When the FragmentPagerAdapter adds a fragment to the FragmentManager, it uses a special tag based on the particular position that the fragment will be placed. FragmentPagerAdapter.getItem(int position) is only called when a fragment for that position does not exist. After rotating, Android will notice that it already created/saved a fragment for this particular position and so it simply tries to reconnect with it with FragmentManager.findFragmentByTag(), instead of creating a new one. All of this comes free when using the FragmentPagerAdapter and is why it is usual to have your fragment initialisation code inside the getItem(int) method.

即使我们没有使用FragmentPagerAdapter，每次在Activity.onCreate(Bundle)中创建一个新的片段也不是一个好主意。正如你所注意到的，当一个片段被添加到FragmentManager时，它将在旋转后为你重新创建，不需要再次添加它。这样做是处理片段时出现错误的常见原因。

在处理片段时，通常的方法是:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    ...

    CustomFragment fragment;
    if (savedInstanceState != null) {
        fragment = (CustomFragment) getSupportFragmentManager().findFragmentByTag("customtag");
    } else {
        fragment = new CustomFragment();
        getSupportFragmentManager().beginTransaction().add(R.id.container, fragment, "customtag").commit(); 
    }

    ...

}

When using a FragmentPagerAdapter, we relinquish fragment management to the adapter, and do not have to perform the above steps. By default, it will only preload one Fragment in front and behind the current position (although it does not destroy them unless you are using FragmentStatePagerAdapter). This is controlled by ViewPager.setOffscreenPageLimit(int). Because of this, directly calling methods on the fragments outside of the adapter is not guaranteed to be valid, because they may not even be alive.

To cut a long story short, your solution to use putFragment to be able to get a reference afterwards is not so crazy, and not so unlike the normal way to use fragments anyway (above). It is difficult to obtain a reference otherwise because the fragment is added by the adapter, and not you personally. Just make sure that the offscreenPageLimit is high enough to load your desired fragments at all times, since you rely on it being present. This bypasses lazy loading capabilities of the ViewPager, but seems to be what you desire for your application.

另一种方法是重写FragmentPageAdapter。instantiateItem(View, int)，并在返回之前保存对从super调用返回的片段的引用(如果已经存在，则具有查找片段的逻辑)。

为了获得更完整的图片，请查看FragmentPagerAdapter(短)和ViewPager(长)的一些源代码。

BasePagerAdapter是什么?你应该使用一个标准的寻页适配器——FragmentPagerAdapter或FragmentStatePagerAdapter，这取决于你是否想要ViewPager不再需要的片段被保留(前者)或保存它们的状态(后者)并在再次需要时重新创建。

使用ViewPager的示例代码可以在这里找到

的确，跨活动实例管理视图寻呼机中的片段有点复杂，因为框架中的FragmentManager负责保存状态并恢复寻呼机所生成的任何活动片段。这实际上意味着，适配器在初始化时需要确保它与任何恢复的片段重新连接。您可以查看FragmentPagerAdapter或FragmentStatePagerAdapter的代码，以了解这是如何实现的。

我想出了这个简单而优雅的解决方案。它假设活动负责创建片段，而适配器只是为它们服务。

这是适配器的代码(这里没有什么奇怪的，除了mFragments是由Activity维护的片段列表)

class MyFragmentPagerAdapter extends FragmentStatePagerAdapter {

    public MyFragmentPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        return mFragments.get(position);
    }

    @Override
    public int getCount() {
        return mFragments.size();
    }

    @Override
    public int getItemPosition(Object object) {
        return POSITION_NONE;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        TabFragment fragment = (TabFragment)mFragments.get(position);
        return fragment.getTitle();
    }
} 

这个线程的整个问题是获得“旧”片段的引用，所以我在活动的onCreate中使用这段代码。

    if (savedInstanceState!=null) {
        if (getSupportFragmentManager().getFragments()!=null) {
            for (Fragment fragment : getSupportFragmentManager().getFragments()) {
                mFragments.add(fragment);
            }
        }
    }

当然，如果需要，您还可以进一步优化此代码，例如确保片段是特定类的实例。

要获得方向改变后的片段，必须使用. gettag()。

    getSupportFragmentManager().findFragmentByTag("android:switcher:" + viewPagerId + ":" + positionOfItemInViewPager)

为了更多的处理，我为我的PageAdapter写了我自己的数组列表，通过viewPagerId和FragmentClass在任何位置获得片段:

public class MyPageAdapter extends FragmentPagerAdapter implements Serializable {
private final String logTAG = MyPageAdapter.class.getName() + ".";

private ArrayList<MyPageBuilder> fragmentPages;

public MyPageAdapter(FragmentManager fm, ArrayList<MyPageBuilder> fragments) {
    super(fm);
    fragmentPages = fragments;
}

