java - Android 在每次方向改变时都会创建额外的 ListFragment

Question

我对 Android 相当陌生，我参加了 coursera 网站上的类(class)。我有一个应用程序需要在平板电脑上以两 Pane 模式运行，在普通设备上以单 Pane 模式运行。作为奖励练习之一，我需要在设备方向更改时保持两 Pane 模式的状态(记住在左侧 ListFragment 中选择了哪个项目以及在右侧 Fragment 中显示了哪些文本)。

我最终做了这个练习，但是我注意到应用程序没有正确销毁 ListFragment - 查看日志。应用程序运行后日志输出被清除，这只是由于方向改变。

正如你所看到的，fragment 被破坏了，但随后 android 又不知从哪里附加了它。

我只包含了单 Pane 代码和布局的代码 - 问题就在那里。两 Pane 布局工作正常。

02-10 07:19:20.845: I/DEBUG(1297): ListFragment onPause()
02-10 07:19:20.845: I/DEBUG(1297): Activity onPause()
02-10 07:19:20.855: I/DEBUG(1297): ListFragment onStop()
02-10 07:19:20.855: I/DEBUG(1297): Activity onStop()
02-10 07:19:20.855: I/DEBUG(1297): ListFragment onDestroyView()
02-10 07:19:20.875: I/DEBUG(1297): ListFragment onDestroy()
02-10 07:19:20.875: I/DEBUG(1297): ListFragment onDetach()
02-10 07:19:20.905: I/DEBUG(1297): Activity onDestroy()
02-10 07:19:21.115: I/DEBUG(1297): ListFragment onAttach()
02-10 07:19:21.115: I/DEBUG(1297): ListFragment onCreate()
02-10 07:19:21.115: I/DEBUG(1297): Activity onCreate()
02-10 07:19:21.315: I/DEBUG(1297): ListFragment onCreateView()
02-10 07:19:21.455: I/DEBUG(1297): ListFragment onActivityCreated()
02-10 07:19:21.466: I/DEBUG(1297): ListFragment onAttach()
02-10 07:19:21.466: I/DEBUG(1297): ListFragment onCreate()
02-10 07:19:21.466: I/DEBUG(1297): ListFragment onCreateView()
02-10 07:19:21.535: I/DEBUG(1297): ListFragment onActivityCreated()
02-10 07:19:21.535: I/DEBUG(1297): Activity onStart()
02-10 07:19:21.535: I/DEBUG(1297): ListFragment onStart()
02-10 07:19:21.535: I/DEBUG(1297): ListFragment onStart()
02-10 07:19:21.555: I/DEBUG(1297): Activity onResum()
02-10 07:19:21.555: I/DEBUG(1297): ListFragment onResume()
02-10 07:19:21.555: I/DEBUG(1297): ListFragment onResume()

MainActivity.java

public class MainActivity extends Activity implements
        FriendsFragment.SelectionListener {

    private static final String TAG = "Lab-Fragments";
    private static final String DEBUG = "DEBUG";
    private static final String STRING_DEBUG = "Activity ";

    private FriendsFragment mFriendsFragment;
    private FeedFragment mFeedFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.i(DEBUG, STRING_DEBUG + "onCreate()");
        setContentView(R.layout.main_activity);
        // If the layout is single-pane, create the FriendsFragment 
        // and add it to the Activity

        if (!isInTwoPaneMode()) {           

            if (mFriendsFragment == null) {//TO REMOVE
                mFriendsFragment = new FriendsFragment();
            }

            //TODO 1 - add the FriendsFragment to the fragment_container
            FragmentTransaction transaction = getFragmentManager().beginTransaction();
            transaction.add(R.id.fragment_container, mFriendsFragment);
            transaction.commit();       

        } else {

            // Otherwise, save a reference to the FeedFragment for later use
            mFeedFragment = (FeedFragment) getFragmentManager().findFragmentById(R.id.feed_frag);
        }
    }

    // If there is no fragment_container ID, then the application is in
    // two-pane mode

    private boolean isInTwoPaneMode() {
        return findViewById(R.id.fragment_container) == null;
    }

