一、情景介绍

首先说下什么情况会用到Fragment的回退栈，假设现在要在一个Activity上不停的切换Fragment（在一个Fragment里不停切换Fragment也可以），但是要求我们可以用安卓的返回键返回上一个Fragment这个时候就要用到Fragment的回退栈了

效果图： 

二、详细说明

这个回退栈类似于安卓为Activity维护的任务栈，我们也可以通过Activity维护一个回退栈来保存每次Fragment事务发生的变化。如果你将Fragment任务添加到回退栈，当用户点击后退按钮时，将看到上一次的保存的Fragment。一旦Fragment完全从后退栈中弹出，用户再次点击后退键，则退出当前Activity。 Fragment的生命周期是依赖Activity的，要是用我理解的话说就是Activity是房子的话，那么Fragment就是房子里面的房间了，看下Fragment的生命周期：

三、功能说明和动手操作

要求:
    1.我们用一个Activity同时切换3个Fragment
    2.需要通过返回键能返回上一个Fragment

首先我们需要3个Fragment

public class Fragment1 extends Fragment implements View.OnClickListener {

@Override

public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment1, container, false);
TextView mTextView = (TextView) view.findViewById(R.id.tv_fragment_content);
mTextView.setText("Fragment1");
Button mButton = (Button) view.findViewById(R.id.btn_toggle_fragment);
mButton.setOnClickListener(this);
return view;
}

@Override

public void onClick(View view) {
Fragment2 f2 = new Fragment2();
FragmentManager mManager = getFragmentManager();
FragmentTransaction ft1 = mManager.beginTransaction();
ft1.addToBackStack(null);//这里将我们的Fragment加入到返回栈 ft1.replace(R.id.lv_fragment_container, f2);
ft1.commit();
}
}

public class Fragment2 extends Fragment implements View.OnClickListener{

    @Override
    public View onCreateView(LayoutInflater inflater,  ViewGroup container,  Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment1,container,false);
        TextView mTextView = (TextView) view.findViewById(R.id.tv_fragment_content);
        mTextView.setText("Fragment2");
        mTextView.setOnClickListener(this);
        Button mButton = (Button) view.findViewById(R.id.btn_toggle_fragment);
        mButton.setOnClickListener(this);
        return view;
    }

    @Override
    public void onClick(View view) {
        Fragment3 f3 = new Fragment3();
        FragmentManager fm3 = getFragmentManager();
        FragmentTransaction ft3 = fm3.beginTransaction();
        ft3.addToBackStack(null);
        ft3.replace(R.id.lv_fragment_container,f3);
        ft3.commit();

    }
}


public class Fragment2 extends Fragment implements View.OnClickListener{

    @Override
    public View onCreateView(LayoutInflater inflater,  ViewGroup container,  Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment1,container,false);
        TextView mTextView = (TextView) view.findViewById(R.id.tv_fragment_content);
        mTextView.setText("Fragment2");
        mTextView.setOnClickListener(this);
        Button mButton = (Button) view.findViewById(R.id.btn_toggle_fragment);
        mButton.setOnClickListener(this);
        return view;
    }

    @Override
    public void onClick(View view) {
        Fragment3 f3 = new Fragment3();
        FragmentManager fm3 = getFragmentManager();
        FragmentTransaction ft3 = fm3.beginTransaction();
        ft3.addToBackStack(null);
        ft3.replace(R.id.lv_fragment_container,f3);
        ft3.commit();

    }
}


public class Fragment3 extends Fragment{

    @Override
    public View onCreateView(LayoutInflater inflater,  ViewGroup container,  Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment1,container,false);
        TextView mTextView = (TextView) view.findViewById(R.id.tv_fragment_content);
        mTextView.setText("Fragment3");


        return view;
    }

}


//3个Fragment用同一个Xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:id="@+id/tv_fragment_content"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:textSize="40sp"
        android:text="--"
        android:layout_weight="1"
        />
    <Button
        android:id="@+id/btn_toggle_fragment"
        android:layout_width="match_parent"
        android:layout_height="40dp"
        android:text="toggle"
        />
</LinearLayout>


//3个Fragment用同一个Xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:id="@+id/tv_fragment_content"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:textSize="40sp"
        android:text="--"
        android:layout_weight="1"
        />
    <Button
        android:id="@+id/btn_toggle_fragment"
        android:layout_width="match_parent"
        android:layout_height="40dp"
        android:text="toggle"
        />
</LinearLayout>

核心代码:

public class MainActivity extends FragmentActivity {
    private Fragment f1;
    @Override
    protected void onCreate( Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Window mWindow = getWindow();
        mWindow.setContentView(R.layout.fragment_layout);
        f1 = new Fragment1();
        getSupportFragmentManager().beginTransaction().add(R.id.lv_fragment_container,f1,"first").commit();
    }

    @Override
    public void onBackPressed() {
        Toast.makeText(MainActivity.this, "left count:"+getSupportFragmentManager().getBackStackEntryCount(), Toast.LENGTH_SHORT).show();
        if(getSupportFragmentManager().getBackStackEntryCount() <= 0)//这里是取出我们返回栈存在Fragment的个数
            finish();
        else
            getSupportFragmentManager().popBackStack();
            //取出我们返回栈保存的Fragment,这里会从栈顶开始弹栈

    }
}

    //activity_main

    <?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent"
    >
    <LinearLayout
        android:id="@+id/lv_fragment_container"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:orientation="vertical"
    />


</LinearLayout>