@Override
public Fragment getItem(int position) {
    return this.fragmentPages.get(position).getFragment();
}

@Override
public CharSequence getPageTitle(int position) {
    return this.fragmentPages.get(position).getPageTitle();
}

@Override
public int getCount() {
    return this.fragmentPages.size();
}


public int getItemPosition(Object object) {
    //benötigt, damit bei notifyDataSetChanged alle Fragemnts refrehsed werden

    Log.d(logTAG, object.getClass().getName());
    return POSITION_NONE;
}

public Fragment getFragment(int position) {
    return getItem(position);
}

public String getTag(int position, int viewPagerId) {
    //getSupportFragmentManager().findFragmentByTag("android:switcher:" + R.id.shares_detail_activity_viewpager + ":" + myViewPager.getCurrentItem())

    return "android:switcher:" + viewPagerId + ":" + position;
}

public MyPageBuilder getPageBuilder(String pageTitle, int icon, int selectedIcon, Fragment frag) {
    return new MyPageBuilder(pageTitle, icon, selectedIcon, frag);
}


public static class MyPageBuilder {

    private Fragment fragment;

    public Fragment getFragment() {
        return fragment;
    }

    public void setFragment(Fragment fragment) {
        this.fragment = fragment;
    }

    private String pageTitle;

    public String getPageTitle() {
        return pageTitle;
    }

    public void setPageTitle(String pageTitle) {
        this.pageTitle = pageTitle;
    }

    private int icon;

    public int getIconUnselected() {
        return icon;
    }

    public void setIconUnselected(int iconUnselected) {
        this.icon = iconUnselected;
    }

    private int iconSelected;

    public int getIconSelected() {
        return iconSelected;
    }

    public void setIconSelected(int iconSelected) {
        this.iconSelected = iconSelected;
    }

    public MyPageBuilder(String pageTitle, int icon, int selectedIcon, Fragment frag) {
        this.pageTitle = pageTitle;
        this.icon = icon;
        this.iconSelected = selectedIcon;
        this.fragment = frag;
    }
}

public static class MyPageArrayList extends ArrayList<MyPageBuilder> {
    private final String logTAG = MyPageArrayList.class.getName() + ".";

    public MyPageBuilder get(Class cls) {
        // Fragment über FragmentClass holen
        for (MyPageBuilder item : this) {
            if (item.fragment.getClass().getName().equalsIgnoreCase(cls.getName())) {
                return super.get(indexOf(item));
            }
        }
        return null;
    }

    public String getTag(int viewPagerId, Class cls) {
        // Tag des Fragment unabhängig vom State z.B. nach bei Orientation change
        for (MyPageBuilder item : this) {
            if (item.fragment.getClass().getName().equalsIgnoreCase(cls.getName())) {
                return "android:switcher:" + viewPagerId + ":" + indexOf(item);
            }
        }
        return null;
    }
}

所以只要用这些片段创建一个MyPageArrayList:

    myFragPages = new MyPageAdapter.MyPageArrayList();

    myFragPages.add(new MyPageAdapter.MyPageBuilder(
            getString(R.string.widget_config_data_frag),
            R.drawable.ic_sd_storage_24dp,
            R.drawable.ic_sd_storage_selected_24dp,
            new WidgetDataFrag()));

    myFragPages.add(new MyPageAdapter.MyPageBuilder(
            getString(R.string.widget_config_color_frag),
            R.drawable.ic_color_24dp,
            R.drawable.ic_color_selected_24dp,
            new WidgetColorFrag()));

    myFragPages.add(new MyPageAdapter.MyPageBuilder(
            getString(R.string.widget_config_textsize_frag),
            R.drawable.ic_settings_widget_24dp,
            R.drawable.ic_settings_selected_24dp,
            new WidgetTextSizeFrag()));

并将它们添加到viewPager:

    mAdapter = new MyPageAdapter(getSupportFragmentManager(), myFragPages);
    myViewPager.setAdapter(mAdapter);

在此之后，你可以通过使用它的类来获得正确的方向改变片段:

        WidgetDataFrag dataFragment = (WidgetDataFrag) getSupportFragmentManager()
            .findFragmentByTag(myFragPages.getTag(myViewPager.getId(), WidgetDataFrag.class));

When the FragmentPagerAdapter adds a fragment to the FragmentManager, it uses a special tag based on the particular position that the fragment will be placed. FragmentPagerAdapter.getItem(int position) is only called when a fragment for that position does not exist. After rotating, Android will notice that it already created/saved a fragment for this particular position and so it simply tries to reconnect with it with FragmentManager.findFragmentByTag(), instead of creating a new one. All of this comes free when using the FragmentPagerAdapter and is why it is usual to have your fragment initialisation code inside the getItem(int) method.

