我在 FragmentActivity 中使用了 ViewPager。当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获取的，我必须在 onResume() 中调用从数据库中获取数据的 Sysnctask。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 fragment 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>

我在方向更改时遇到以下崩溃:

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

最佳答案

下面几行有一些提示:

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

和:

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

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 依赖于使用反射调用非参数化构造函数，以便在配置更改时恢复 fragment 的状态(除其他外)。

如果您想为 fragment 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)。 .看看 DetailsFragment and CountingFragment in the documentation有关如何执行此操作的示例。

这个问题在另一种情况下也可能出现，对于不太熟悉Java的人来说可能不太明显。如果您的 fragment 是一个嵌套类，比如一些 Activity，请确保声明内部类 static。例如，在上述问题的上下文中，它应该是一个 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，并且可以有自己的生命周期，而不依赖于外部类。如果没有 static 修饰符，则在不实例化外部类的情况下无法实例化嵌套类，这意味着 Android 在尝试重新实例化 fragment 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，任何外部类依赖项都会自动删除。

关于android - FragmentActivity 与 ViewPager : Crash on orientation change，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/16977431/