1.首先贴上项目结构图：
2.先添加一个接口文件BackHandledInterface.java,定义一个setSelectedFragment方法用于设置当前加载的Fragment在栈顶，主界面MainActivity须实现此接口，代码如下：
package com.example.testdemo; public interface BackHandledInterface { public abstract void setSelectedFragment(BackHandledFragment selectedFragment); }
3.定义一个抽象类BackHandledFragment继承自Fragment，后面跳转的Fragment界面都要继承自BackHandledFragment。抽象类BackHandledFragment中定义一个返回值为boolean类型的onBackPressed方法，用于处理点击返回按键（物理Back键）时的逻辑，若该方法返回false，表示当前Fragment不消费返回事件，而由Fragment所属的FragmentActivity来处理这个事件。代码如下：
1 package com.example.testdemo; 2
3 import android.os.Bundle; 4 import android.support.v4.app.Fragment; 5
6 public abstract class BackHandledFragment extends Fragment { 7
8 protected BackHandledInterface mBackHandledInterface; 9
10 /**
11 * 所有继承BackHandledFragment的子类都将在这个方法中实现物理Back键按下后的逻辑 12 */
13 protected abstract boolean onBackPressed(); 14
15 @Override 16 public void onCreate(Bundle savedInstanceState) { 17 super.onCreate(savedInstanceState); 18 if (!(getActivity() instanceof BackHandledInterface)) { 19 throw new ClassCastException( 20 "Hosting Activity must implement BackHandledInterface"); 21 } else { 22 this.mBackHandledInterface = (BackHandledInterface) getActivity(); 23 } 24 } 25
26 @Override 27 public void onStart() { 28 super.onStart(); 29 // 告诉FragmentActivity，当前Fragment在栈顶
30 mBackHandledInterface.setSelectedFragment(this); 31 } 32
33 }
4.主界面MainActivity要继承FragmentActivity才能调用getSupportFragmentManager()方法来处理Fragment。MainActivity还需重写onBackPressed方法用来捕捉返回键（Back Key）事件，代码如下：
1 package com.example.testdemo; 2
3 import android.os.Bundle; 4 import android.support.v4.app.FragmentActivity; 5 import android.support.v4.app.FragmentManager; 6 import android.support.v4.app.FragmentTransaction; 7 import android.view.View; 8 import android.view.View.OnClickListener; 9 import android.widget.Button; 10
11 public class MainActivity extends FragmentActivity implements
12 BackHandledInterface { 13 private static MainActivity mInstance; 14 private BackHandledFragment mBackHandedFragment; 15 private Button btnSecond; 16
17 @Override 18 public void onCreate(Bundle savedInstanceState) { 19 super.onCreate(savedInstanceState); 20 setContentView(R.layout.activity_main); 21 btnSecond = (Button) findViewById(R.id.btnSecond); 22 btnSecond.setOnClickListener(new OnClickListener() { 23
24 @Override 25 public void onClick(View v) { 26 FirstFragment first = new FirstFragment(); 27 loadFragment(first); 28 btnSecond.setVisibility(View.GONE); 29 } 30 }); 31
32 } 33
34 public static MainActivity getInstance() { 35 if (mInstance == null) { 36 mInstance = new MainActivity(); 37 } 38 return mInstance; 39 } 40
41 public void loadFragment(BackHandledFragment fragment) { 42 BackHandledFragment second = fragment; 43 FragmentManager fm = getSupportFragmentManager(); 44 FragmentTransaction ft = fm.beginTransaction(); 45 ft.replace(R.id.firstFragment, second, "other"); 46 ft.addToBackStack("tag"); 47 ft.commit(); 48 } 49
50 @Override 51 public void setSelectedFragment(BackHandledFragment selectedFragment) { 52 this.mBackHandedFragment = selectedFragment; 53 } 54
55 @Override 56 public void onBackPressed() { 57 if (mBackHandedFragment == null || !mBackHandedFragment.onBackPressed()) { 58 if (getSupportFragmentManager().getBackStackEntryCount() == 0) { 59 super.onBackPressed(); 60 } else { 61 if (getSupportFragmentManager().getBackStackEntryCount() == 1) { 62 btnSecond.setVisibility(View.VISIBLE); 63 } 64 getSupportFragmentManager().popBackStack(); 65 } 66 } 67 } 68 }
5.分别添加两个子级Fragment，FirstFragment.java和SecondFragment.java,代码分别如下：
FirstFragment.java
1 package com.example.testdemo; 2
