带有 ViewPager 的 FragmentActivity：方向更改时崩溃

Question

我在 FragmentActivity 中使用了 ViewPager。方向改变时我会崩溃。

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_list);
    context = this;
    loadInitialData();
    setActionBar();
    initializeUiComponents();
}

private void initializeUiComponents() {
    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.pager);
    }

我的数据是动态地从数据库中来的，我必须调用 Sysnctask，它在 onResume() 中从数据库中获取数据。这是 Asynctask 的 onPostExecute()。

protected void onPostExecute(Void result) {
        
            mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
            mViewPager.setAdapter(mSectionsPagerAdapter);
        }
        mSectionsPagerAdapter.notifyDataSetChanged();
        }
}

添加 FragmentAdapterCode：

public class SectionsPagerAdapter extends FragmentPagerAdapter {
    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }
    
    @Override
    public Fragment getItem(int position) {
        // getItem is called to instantiate the fragment for the given page.
        // Return a DummySectionFragment (defined as a static inner class
        // below) with the page number as its lone argument.
        Fragment fragment = new PayeeListSectionFragment();
        Bundle args = new Bundle();
        args.putInt(PayeeListSectionFragment.ARG_SECTION_NUMBER, position + 1);
        fragment.setArguments(args);
        return fragment;
    }
    
    @Override
    public int getCount() {
        return 2;
    }
    
    @Override
    public CharSequence getPageTitle(int position) {
        
        return string;
    }
}

添加片段代码：

public class PayeeListSectionFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this fragment.
     */
    public static final String ARG_SECTION_NUMBER = "section_number";
    
    public PayeeListSectionFragment() {
    }
    
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View rootView = null;
        \\ rootView inflated
        return rootView;
    }
}

使用的xml是：

 <android.support.v4.view.ViewPager
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".PayeeListActivity" >

    <!--
This title strip will display the currently visible page title, as well as the page
titles for adjacent pages.

    -->

    <android.support.v4.view.PagerTitleStrip
        android:id="@+id/pager_title_strip"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="top"
        android:background="#33b5e5"
        android:paddingBottom="4dp"
        android:paddingTop="4dp"
        android:textColor="#fff" />
</android.support.v4.view.ViewPager>

我在改变方向时遇到以下崩溃：

06-07 11:01:57.834: E/AndroidRuntime(766): FATAL EXCEPTION: main  
06-07 11:01:57.834: E/AndroidRuntime(766): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.walletv2.activity/com.walletv2.activity.PayeeListActivity}: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment com.walletv2.activity.PayeeListActivity$PayeeListSectionFragment: make sure class name exists, is public, and has an empty constructor that is public  
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread.handleRelaunchActivity(ActivityThread.java:3692)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread.access$700(ActivityThread.java:141)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1240)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.os.Handler.dispatchMessage(Handler.java:99)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.os.Looper.loop(Looper.java:137)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread.main(ActivityThread.java:5041)
06-07 11:01:57.834: E/AndroidRuntime(766):  at java.lang.reflect.Method.invokeNative(Native Method)
06-07 11:01:57.834: E/AndroidRuntime(766):  at java.lang.reflect.Method.invoke(Method.java:511)
06-07 11:01:57.834: E/AndroidRuntime(766):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
06-07 11:01:57.834: E/AndroidRuntime(766):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
06-07 11:01:57.834: E/AndroidRuntime(766):  at dalvik.system.NativeStart.main(Native Method)
06-07 11:01:57.834: E/AndroidRuntime(766): Caused by: android.support.v4.app.Fragment$InstantiationException: Unable to instantiate fragment com.walletv2.activity.PayeeListActivity$PayeeListSectionFragment: make sure class name exists, is public, and has an empty constructor that is public
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.support.v4.app.Fragment.instantiate(Fragment.java:405)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.support.v4.app.FragmentState.instantiate(Fragment.java:97)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.support.v4.app.FragmentManagerImpl.restoreAllState(FragmentManager.java:1767)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.support.v4.app.FragmentActivity.onCreate(FragmentActivity.java:208)
06-07 11:01:57.834: E/AndroidRuntime(766):  at com.walletv2.activity.PayeeListActivity.onCreate(PayeeListActivity.java:78)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.Activity.performCreate(Activity.java:5104)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
06-07 11:01:57.834: E/AndroidRuntime(766):  ... 12 more
06-07 11:01:57.834: E/AndroidRuntime(766): Caused by: java.lang.InstantiationException: can't instantiate class com.walletv2.activity.PayeeListActivity$PayeeListSectionFragment; no empty constructor
06-07 11:01:57.834: E/AndroidRuntime(766):  at java.lang.Class.newInstanceImpl(Native Method)
06-07 11:01:57.834: E/AndroidRuntime(766):  at java.lang.Class.newInstance(Class.java:1319)
06-07 11:01:57.834: E/AndroidRuntime(766):  at android.support.v4.app.Fragment.instantiate(Fragment.java:394)
06-07 11:01:57.834: E/AndroidRuntime(766):  ... 19 more

Answer 1

以下几行有一些提示：

原因：android.support.v4.app.Fragment$InstantiationException：无法实例化片段 com.walletv2.activity.PayeeListActivity$PayeeListSectionFragment：确保类名存在，是公共的，并且有一个空的构造函数公开

还有：

06-07 11:01:57.834: E/AndroidRuntime(766): Caused by: java.lang.InstantiationException: can't instantiate class com.walletv2.activity.PayeeListActivity$PayeeListSectionFragment; 没有空的构造函数

这表明您已将参数化构造函数添加到 PayeeListSectionFragment 内部类。不幸的是，这行不通，因为 Android 依赖于使用反射来调用非参数化构造函数，以便在配置更改时恢复片段的状态（除其他外）。

如果您想为片段提供一个或多个参数，您必须将它们插入Bundle 并将其设置为参数，使用setArguments(Bundle)。查看DetailsFragment and CountingFragment in the documentation 以获取有关如何执行此操作的示例。

---

这个问题也可能出现在另一种情况下，这对于那些不太熟悉 Java 的人来说可能不太明显。如果你的片段是一个嵌套类，比如Activity，请确保你声明了内部类static。例如，在上述问题的上下文中，它应该是public static class PayeeListSectionFragment（强调static 修饰符）。这样，内部类就不会保留对外部类的引用，并且可以拥有自己的生命周期，而不依赖于外部类。如果没有 static 修饰符，嵌套类不能在不实例化外部类的情况下被实例化，这意味着 Android 在尝试重新实例化您的片段类时会遇到问题。

另一种解决方案是将内部类移动到它自己的.java 文件中。这样，所有外部类依赖项都会自动删除。