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" />
最佳答案
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();
}
}
...
关于java - Android 在每次方向改变时都会创建额外的 ListFragment,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/21677605/