本文实例讲述了Android编程使用Fragment界面向下跳转并一级级返回的实现方法。分享给大家供大家参考，具体如下：
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;
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>7.最后奉上实例链接：
完整实例代码代码点击此处本站下载。
希望本文所述对大家Android程序设计有所帮助。
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