Android Fragment 探索（二）— 示例

下面提供了一个 Fragment 示例，其中的 Activity 使用两个 Fragment 来创建一个双窗格布局。一个用于显示莎士比亚戏剧标题列表，另一个用于显示其摘要。此外，它还展示了如何根据屏幕配置提供不同的 Fragment 配置。

设置 MainActivity 的根布局：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.fragment_layout);
}

fragment_layout.xml 的代码如下:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent" android:layout_height="match_parent">

    <fragment class="com.example.android.apis.app.FragmentLayout$TitlesFragment"
            android:id="@+id/titles" android:layout_weight="1"
            android:layout_width="0px" android:layout_height="match_parent" />

    <FrameLayout android:id="@+id/details" android:layout_weight="1"
            android:layout_width="0px" android:layout_height="match_parent"
            android:background="?android:attr/detailsElementBackground" />

</LinearLayout>

使用此布局，系统可在 Activity 加载布局时立即实例化 TitlesFragment（列出戏剧标题），而 FrameLayout（用于显示戏剧摘要）则会占用屏幕右侧的空间，但一开始处于空白状态。用户从列表中选择某个项目后，系统才会将 Fragment 放入 FrameLayout。

不过，并非所有屏幕配置都具有足够的宽度并排显示戏剧列表和摘要。因此，以上布局仅用于横向屏幕配置（布局保存在 res/layout-land/fragment_layout.xml）。

因此，当屏幕纵向显示时，系统会使用以下布局（保存在 res/layout/fragment_layout.xml）：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="match_parent">
    <fragment class="com.example.android.apis.app.FragmentLayout$TitlesFragment"
            android:id="@+id/titles"
            android:layout_width="match_parent" android:layout_height="match_parent" />
</FrameLayout>

此布局仅包括 TitlesFragment 一个 Fragment。也就是说，当设备纵向显示时，只有戏剧标题列表可见。因此，当用户点击某个列表项时，应用会启动一个新 Activity 来显示摘要，而不是加载另一个 Fragment。

接下来，我们来实现这两个 Fragment 的功能。第一个是 TitlesFragment，它显示莎士比亚戏剧标题列表。该 Fragment 扩展了 ListFragment，并依靠它来处理大多数列表视图工作。

请注意，用户点击列表项时可能会出现两种行为，具体取决于这两个布局中哪一个处于活动状态：

系统可能会创建一个新 Fragment 并在同一个 Activity 中显示戏剧摘要（将 Fragment 添加到 FrameLayout），
也可能会启动一个新 Activity（在该 Activity 中显示 Fragment）

public static class TitlesFragment extends ListFragment {
    boolean mDualPane;
    int mCurCheckPosition = 0;

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        // Populate list with our static array of titles.
        setListAdapter(new ArrayAdapter<String>(getActivity(),
                android.R.layout.simple_list_item_activated_1, Shakespeare.TITLES));

        // Check to see if we have a frame in which to embed the details
        // fragment directly in the containing UI.
        View detailsFrame = getActivity().findViewById(R.id.details);
        mDualPane = detailsFrame != null && detailsFrame.getVisibility() == View.VISIBLE;

        if (savedInstanceState != null) {
            // Restore last state for checked position.
            mCurCheckPosition = savedInstanceState.getInt("curChoice", 0);
        }

        if (mDualPane) {
            // In dual-pane mode, the list view highlights the selected item.
            getListView().setChoiceMode(ListView.CHOICE_MODE_SINGLE);
            // Make sure our UI is in the correct state.
            showDetails(mCurCheckPosition);
        }
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt("curChoice", mCurCheckPosition);
    }

    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        showDetails(position);
    }

    /**
     * Helper function to show the details of a selected item, either by
     * displaying a fragment in-place in the current UI, or starting a
     * whole new activity in which it is displayed.
     */
    void showDetails(int index) {
        mCurCheckPosition = index;

        if (mDualPane) {
            // We can display everything in-place with fragments, so update
            // the list to highlight the selected item and show the data.
            getListView().setItemChecked(index, true);

            // Check what fragment is currently shown, replace if needed.
            DetailsFragment details = (DetailsFragment)
                    getSupportFragmentManager().findFragmentById(R.id.details);
            if (details == null || details.getShownIndex() != index) {
                // Make new fragment to show this selection.
                details = DetailsFragment.newInstance(index);

                // Execute a transaction, replacing any existing fragment
                // with this one inside the frame.
                FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
                if (index == 0) {
                    ft.replace(R.id.details, details);
                } else {
                    ft.replace(R.id.a_item, details);
                }
                ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
                ft.commit();
            }

        } else {
            // Otherwise we need to launch a new activity to display
            // the dialog fragment with selected text.
            Intent intent = new Intent();
            intent.setClass(getActivity(), DetailsActivity.class);
            intent.putExtra("index", index);
            startActivity(intent);
        }
    }
}

DetailsFragment 显示从 TitlesFragment 的列表中选择的项目的戏剧摘要：

public static class DetailsFragment extends Fragment {
    /**
     * Create a new instance of DetailsFragment, initialized to
     * show the text at 'index'.
     */
    public static DetailsFragment newInstance(int index) {
        DetailsFragment f = new DetailsFragment();

        // Supply index input as an argument.
        Bundle args = new Bundle();
        args.putInt("index", index);
        f.setArguments(args);

        return f;
    }

    public int getShownIndex() {
        return getArguments().getInt("index", 0);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        if (container == null) {
            // We have different layouts, and in one of them this
            // fragment's containing frame doesn't exist. The fragment
            // may still be created from its saved state, but there is
            // no reason to try to create its view hierarchy because it
            // isn't displayed. Note this isn't needed -- we could just
            // run the code below, where we would create and return the
            // view hierarchy; it would just never be used.
            return null;
        }

        ScrollView scroller = new ScrollView(getActivity());
        TextView text = new TextView(getActivity());
        int padding = (int)TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,
                4, getActivity().getResources().getDisplayMetrics());
        text.setPadding(padding, padding, padding, padding);
        scroller.addView(text);
        text.setText(Shakespeare.DIALOGUE[getShownIndex()]);
        return scroller;
    }
}

如果用户点击某个列表项，且当前布局不包括 R.id.details 视图，则应用会启动 DetailsActivity 以显示该项目的内容。

以下是 DetailsActivity 的代码，它简单地嵌入了 DetailsFragment，以在屏幕为纵向时显示所选的戏剧摘要：

public static class DetailsActivity extends FragmentActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        if (getResources().getConfiguration().orientation
                == Configuration.ORIENTATION_LANDSCAPE) {
            // If the screen is now in landscape mode, we can show the
            // dialog in-line with the list so we don't need this activity.
            finish();
            return;
        }

        if (savedInstanceState == null) {
            // During initial setup, plug in the details fragment.
            DetailsFragment details = new DetailsFragment();
            details.setArguments(getIntent().getExtras());
            getSupportFragmentManager().beginTransaction().add(android.R.id.content, details).commit();
        }
    }
}

可以看到，如果当前屏幕配置为横向，则此 Activity 会自行销毁，以便展示横向视图。如果用户在纵向显示时启动 DetailsActivity，但随后旋转为横向（这会重启当前 Activity），就可能出现这种情况。