Android 使用Fragment界面向下跳转并一级级返回

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：

package com.example.testdemo;

import android.os.Bundle;
import android.support.v4.app.Fragment;

public abstract class BackHandledFragment extends Fragment {

    protected BackHandledInterface mBackHandledInterface;

    /**
     * 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
     */
    protected abstract boolean onBackPressed();

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        if (!(getActivity() instanceof BackHandledInterface)) {
            throw new ClassCastException(
                    "Hosting Activity must implement BackHandledInterface");
        } else {
            this.mBackHandledInterface = (BackHandledInterface) getActivity();
        }
    }

    @Override
    public void onStart() {
        super.onStart();
        // 告诉FragmentActivity，当前Fragment在栈顶
        mBackHandledInterface.setSelectedFragment(this);
    }

}

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：

package com.example.testdemo;

import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;

public class MainActivity extends FragmentActivity implements
        BackHandledInterface {
    private static MainActivity mInstance;
    private BackHandledFragment mBackHandedFragment;
    private Button btnSecond;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        btnSecond = (Button) findViewById(R.id.btnSecond);
        btnSecond.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                FirstFragment first = new FirstFragment();
                loadFragment(first);
                btnSecond.setVisibility(View.GONE);
            }
        });

    }

    public static MainActivity getInstance() {
        if (mInstance == null) {
            mInstance = new MainActivity();
        }
        return mInstance;
    }

    public void loadFragment(BackHandledFragment fragment) {
        BackHandledFragment second = fragment;
        FragmentManager fm = getSupportFragmentManager();
        FragmentTransaction ft = fm.beginTransaction();
        ft.replace(R.id.firstFragment, second, "other");
        ft.addToBackStack("tag");
        ft.commit();
    }

    @Override
    public void setSelectedFragment(BackHandledFragment selectedFragment) {
        this.mBackHandedFragment = selectedFragment;
    }

    @Override
    public void onBackPressed() {
        if (mBackHandedFragment == null || !mBackHandedFragment.onBackPressed()) {
            if (getSupportFragmentManager().getBackStackEntryCount() == 0) {
                super.onBackPressed();
            } else {
                if (getSupportFragmentManager().getBackStackEntryCount() == 1) {
                    btnSecond.setVisibility(View.VISIBLE);
                }
                getSupportFragmentManager().popBackStack();
            }
        }
    }
}

5.分别添加两个子级Fragment，FirstFragment.java和SecondFragment.java,代码分别如下：

FirstFragment.java

package com.example.testdemo;

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.view.LayoutInflater;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.ViewGroup;
import android.widget.Button;

public class FirstFragment extends BackHandledFragment {
    private View myView;
    private Button btnSecond;

    @Override
    public View onCreateView(LayoutInflater inflater,
            @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        myView = inflater.inflate(R.layout.fragment_first, null);
        initView();
        return myView;
    }

    private void initView() {
        btnSecond = (Button) myView.findViewById(R.id.btnSecond);
        btnSecond.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                SecondFragment second = new SecondFragment();
                FragmentManager fm = getFragmentManager();
                FragmentTransaction ft = fm.beginTransaction();
                ft.replace(R.id.firstFragment, second);
                ft.addToBackStack("tag");
                ft.commit();
            }
        });
    }

    @Override
    protected boolean onBackPressed() {
        return false;
    }

}

SecondFragment.java

package com.example.testdemo;

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class SecondFragment extends BackHandledFragment {

    private View mView;

    @Override
    public View onCreateView(LayoutInflater inflater,
            @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        mView = inflater.inflate(R.layout.fragment_second, null);
        return mView;
    }

    @Override
    protected boolean onBackPressed() {
        return false;
    }

}

6.三个布局文件代码如下：
activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="FragmentActivity 父界面"
        android:textSize="26sp" />

    <Button
        android:id="@+id/btnSecond"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:text="跳转到FirstFragment" />

    <FrameLayout
        android:id="@+id/firstFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent" >
    </FrameLayout>

</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

<?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="SecondFragment"
        android:textColor="#000000"
        android:textSize="26sp" />

</RelativeLayout>