarchy to the container view group in the onCreateView method because the activity will handle that.

For the book description fragment, we'll define a class called BookDescFragment. This class is identical to the BookListFragment class except the BookDescFragment class uses the R.layout.fragment_book_desc layout resource as follows:

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

When using the Android Support Library to provide pre-API Level 11 (Android 3.0) fragment support, we have one additional step. In this case, we have to make one small, but important change to our activity. We must change the MainActivity class' base class from the Activity class to the android.support.v4.app.FragmentActivity class. Because the pre-API Level 11 Activity class doesn't understand fragments, we use the FragmentActivity class from the Android Support Library to add fragment support to our MainActivity class.