我正在编写一个在MainActivity中有2个标签的应用程序。我使用适用于Eclipse的Android插件创建了活动模板并对其进行了调整。
无论如何，当我在一段时间后恢复应用程序时，片段将为空。
以下是我的活动代码（我删除了“不重要的”部分）
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();
}
}
}
我问了一位同事他告诉我，将两个片段作为成员放在我的Activity中并不好，因为Android最终会创建片段onResume的新实例，这就是为什么我的引用是null。但他无法告诉我如何解决我的问题。
我正在考虑编写一个getFragment()方法，该方法将使用FragmentManager来获取片段，但我不知道如何将片段置于一个状态，我可以使用{{{ 1}}。
提前谢谢！
答案 0 :(得分：8)
您的同事是对的：在您的活动变量中使用对片段的引用并不是一个好习惯。您的活动将被销毁，例如当您旋转屏幕时，引用可能会导致内存泄漏。
至于您的问题，请将初始化ViewPager（您的initSectionsPagerAdapter()方法）的代码移至onResume。它将在活动首次启动时以及何时变为可见时被调用，例如，当另一个在顶部的应用程序关闭时。您不需要onResume中的当前代码。
编辑：
创建片段时，不要在活动的变量中存储对它的引用。要稍后访问片段，您可以使用：
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) {
}
}