Fragment和Activity有很多相似之处，例如可以不带UI，但这样做对两者似乎都没什么意义。他们的创建方式也很相似，例如下面的代码：

package test.fragments;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
public class MySkeletonFragment extends Fragment {
@Override
public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState) {// Create, or inflate the Fragment’s UI, and return it.// If this Fragment has no UI then return null.return inflater.inflate(R.layout.my_fragment, container, false);}
}

Fragment生命周期

Fragment的生命周期和它的宿主Activity密切相关，几乎和宿主Activity的生命周期一致，他们之间最大的不同在于Activity可以增加或删除Fragment。下图总结了Fragment的生命周期：

使用FragmentTransaction的add方法可以添加一个新的Fragment，add()方法的主要参数是Fragment的容器View（或其ID）及Fragment实例，例如：

FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.add(R.id.ui_container, new MyListFragment());
fragmentTransaction.commit();

删除Fragment需要FragmentTransaction的remove()方法，参数为Fragment对象，Fragment对象可以通过FragmentManager的findFragmentById()方法获得。

FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
Fragment fragment = fragmentManager.findFragmentById(R.id.details_fragment);
fragmentTransaction.remove(fragment);
fragmentTransaction.commit();

替换Fragment使用的是FragmentTransaction的replace()方法，参数分别为所要替代Fragment所在容器的ID和新的Fragment：

FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.replace(R.id.details_fragment, new DetailFragment(selected_index));
fragmentTransaction.commit();

获取指定的Fragment

有两种方法可以获取某个特定的Fragment，如果这个Fragment已经被添加到某个layout文件中，则可以使用xml文件中的id作为参数：

MyFragment myFragment = (MyFragment)fragmentManager.findFragmentById(R.id.MyFragment);

也可以通过创建Fragment时添加的tag获取特定的Fragment：

MyFragment myFragment = (MyFragment)fragmentManager.findFragmentByTag(MY_FRAGMENT_TAG);

删除Fragment容器

在配置文件中将visibility的属性设为"gone"，即可删除某个Fragment，例如：

<?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"><FrameLayoutandroid:id="@+id/ui_container"android:layout_width="match_parent"android:layout_height="match_parent"android:layout_weight="1"/><FrameLayoutandroid:id="@+id/details_container"android:layout_width="match_parent"android:layout_height="match_parent"android:layout_weight="3"android:visibility="gone"/>
</LinearLayout>

Fragment和Back Stack

Activity拥有Activity Stack，从而在用户按”返回”按钮时，回到前一个Activity。Fragment也可以响应”返回”事件，方法是FragmentTransaction在commit之前调用addToBackStack()方法。这样，在用户按返回键后，Android会首先重现之前的UI布局。

FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.add(R.id.ui_container, new MyListFragment());
Fragment fragment = fragmentManager.findFragmentById(R.id.details_fragment);
fragmentTransaction.remove(fragment);
String tag = null;
fragmentTransaction.addToBackStack(tag);
fragmentTransaction.commit();

原理和Activity类似，调用addToBackStack()后，Fragment会被push到back stack中，而不是销毁。

Fragment Transaction的动画效果

Fragment Transaction有两种方法实现动画效果，分别是：

fragmentTransaction.setCustomAnimations(R.animator.slide_in_left, R.animator.slide_out_right);

Fragment和宿主Activity之间的接口

TextView textView = (TextView)getActivity().findViewById(R.id.textview);

public interface OnSeasonSelectedListener {public void onSeasonSelected(Season season);
}
private OnSeasonSelectedListener onSeasonSelectedListener;private Season currentSeason;@Overridepublic void onAttach(Activity activity) {super.onAttach(activity);try {onSeasonSelectedListener = (OnSeasonSelectedListener)activity;} catch (ClassCastException e) {throw new ClassCastException(activity.toString() +"must implement OnSeasonSelectedListener");}}private void setSeason(Season season) {currentSeason = season;onSeasonSelectedListener.onSeasonSelected(season);
}

public class NewItemFragment extends Fragment {@Overridepublic void onAttach(Activity activity) {  super.onAttach(activity);   // Get a type-safe reference to the parent Activity.}@Overridepublic void onCreate(Bundle savedInstanceState) {
    　　super.onCreate(savedInstanceState);
    　　// Create background worker threads and tasks.}@Overridepublic void onActivityCreated(Bundle savedInstanceState) {
    　　super.onActivityCreated(savedInstanceState);
    　　// Initiate worker threads and tasks.}
}