我正在编写一个在其 MainActivity 中有 2 个选项卡的应用程序。我使用适用于 Eclipse 的 Android 插件创建了 Activity 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， fragment 将为空。

这是我的 Activity 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();
    }
}   

 }

我问了一个同事，他说我的Activity 中保留这两个 Fragments 作为成员是不好的，因为 Android 最终会创建一个 Fragments 的新实例 onResume，这就是为什么我的引用为空。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 fragment ，但我不知道如何让我的 fragment 处于某个状态，其中我可以使用 FragmentManager 阅读它们。

提前致谢!

最佳答案

您的同事是对的:在您的 Activity 变量中使用对 fragment 的引用不是一个好习惯。您的 Activity 将被破坏，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，将初始化 ViewPager 的代码(您的 initSectionsPagerAdapter() 方法)移动到 onResume。它会在 Activity 首次启动时以及它变得可见时被调用，例如当位于顶部的另一个应用程序关闭时。您不需要 onResume 中的当前代码。

编辑:

当您创建一个 fragment 时，不要将对它的引用存储在 Activity 的变量中。稍后要访问 fragment ，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) {
    }
}

关于java - onresume 后 ViewPager 中的 fragment 为空，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/20280811/