即使我们没有使用FragmentPagerAdapter，每次在Activity.onCreate(Bundle)中创建一个新的片段也不是一个好主意。正如你所注意到的，当一个片段被添加到FragmentManager时，它将在旋转后为你重新创建，不需要再次添加它。这样做是处理片段时出现错误的常见原因。

在处理片段时，通常的方法是:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    ...

    CustomFragment fragment;
    if (savedInstanceState != null) {
        fragment = (CustomFragment) getSupportFragmentManager().findFragmentByTag("customtag");
    } else {
        fragment = new CustomFragment();
        getSupportFragmentManager().beginTransaction().add(R.id.container, fragment, "customtag").commit(); 
    }

    ...

}

When using a FragmentPagerAdapter, we relinquish fragment management to the adapter, and do not have to perform the above steps. By default, it will only preload one Fragment in front and behind the current position (although it does not destroy them unless you are using FragmentStatePagerAdapter). This is controlled by ViewPager.setOffscreenPageLimit(int). Because of this, directly calling methods on the fragments outside of the adapter is not guaranteed to be valid, because they may not even be alive.

To cut a long story short, your solution to use putFragment to be able to get a reference afterwards is not so crazy, and not so unlike the normal way to use fragments anyway (above). It is difficult to obtain a reference otherwise because the fragment is added by the adapter, and not you personally. Just make sure that the offscreenPageLimit is high enough to load your desired fragments at all times, since you rely on it being present. This bypasses lazy loading capabilities of the ViewPager, but seems to be what you desire for your application.

另一种方法是重写FragmentPageAdapter。instantiateItem(View, int)，并在返回之前保存对从super调用返回的片段的引用(如果已经存在，则具有查找片段的逻辑)。

为了获得更完整的图片，请查看FragmentPagerAdapter(短)和ViewPager(长)的一些源代码。

我想为一个可能略有不同的情况提供一个替代解决方案，因为我对答案的许多搜索一直将我引向这个主题。

我的情况 -我动态创建/添加页面，并将它们滑动到ViewPager，但当旋转(onConfigurationChange)，我最终会有一个新页面，因为当然OnCreate再次被调用。但是我想保持对旋转之前创建的所有页面的引用。

问题 -我没有为我创建的每个片段提供唯一的标识符，所以引用的唯一方法是以某种方式将引用存储在数组中，以便在旋转/配置更改后恢复。

解决方案 -关键的概念是让Activity(显示片段)也管理现有片段的引用数组，因为这个Activity可以利用onSaveInstanceState中的Bundles

public class MainActivity extends FragmentActivity

因此，在这个Activity中，我声明了一个私有成员来跟踪打开的页面

private List<Fragment> retainedPages = new ArrayList<Fragment>();

每次onSaveInstanceState被调用时都会更新，并在onCreate中恢复

@Override
protected void onSaveInstanceState(Bundle outState) {
    retainedPages = _adapter.exportList();
    outState.putSerializable("retainedPages", (Serializable) retainedPages);
    super.onSaveInstanceState(outState);
}

.．.所以一旦它被存储，它就可以被检索…

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if (savedInstanceState != null) {
        retainedPages = (List<Fragment>) savedInstanceState.getSerializable("retainedPages");
    }
    _mViewPager = (CustomViewPager) findViewById(R.id.viewPager);
    _adapter = new ViewPagerAdapter(getApplicationContext(), getSupportFragmentManager());
    if (retainedPages.size() > 0) {
        _adapter.importList(retainedPages);
    }
    _mViewPager.setAdapter(_adapter);
    _mViewPager.setCurrentItem(_adapter.getCount()-1);
}

这些是对主活动的必要更改，因此我需要FragmentPagerAdapter中的成员和方法来工作，因此在

public class ViewPagerAdapter extends FragmentPagerAdapter

一个相同的构造(如上面的MainActivity中所示)

private List<Fragment> _pages = new ArrayList<Fragment>();

这种同步(如上面在onSaveInstanceState中使用的那样)是由方法特别支持的

public List<Fragment> exportList() {
    return _pages;
}

public void importList(List<Fragment> savedPages) {
    _pages = savedPages;
}

最后，在fragment类中

public class CustomFragment extends Fragment

为了使这一切顺利进行，有两个改变，首先

public class CustomFragment extends Fragment implements Serializable

然后将其添加到onCreate，这样Fragments就不会被破坏

setRetainInstance(true);

我仍然在思考Fragments和Android生命周期，所以这里需要注意的是，这种方法可能存在冗余/低效率。但它对我有用，我希望它能对其他有类似情况的人有所帮助。