在ViewPager片段后onresume空 [英] Fragments in ViewPager are null after onresume
问题描述
我正在写有2标签在其 MainActivity 的应用程序。我创建的活动模板与Android的Eclipse插件，并适应它。
反正当我一段时间后，恢复该应用程序的片段将是零。
I'm writing an application that has 2 Tabs in its MainActivity. I created the Activity Template with the Android Plugin for Eclipse and adapted it.
Anyway when I resume the Application after a while the Fragments will be null.
下面是我的活动的code（我删除不重要的部分）
Here is the code of my Activity (I removed "unimportant" parts)
public class MainActivity extends FragmentActivity implements
ActionBar.TabListener {
/**
* The {@link android.support.v4.view.PagerAdapter} that will provide
* fragments for each of the sections. We use a
* {@link android.support.v4.app.FragmentPagerAdapter} derivative, which
* will keep every loaded fragment in memory. If this becomes too memory
* intensive, it may be best to switch to a
* {@link android.support.v4.app.FragmentStatePagerAdapter}.
*/
SectionsPagerAdapter mSectionsPagerAdapter;
private static final String ARG_SECTION_NUMBER = "section_number";
/**
* Fragment to display markets on a {@link GoogleMap}.
*/
private GluehweinMapFragment mGluehweinMapFragment;
/**
* Fragment to display available markets as list.
*/
private GluehweinListFragment mGluehweinListFragment;
/**
* The {@link ViewPager} that will host the section contents.
*/
ViewPager mViewPager;
/*
* (non-Javadoc)
*
* @see android.support.v4.app.FragmentActivity#onCreate(android.os.Bundle)
*/
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initSectionsPagerAdapter();
//[...]
}
private void initSectionsPagerAdapter() {
if (mSectionsPagerAdapter == null || mViewPager == null) {
final ActionBar actionBar = getActionBar();
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
// Create the adapter that will return a fragment for each of the
// three
// primary sections of the app.
mSectionsPagerAdapter = new SectionsPagerAdapter(
getSupportFragmentManager());
// Set up the ViewPager with the sections adapter.
mViewPager = (ViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mSectionsPagerAdapter);
// When swiping between different sections, select the corresponding
// tab. We can also use ActionBar.Tab#select() to do this if we have
// a reference to the Tab.
mViewPager
.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
@Override
public void onPageSelected(int position) {
switch (position) {
case 0:
break;
}
refreshFragments();
actionBar.setSelectedNavigationItem(position);
}
});
actionBar.getSelectedNavigationIndex();
// For each of the sections in the app, add a tab to the action bar.
for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
// Create a tab with text corresponding to the page title
// defined by
// the adapter. Also specify this Activity object, which
// implements
// the TabListener interface, as the callback (listener) for
// when
// this tab is selected.
actionBar.addTab(actionBar.newTab()
.setText(mSectionsPagerAdapter.getPageTitle(i))
.setTabListener(this));
}
}
}
/**
* Hides or shows the closed markets on the list and map
*/
private void updateVisibility() {
if (this.mGluehweinMapFragment != null) {
mGluehweinMapFragment.updateVisibility();
}
if (this.mGluehweinListFragment != null) {
mGluehweinListFragment.updateVisibility();
}
}
@Override
public void onTabSelected(ActionBar.Tab tab,
FragmentTransaction fragmentTransaction) {
mViewPager.setCurrentItem(tab.getPosition());
invalidateOptionsMenu();
}
@Override
public void onTabUnselected(ActionBar.Tab tab,
FragmentTransaction fragmentTransaction) {
}
@Override
public void onTabReselected(ActionBar.Tab tab,
FragmentTransaction fragmentTransaction) {
}
/**
* A {@link FragmentPagerAdapter} that returns a fragment corresponding to
* one of the sections/tabs/pages.
*/
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 = null;
Bundle args = new Bundle();
args.putInt(ARG_SECTION_NUMBER, position + 1);
switch (position) {
case 0:
// position 0 is the Map
if (mGluehweinMapFragment == null) {
mGluehweinMapFragment = new GluehweinMapFragment();
}
fragment = mGluehweinMapFragment;
break;
case 1:
// position 1 is the List
if (mGluehweinListFragment == null) {
mGluehweinListFragment = new GluehweinListFragment();
}
fragment = mGluehweinListFragment;
break;
}
fragment.setArguments(args);
return fragment;
}
@Override
public int getCount() {
return 2;
}
@Override
public CharSequence getPageTitle(int position) {
Locale l = Locale.getDefault();
switch (position) {
case 0:
return getString(R.string.tab_map).toUpperCase(l);
case 1:
return getString(R.string.tab_list).toUpperCase(l);
}
return null;
}
}
@Override
protected void onResume() {
// on resume, restart requesting location updates
super.onResume();
//[..]
initSectionsPagerAdapter();
//some calls on the fragments which are null here!!!
refreshFragments();
}
/**
* Notifies the Fragments in the Tabs that data has changed
*/
private void refreshFragments() {
if (this.mGluehweinMapFragment != null) {
mGluehweinMapFragment.updateVisibility();
}
if (this.mGluehweinListFragment != null) {
mGluehweinListFragment.updateVisibility();
}
}
}
我问过一个同事，他说我的不好，以保持两个片段在我的活动，成员由于Android最终将创建片段的新实例 onResume ，这就是为什么我的引用是空。但他不能告诉我怎么解决我的问题。
我在想编写一个将使用 getFragment（）方法 FragmentManager 来获得片段，但我不'知道如何让我的片段在一个国家，在那里我可以阅读它们与 FragmentManager 。
I asked a colleague and he said me that its not good to keep the two Fragments as members in my Activity, because Android eventually would create a new instance of the Fragments onResume and this is why my references are null. But he could not tell me how to fix my problem.
I was thinking about writing a getFragment() method that would use a FragmentManager to get the fragments, but I don't know how to get my fragments in a state, where I can read them with a FragmentManager.
感谢你在前进！
推荐答案
您的同事是正确的：它不是用在您的活动的变量引用片段一个很好的做法。你的活动就会被破坏，例如当你旋转屏幕，并且引用可能导致内存泄漏。
Your colleague is right: it's not a good practice to use references to fragments in your activity's variables. Your activity will be destroyed, e.g. when you rotate the screen, and the references may lead to memory leaks.
至于你的问题，将code表示初始化的 ViewPager （你的 initSectionsPagerAdapter（）方法），以 onResume 。当活动首次启动以及当它成为可见的，例如，将被称为当另一个应用程序，是在顶部是封闭的。你并不需要在 onResume 您目前的code。
As for your problem, move the code that initialises the ViewPager (your initSectionsPagerAdapter() method) to onResume. It will be called when the activity is first started as well as when it becomes visible, e.g. when another app that was on top is closed. You don't need your current code in onResume.
编辑：
当你创建一个片段，不要存放在活动中的变量的引用。以后要进入片段，您可以使用：
When you create a fragment, don't store the reference to it in activity's variables. To access the fragments later, you can use:
FragmentManager fm = this.getFragmentManager();
GluehweinMapFragment f1 = (GluehweinMapFragment)fm.getFragments().get(0); // to get one fragment
for (Fragment f : fm.getFragments()) { // to loop through fragments and checking their type
if (f instanceof GluehweinMapFragment) {
}
}
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