3 import android.os.Bundle; 4 import android.support.annotation.Nullable; 5 import android.support.v4.app.FragmentManager; 6 import android.support.v4.app.FragmentTransaction; 7 import android.view.LayoutInflater; 8 import android.view.View; 9 import android.view.View.OnClickListener; 10 import android.view.ViewGroup; 11 import android.widget.Button; 12
13 public class FirstFragment extends BackHandledFragment { 14 private View myView; 15 private Button btnSecond; 16
17 @Override 18 public View onCreateView(LayoutInflater inflater, 19 @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) { 20 myView = inflater.inflate(R.layout.fragment_first, null); 21 initView(); 22 return myView; 23 } 24
25 private void initView() { 26 btnSecond = (Button) myView.findViewById(R.id.btnSecond); 27 btnSecond.setOnClickListener(new OnClickListener() { 28
29 @Override 30 public void onClick(View v) { 31 SecondFragment second = new SecondFragment(); 32 FragmentManager fm = getFragmentManager(); 33 FragmentTransaction ft = fm.beginTransaction(); 34 ft.replace(R.id.firstFragment, second); 35 ft.addToBackStack("tag"); 36 ft.commit(); 37 } 38 }); 39 } 40
41 @Override 42 protected boolean onBackPressed() { 43 return false; 44 } 45
46 }
SecondFragment.java
1 package com.example.testdemo; 2
3 import android.os.Bundle; 4 import android.support.annotation.Nullable; 5 import android.view.LayoutInflater; 6 import android.view.View; 7 import android.view.ViewGroup; 8
9 public class SecondFragment extends BackHandledFragment { 10
11 private View mView; 12
13 @Override 14 public View onCreateView(LayoutInflater inflater, 15 @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) { 16 mView = inflater.inflate(R.layout.fragment_second, null); 17 return mView; 18 } 19
20 @Override 21 protected boolean onBackPressed() { 22 return false; 23 } 24
25 }
6.三个布局文件代码如下：
activity_main.xml
1 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
2 xmlns:tools="http://schemas.android.com/tools"
3 android:layout_width="match_parent"
4 android:layout_height="match_parent"
5 android:orientation="vertical" >
6
7 <TextView 8 android:layout_width="wrap_content"
9 android:layout_height="wrap_content"
10 android:layout_centerInParent="true"
11 android:text="FragmentActivity 父界面"
12 android:textSize="26sp" />
13
14 <Button 15 android:id="@+id/btnSecond"
16 android:layout_width="wrap_content"
17 android:layout_height="wrap_content"
18 android:layout_alignParentBottom="true"
19 android:text="跳转到FirstFragment" />
20
21 <FrameLayout 22 android:id="@+id/firstFragment"
23 android:layout_width="match_parent"
24 android:layout_height="match_parent" >
25 </FrameLayout>
26
27 </RelativeLayout>
fragment_first.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:background="#e5e5e5" android:orientation="vertical" >
<TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_centerInParent="true" android:text="FirstFragment" android:textColor="#000000" android:textSize="26sp" />
<Button android:id="@+id/btnSecond" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_alignParentBottom="true" android:text="打开SecondFragment" />
</RelativeLayout>
fragment_second.xml
1 <?xml version="1.0" encoding="utf-8"?>
2 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
3 android:layout_width="match_parent"
4 android:layout_height="match_parent"
5 android:background="#e5e5e5"
6 android:orientation="vertical" >
7
8 <TextView 9 android:layout_width="wrap_content"
10 android:layout_height="wrap_content"
11 android:layout_centerInParent="true"
12 android:text="SecondFragment"
13 android:textColor="#000000"
14 android:textSize="26sp" />
15
16 </RelativeLayout>
7.最后奉上实例链接：