    // Display selected Twitter feed

    public void onItemSelected(int position) {

        Log.i(TAG, "Entered onItemSelected(" + position + ")");

        // If there is no FeedFragment instance, then create one

        if (mFeedFragment == null)
            mFeedFragment = new FeedFragment();

        // If in single-pane mode, replace single visible Fragment

        if (!isInTwoPaneMode()) {

            //TODO 2 - replace the fragment_container with the FeedFragment
            FragmentTransaction transaction = getFragmentManager().beginTransaction();
            transaction.replace(R.id.fragment_container, mFeedFragment);
            transaction.addToBackStack(null);
            transaction.commit();
            // execute transaction now
            getFragmentManager().executePendingTransactions();
        }

        // Update Twitter feed display on FriendFragment
        mFeedFragment.updateFeedDisplay(position);

    }

    @Override
    public void onStart() {
        super.onStart();
        Log.i(DEBUG, STRING_DEBUG + "onStart()");
    }

    @Override
    public void onResume() {
        super.onResume();
        Log.i(DEBUG, STRING_DEBUG + "onResum()");
    }

    @Override
    public void onPause() {
        super.onPause();
        Log.i(DEBUG, STRING_DEBUG + "onPause()");
    }

    @Override
    public void onStop() {
        super.onStop();
        Log.i(DEBUG, STRING_DEBUG + "onStop()");
    }

    public void onRestart() {
        super.onRestart();
        Log.i(DEBUG, STRING_DEBUG + "onRestart()");
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.i(DEBUG, STRING_DEBUG + "onDestroy()");
    }
}

ListFragment.java // FriendsFragment

public class FriendsFragment extends ListFragment {

    private static final String[] FRIENDS = { "ladygaga", "msrebeccablack", "taylorswift13" };
    private static final String TAG = "Lab-Fragments";
    private static final String DEBUG = "DEBUG";
    private static final String STRING_DEBUG = "ListFragment ";
    private int currentIndex = -1;

    public interface SelectionListener {
        public void onItemSelected(int position);
    }

    private SelectionListener mCallback;

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        Log.i(DEBUG, STRING_DEBUG + "onAttach()");
        // Make sure that the hosting Activity has implemented
        // the SelectionListener callback interface. We need this
        // because when an item in this ListFragment is selected, 
        // the hosting Activity's onItemSelected() method will be called.       
        try {
            mCallback = (SelectionListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString()
                    + " must implement SelectionListener");
        }
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.i(DEBUG, STRING_DEBUG + "onCreate()");
        // use different layout definition, depending on whether device is pre-
        // or post-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;

        // Set the list adapter for this ListFragment
        setListAdapter(new ArrayAdapter<String>(getActivity(), layout, FRIENDS));

        if (isInTwoPaneMode()) {
            setRetainInstance(true);
        }
    }

    // Note: ListFragments come with a default onCreateView() method.
    // For other Fragments you'll normally implement this method.
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
        Log.i(DEBUG, STRING_DEBUG + "onCreateView()");
        return super.onCreateView(inflater, container, savedInstanceState);
    }


    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        Log.i(DEBUG, STRING_DEBUG + "onActivityCreated()");
        Log.i(TAG, "Entered onActivityCreated()");
        // When using two-pane layout, configure the ListView to highlight the
        // selected list item

        if (isInTwoPaneMode()) {
            getListView().setChoiceMode(ListView.CHOICE_MODE_SINGLE);
        }

        if (isInTwoPaneMode() && -1 != currentIndex) {
            getListView().setItemChecked(currentIndex, true);
        }
    }

    @Override
    public void onStart() {
        super.onStart();
        Log.i(DEBUG, STRING_DEBUG + "onStart()");
    }

    @Override
    public void onResume() {
        super.onResume();
        Log.i(DEBUG, STRING_DEBUG + "onResume()");
    }

    @Override
    public void onPause() {
        super.onPause();
        Log.i(DEBUG, STRING_DEBUG + "onPause()");
    }

    public void onStop() {
        super.onStop();
        Log.i(DEBUG, STRING_DEBUG + "onStop()");
    }

    public void onDestroyView() {
        super.onDestroyView();
        Log.i(DEBUG, STRING_DEBUG + "onDestroyView()");
    }

    public void onDestroy() {
        super.onDestroy();
        Log.i(DEBUG, STRING_DEBUG + "onDestroy()");
    }

    public void onDetach() {
        super.onDetach();
        Log.i(DEBUG, STRING_DEBUG + "onDetach()");
    }

