Android的Fragment介绍

前言

fragment是从android3.0开始提出来的，用来支持大屏幕设备的ui设计。通过将activity划分为多个fragment，不仅提高了设计的灵活性，而且可以在程序运行时改变它们的特征，比如动态的修改，替换已有的fragment等等。

fragment的角色是相当于activity中ui的一个子集或者说是activity中的一个模块，可以通过组合多个fragment来构造一个多个面板的界面。由于fragment嵌入在activity中，因此它的生命周期受到activity的影响，一旦activity停掉，它里面所有的fragment也都会被摧毁。

可以在activity的布局文件中通过声明<fragment>标签来填充它的布局，同时也可以通过代码将它加到已经存在的ViewGroup中，因为fragment布局实际上就是ViewGroup的子树。

新建一个Fragment

首先需要了解的是fragment的生命周期，如下图所示：

可以看出调用的先后顺序是：

• onCreate  这里可以用来初始化一些当fragment停掉时候你仍想保存的组件
• onCreateView  这里可以用来绘制fragment的用户界面，因此必须返回一个view
• onPause  当fragment被移走或者替换时调用，可以在这里commit changes

一般可以继承fragment类来创建自己的fragment，同时亦有几个有用的fragment基类供我们继承：

1. DialogFragment  也就是浮动在Activity上面的fragment，这种对话框可以供用户返回，便于管理，是google推荐的做法。
2. ListFragment  就是包含ListView的Fragment，用来显示列表等。
3. PreferenceFragment  可以用来显示用户选项的设置。

一般来说，要创建Fragment，第一步就是创建一个Fragment的子类，并在onCrateView方法中填充布局：

public static class ExampleFragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        //resourceID,ViewGroup, whether the inflated layout should be
        //attached to the ViewGroup during inflation
        return inflater.inflate(R.layout.example_fragment, container, false);
    }
}

第二步就是将fragment添加到activity中,可以在activity的布局文件中声明,如:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <fragment android:name="com.example.news.ArticleListFragment"
            android:id="@+id/list"
            android:layout_weight="1"
            android:layout_width="0dp"
            android:layout_height="match_parent" />
    <fragment android:name="com.example.news.ArticleReaderFragment"
            android:id="@+id/viewer"
            android:layout_weight="2"
            android:layout_width="0dp"
            android:layout_height="match_parent" />
</LinearLayout>

这样一来,当创建activity的布局时,就会初始化每一个fragment,并且调用它们的onCreateView方法,获取fragment的布局,然后插入到ViewGroup下面。

除了使用布局文件之外，还可以用程序进行添加：

FragmentManager fragmentManager = getFragmentManager()
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
ExampleFragment fragment = new ExampleFragment();
fragmentTransaction.add(R.id.fragment_container, fragment);
fragmentTransaction.commit();

前两行用来获取碎片事务FragmentTransaction，后三行用来添加一个fragment，add方法的第一个参数是要放置的ViewGroup，第二个参数是要添加的fragment。

Fragment的管理

通过fragmentManager可以做一下事情：

• 获取fragment，通过findFragmentById或者findFragmentByTag
• 将Fragment从栈中移除，通过popBackStack方法
• 为back stack注册监听器

相信这里的细节还有很多，暂时先不介绍。

Fragment的事务

fragment的一大亮点就是可以动态增加，删除，替换fragment，而这是通过fragmentTransaction来实现的。一旦获取了事务的实例，就可以通过调用add，remove，replace方法完成上述功能，接下来通过commit方法提交事务就可以了。

// Create new fragment and transaction
Fragment newFragment = new ExampleFragment();
FragmentTransaction transaction = getFragmentManager().beginTransaction();

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

// Commit the transaction
transaction.commit();

除此之外还可以调用addToBackStack方法将fragment添加到back stack中，用户按下返回键时，就会跳到back stack顶部的fragment。调不调用它的区别在于，如果没有它，fragment被remove后就彻底销毁了，再也不能恢复，如果调用了它，fragment就不会销毁而是stop，这样就可以恢复，因为fragment的状态都被保存在了back stack上面。

commit方法的作用是调度到activity的主线程中执行，并且事务的提交必须要在onSaveInstanceState之前。

和Activity的通信

fragment也是可以和它所在的activity通信的，比如可以通过getActivity()方法得到所在的activity，相应的，activity也可以调用FragmentManager的findFragmentById方法获取某一个fragment的引用。

有时候需要在fragment和activity之间共享事件，如activity有一个fragment，是用来显示文章的列表的，当用户点击了某一项，activity需要检测到这个事件，同时更改另一个fragment从而显示出相应的文章内容。这时可以这么做，在fragment中定义事件监听的接口，然后让activity实现这个接口。然后，这个fragment就可以调用接口中的某些方法来通知activity了。fragment中可以在onAttach里面将activity强制转化为接口的实现类，从而确保能成功调用接口中的方法。

public static class FragmentA extends ListFragment {

    OnArticleSelectedListener mListener;
    ...
    // Container Activity must implement this interface
    public interface OnArticleSelectedListener {
        public void onArticleSelected(Uri articleUri);
    }
    ...
    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        try {
            mListener = (OnArticleSelectedListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString() + " must implement OnArticleSelectedListener");
        }
    }
    ...
    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        // Append the clicked item's row ID with the content provider Uri
        Uri noteUri = ContentUris.withAppendedId(ArticleColumns.CONTENT_URI, id);
        // Send the event and Uri to the host activity
        mListener.onArticleSelected(noteUri);
    }
    ...
}

例子

该例子来源于api demo，用到了很典型的两个fragment，一个是TitlesFragment，用来显示一个列表，另一个是DetailsFragment，用来显示选中item的详细信息。此外，该程序还考虑到了屏幕的横竖屏问题，如果是竖屏的话，没有足够的地方显示第二个Fragment，就单独启动一个Activity。源码如下：

/**
 * Demonstration of using fragments to implement different activity layouts.
 * This sample provides a different layout (and activity flow) when run in
 * landscape.
 */
public class FragmentLayout extends Activity {

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

        setContentView(R.layout.fragment_layout);
    }

    /**
     * This is a secondary activity, to show what the user has selected
     * when the screen is not large enough to show it all in one activity.
     */

    public static class DetailsActivity extends Activity {

        @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());
                getFragmentManager().beginTransaction().add(android.R.id.content, details).commit();
            }
        }
    }

    /**
     * This is the "top-level" fragment, showing a list of items that the
     * user can pick.  Upon picking an item, it takes care of displaying the
     * data to the user as appropriate based on the currrent UI layout.
     */

    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)
                        getFragmentManager().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 = getFragmentManager().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);
            }
        }
    }

    /**
     * This is the secondary fragment, displaying the details of a particular
     * item.
     */

    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
                // won't be displayed.  Note this is not 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;
        }
    }

}