我遇到了这个奇怪的问题，我的列表 fragment 被创建了两次，一次是在父 Activity 上调用 super.oncreate 时，一次是在同一父 Activity 上调用 setContentView 时。这是一个简单的应用程序，我在纵向和横向方向上使用不同的布局。

这是主要 Activity :

private HeadlinesFragment headlines;

@Override
public void onCreate(Bundle savedInstanceState) {
    Log.w("MainActivity", "Before super.onCreate: " + this.toString());
    super.onCreate(savedInstanceState);
    Log.w("MainActivity", "Before setContentView: " + this.toString());
    setContentView(R.layout.news_articles);

    //check to see if its portrait
    if (findViewById(R.id.fragment_container) != null) {
        if(getSupportFragmentManager().findFragmentById(R.id.fragment_container) == null) {
            headlines = new HeadlinesFragment();
            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, headlines).commit();
        }
    }
}

这是 layout-land 文件夹中的新闻文章:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="match_parent">

<fragment android:name="com.example.android.fragments.HeadlinesFragment"
          android:id="@+id/headlines_fragment"
          android:layout_weight="1"
          android:layout_width="0dp"
          android:layout_height="match_parent" />

<fragment android:name="com.example.android.fragments.ArticleFragment"
          android:id="@+id/article_fragment"
          android:layout_weight="2"
          android:layout_width="0dp"
          android:layout_height="match_parent" />

这是布局文件夹中的新闻文章(纵向)

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/fragment_container"
android:layout_width="match_parent"
android:layout_height="match_parent" />

这是创建了两次的 headlinesfragment

public class HeadlinesFragment extends ListFragment {
OnHeadlineSelectedListener mCallback;

// The container Activity must implement this interface so the frag can deliver messages
public interface OnHeadlineSelectedListener {
    /** Called by HeadlinesFragment when a list item is selected */
    public void onArticleSelected(int position);
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    Log.w("HeadlinesFragment", "inside onCreate: " + this.toString());

    // We need to use a different list item layout for devices older than Honeycomb
    int layout = Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB ?
            android.R.layout.simple_list_item_activated_1 : android.R.layout.simple_list_item_1;

    // Create an array adapter for the list view, using the Ipsum headlines array
    setListAdapter(new ArrayAdapter<String>(getActivity(), layout, Ipsum.Headlines));
}

@Override
public void onStart() {
    super.onStart();

    // When in landscape layout, set the listview to highlight the selected list item
    // (We do this during onStart because at the point the listview is available.)
    if (getFragmentManager().findFragmentById(R.id.article_fragment) != null) {
        getListView().setChoiceMode(ListView.CHOICE_MODE_SINGLE);
    }
}

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);

    // This makes sure that the container activity has implemented
    // the callback interface. If not, it throws an exception.
    try {
        mCallback = (OnHeadlineSelectedListener) activity;
    } catch (ClassCastException e) {
        throw new ClassCastException(activity.toString()
                + " must implement OnHeadlineSelectedListener");
    }
}


@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
    // TODO Auto-generated method stub
    super.onCreateOptionsMenu(menu, inflater);
}

@Override
public void onDestroy() {
    Log.w("HeadlinesFragment", "inside onDestroy: " + this.toString());
    super.onDestroy();
}
}

这是文章 fragment

public class ArticleFragment extends Fragment {
final static String ARG_POSITION = "position";
int mCurrentPosition = 0;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, 
    Bundle savedInstanceState) {
    Log.w("ArticleFragment", "inside onCreateView: " + this.toString());

    if (savedInstanceState != null) {
        mCurrentPosition = savedInstanceState.getInt(ARG_POSITION);
    }

    // Inflate the layout for this fragment
    View view = inflater.inflate(R.layout.article_view, container, false);
    return view;
}

@Override
public void onStart() {
    super.onStart();
    Bundle args = getArguments();
    if (args != null) {
        // Set article based on argument passed in
        updateArticleView(args.getInt(ARG_POSITION));
    } else if (mCurrentPosition != -1) {
        // Set article based on saved instance state defined during onCreateView
        updateArticleView(mCurrentPosition);
    }
}

@Override
public void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);

    // Save the current article selection in case we need to recreate the fragment
    outState.putInt(ARG_POSITION, mCurrentPosition);
}

@Override
public void onDestroy() {
    Log.w("ArticleFragment", "inside onDestroy: " + this.toString());
    super.onDestroy();
}

详细的问题是这样的:

1) 以纵向启动应用程序 2) 调用 setContentView 并加载 news_articles 但它是带有 fragment_container 的那个。 3) headlinesfragment 被创建//到目前为止的正常行为 4)将方向更改为横向 5) mainActivity 被销毁 -> headlinefragment 被销毁 6) super.oncreate oncreate on mainactivity 被调用 7)创建标题 fragment 8) 调用 mainactivity 上的 setcontentview 9) 另一个标题 fragment 被创建//问题

我已经放置了日志，如上面的代码所示，这是我以纵向模式启动应用程序并更改为横向模式时的输出。

W/MainActivity(6925): Before super.onCreate: MainActivity@41d81238
W/MainActivity(6925): Before setContentView: MainActivity@41d81238
W/HeadlinesFragment(6925): inside onCreate: HeadlinesFragment{41d8d4d8 #0 id=0x7f050001}
W/MainActivity(6925): inside onDestroy: MainActivity@41d81238
W/HeadlinesFragment(6925): inside onDestroy: HeadlinesFragment{41d8d4d8 # 0id=0x7f050001}
W/MainActivity(6925): Before super.onCreate: MainActivity@41ea6258
W/HeadlinesFragment(6925): inside onCreate: HeadlinesFragment{41ea7290 #0 id=0x7f050001}
W/MainActivity(6925): Before setContentView: MainActivity@41ea6258
W/HeadlinesFragment(6925): inside onCreate: HeadlinesFragment{41eb1f30 #1 id=0x7f050002}
W/ArticleFragment(6925): inside onCreateView: ArticleFragment{41eb5f20 #2 id=0x7f050003}

我希望我已经清楚我的代码和日志，在我看来 super.oncreate 和 setcontentview 都创建了一个 headlinesfragment；至少我认为。

我的问题是为什么要创建 2 个 headlinesfragment 实例以及如何避免这种情况。

非常感谢您对此的任何帮助

最佳答案

在您的 Activity 的 onCreate 中，您可以检查 savedInstanceState 包的状态。如果它不为 null，则意味着发生了配置更改(在您的情况下，屏幕方向发生了更改)并且您不需要重新创建 Fragment。

您犯的另一个错误是您试图使用 findFragmentById 检索您的 Fragment。您没有传递 Fragment id，而是给它附加到 Fragment 的 View 的 id，这是不同的(这就是为什么我猜测它总是返回 null 的原因)。

正确的实现应该更像这样(这是你的Activity):

    //check to see if its portrait
    if (findViewById(R.id.fragment_container) != null) {
        if(savedInstanceState == null) {
            headlines = new HeadlinesFragment();
            getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, headlines, FRAGMENT_TAG_STRING).commit(); // Use tags, it's simpler to deal with
        } else {
            headlines = getSupportFragmentManager().findFragmentByTag(FRAGMENT_TAG_STRING);
        } 
   }

关于android fragment 在方向改变时创建了两次，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/16265482/