Android 为什么不能在 Fragment 中使用 ViewPager？它实际上是

Why it is not possible to use ViewPager within a Fragment? It actually is

提问by Eugene

There are information that it is impossible to use ViewPagerwithin a Fragmentin many sources like "The Busy Coders Guide for Android Developers" by Mark Murphy, or posts like thison SO. I'm confused because I don't have such a problem and I successfully use ViewPagerwithin my Fragment. The only distinction is that I instantiate a ViewPagernot in onCreateView()method but in onActivityCreated(). And everything works perfectly fine.

有信息，这是不可能使用ViewPager内Fragment像“许多来源的忙打码机指南Android开发”由马克·墨菲，或职位像这样的SO。我很困惑，因为我没有这样的问题，而且我ViewPager在我的Fragment. 唯一的区别是我ViewPager不是在onCreateView()方法中而是在onActivityCreated(). 一切正常。

So the question is - may be I just don't know something and this is not recommended for some reason to make UI instantiations in onActivityCreated()? But again - everything works just fine.

所以问题是 - 可能我只是不知道一些事情，并且出于某种原因不建议在 中进行 UI 实例化onActivityCreated()？但同样 - 一切正常。

Here is the listing of the class and xml:

这是类和 xml 的列表：

Class:

班级：

public class ViewPagerFragment extends Fragment {

    static final int NUM_ITEMS = 2;

    private ViewPagerAdapter mAdapter;
    private ViewPager mPager;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.view_pager_fragment, container, false);
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        mAdapter = new ViewPagerAdapter(getFragmentManager());

        mPager = (ViewPager) getView().findViewById(R.id.pager);
        mPager.setAdapter(mAdapter);
    }

    public static class ViewPagerAdapter extends FragmentPagerAdapter {
        public ViewPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int num) {
            if (num == 0) {
                return new ItemsListFragment();
            } else {
                return new FavsListFragment();
            }
        }

        @Override
        public int getCount() {
            return NUM_ITEMS;
        }
    }
}

Layout:

布局：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="fill_parent"
              android:layout_height="fill_parent"
              android:orientation="vertical">

    <android.support.v4.view.ViewPager
        android:id="@+id/pager"
        android:layout_width="fill_parent"
        android:layout_height="0dp"
        android:layout_weight="1"/>

</LinearLayout>

回答by Rajkumar

To implement the View pager within a fragment use getChildFragmentManager()instead of getFragmentManager(). You can call setAdapter()for the ViewPager from onCreateView()or onActivityCreated(), that is not a matter.

要在片段中实现视图分页器，请使用getChildFragmentManager()而不是getFragmentManager()。您可以从onCreateView()或onActivityCreated ()为 ViewPager调用setAdapter()，这不是问题。

I hope this will help you guys.

我希望这会帮助你们。

回答by CommonsWare

UPDATE: Since this answer was originally written, you now can have nested fragments, which means it is possible to have a ViewPageruse fragments for pages andbe in a fragment itself. This sample projectdemonstrates the technique.

更新：由于这个答案最初是写的，你现在可以有嵌套的片段，这意味着可以有一个ViewPager用于页面的片段并在片段本身中。此示例项目演示了该技术。

I now return you to your regularly-scheduled answer, presented in its entirety...

我现在让你回到你定期安排的答案，完整地呈现......

---

---

Quoting myself from the book:

引用书中自己的话：

The simplest way to use a ViewPager is to have it page fragments in and out of the screen based on user swipes. This only works if the ViewPager itself is not contained within a fragment, as you cannot have fragments nested inside of other fragments.

使用 ViewPager 的最简单方法是根据用户的滑动让页面片段进出屏幕。这仅在 ViewPager 本身不包含在片段中时才有效，因为您不能将片段嵌套在其他片段中。

Quoting Dianne Hackborn:

引用黛安·哈克伯恩的话：

Nested fragments are not currently supported. Trying to put a fragment within the UI of another fragment will result in undefined and likely broken behavior.

当前不支持嵌套片段。尝试将一个片段放在另一个片段的 UI 中将导致未定义和可能损坏的行为。

It is perfectly possible to put a ViewPagerinside a Fragment, so long as the contentsof the ViewPagerdo not themselves contain fragments. Since the concrete implementations of PagerAdaptersupplied by the Android Support package use fragments, you have to roll your own fragment-less PagerAdapterto put the ViewPagerin a fragment.

这是完全可能把一个ViewPager内部的Fragment，只要内容的ViewPager文件本身并不包含片段。由于PagerAdapterAndroid 支持包提供的具体实现使用片段，因此您必须滚动自己的无片段PagerAdapter以将其ViewPager放入片段中。

I will endeavor to make this point clearer in the next edition of the book (unless you're British, in which case I'll endeavour to make this point clearer :-).

我将努力在本书的下一版中更清楚地说明这一点（除非您是英国人，在这种情况下，我将努力使这一点更清楚:-）。

回答by hims_3009

While Initializing the SetionPageAdapter for ViewPager use "getChildFragmentManager()".

在为 ViewPager 初始化 SetionPageAdapter 时，请使用“getChildFragmentManager()”。

mSectionsPagerAdapter = new SectionsPagerAdapter(getChildFragmentManager());

mSectionsPagerAdapter = new SectionsPagerAdapter(getChildFragmentManager());

回答by mukesh mali

it is possible try to do this code and save view_pager_fragment.xml

可以尝试执行此代码并保存 view_pager_fragment.xml

<android.support.design.widget.AppBarLayout
    android:id="@+id/appBarLayout"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">

    <android.support.design.widget.TabLayout
        android:id="@+id/tab"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

</android.support.design.widget.AppBarLayout>

<android.support.v4.view.ViewPager
    android:id="@+id/viewpager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior" />

ViewPagerFragment

ViewPagerFragment

ViewPager viewPager;
TabLayout tabLayout;

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.tracks, container, false);

    viewPager = (ViewPager) view.findViewById(R.id.viewpager);
    tabLayout = (TabLayout) view.findViewById(R.id.tab);

    return view;
}

@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);

    setupViewPager(viewPager);
    tabLayout.setupWithViewPager(viewPager);

    tabLayout.setOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
        @Override
        public void onTabSelected(TabLayout.Tab tab) {

        }

        @Override
        public void onTabUnselected(TabLayout.Tab tab) {

        }

        @Override
        public void onTabReselected(TabLayout.Tab tab) {

        }
    });
}

private void setupViewPager(ViewPager viewPager) {
    ViewPagerAdapter viewPagerAdapter = new ViewPagerAdapter(getChildFragmentManager());``

    viewPagerAdapter.addFragment(new FirstFragment(), "First");
    viewPagerAdapter.addFragment(new SecoundFragment(), "Secound");
    viewPagerAdapter.addFragment(new LoginFragment(), "Login");

    viewPager.setAdapter(viewPagerAdapter);
}

private class ViewPagerAdapter extends FragmentPagerAdapter {

    List<Fragment> fragmentList = new ArrayList<>();
    List<String> fragmentTitles = new ArrayList<>();

    public ViewPagerAdapter(FragmentManager fragmentManager) {
        super(fragmentManager);
    }

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

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

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

    public void addFragment(Fragment fragment, String name) {
        fragmentList.add(fragment);
        fragmentTitles.add(name);
    }
}