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