    @Override
    public void onListItemClick(ListView l, View view, int position, long id) {
        // Notify the hosting Activity that a selection has been made.
        currentIndex = position;
        mCallback.onItemSelected(position);
    }

    // If there is a FeedFragment, then the layout is two-pane 
    private boolean isInTwoPaneMode() {
        return getFragmentManager().findFragmentById(R.id.feed_frag) != null;
    }
}

Fragment // QuoteFragment //DisplayFragment

package course.labs.fragmentslab;

import android.app.Activity;
import android.app.Fragment;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

public class FeedFragment extends Fragment {

    private static final String TAG = "Lab-Fragments";

    private TextView mTextView;
    private static FeedFragmentData feedFragmentData;

    private static final String DEBUG = "DEBUG";
    private static final String STRING_DEBUG = "Fragment ";

    //Index of current selected item to display in FeedFragment
    private int currentIndex = -1;

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        Log.i(DEBUG, STRING_DEBUG + "onAttach()");
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.i(DEBUG, STRING_DEBUG + "onCreate()");
        if (isInTwoPaneMode()) {
            setRetainInstance(true);
        }
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        Log.i(DEBUG, STRING_DEBUG + "onCreateView()");
        return inflater.inflate(R.layout.feed, container, false);
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        Log.i(DEBUG, STRING_DEBUG + "onActivityCreated()");
        mTextView = (TextView) getView().findViewById(R.id.feed_view);  
        // Read in all Twitter feeds 
        if (null == feedFragmentData) { 

            feedFragmentData = new FeedFragmentData(getActivity());

        }       
        Log.d("DEBUG", "Index = " + currentIndex);
        if (-1 != currentIndex) {
            updateFeedDisplay(currentIndex);
        }

    }


    // Display Twitter feed for selected feed
    void updateFeedDisplay(int position) {
        Log.i(TAG, "Entered updateFeedDisplay()");
        currentIndex = position;
        mTextView.setText(feedFragmentData.getFeed(position));
    }

    // If there is a FeedFragment, then the layout is two-pane 
    private boolean isInTwoPaneMode() {
        return getFragmentManager().findFragmentById(R.id.feed_frag) != null;
    }

    @Override
    public void onStart() {
        super.onStart();
        Log.i(DEBUG, STRING_DEBUG + "onStart()");
    }

    @Override
    public void onResume() {
        super.onResume();
        Log.i(DEBUG, STRING_DEBUG + "onResume()");
    }

    @Override
    public void onPause() {
        super.onPause();
        Log.i(DEBUG, STRING_DEBUG + "onPause()");
    }

    public void onStop() {
        super.onStop();
        Log.i(DEBUG, STRING_DEBUG + "onStop()");
    }

    public void onDestroyView() {
        super.onDestroyView();
        Log.i(DEBUG, STRING_DEBUG + "onDestroyView()");
    }

    public void onDestroy() {
        super.onDestroy();
        Log.i(DEBUG, STRING_DEBUG + "onDestroy()");
    }

    public void onDetach() {
        super.onDetach();
        Log.i(DEBUG, STRING_DEBUG + "onDetach()");
    }

}

main_activity.xml

<?xml version="1.0" encoding="utf-8"?>
<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" />

feed.xml

<TextView
    android:id="@+id/feed_view"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="@string/greeting" />

Answer 1

在 fragment Activity 的 onCreate 方法中，您可以检查是否 savedInstanceState == null 并仅在该条件为true。否则 Android 将自动重新附加现有 fragment 。我建议使用以下代码:

...
if (!isInTwoPaneMode()) {           
    if (savedInstanceState == null) {
        mFriendsFragment = new FriendsFragment();
        FragmentTransaction transaction = getFragmentManager().beginTransaction();
        transaction.add(R.id.fragment_container, mFriendsFragment);
        transaction.commit();       
    }
}
...