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：
- 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。
- ListFragment 就是包含ListView的Fragment，用来显示列表等。
- 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; } } }
