Viewpager 未显示在约束布局中

我为你安排了一切。你需要有一个BaseFragment可以保存ViewPager. 该Viewpager调用2个的其它片段。在这里FirstFragment（电话）和SecondFragment（电子邮件）。所以首先定义你BaseFragment.java这样的：

public class BaseFragment extends Fragment {

    public BaseFragment(){
    }

    public static FirstFragment firstFragment;
    public static SecondFragment secondFragment;

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


        TabLayout tabLayout = view.findViewById(R.id.tab_layout);
        ViewPager viewPager = view.findViewById(R.id.login_view_pager);
        firstFragment = new FirstFragment();
        secondFragment = new SecondFragment();
        tabLayout.setupWithViewPager(viewPager);

        ViewPagerAdapter viewPagerAdapter = new ViewPagerAdapter(getChildFragmentManager(), 0);
        viewPagerAdapter.addFragment(firstFragment,"Phone");
        viewPagerAdapter.addFragment(secondFragment,"Email");
        viewPager.setAdapter(viewPagerAdapter);

        return view;
    }

    private class ViewPagerAdapter extends FragmentPagerAdapter {

        private List<Fragment> fragments = new ArrayList<>();
        private List<String> fragmentTitle = new ArrayList<>();

        public ViewPagerAdapter(@NonNull FragmentManager fm, int behavior) {
            super(fm, behavior);
        }

        public void addFragment(Fragment fragment, String title) {
            fragments.add(fragment);
            fragmentTitle.add(title);
        }

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

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

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


    }
}

并且还fragment_base.xml用你给我们的东西来定义它：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    tools:context=".BaseFragment">


    <ScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:fillViewport="true"
        tools:ignore="MissingConstraints">

        <androidx.constraintlayout.widget.ConstraintLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content">

            <LinearLayout
                android:orientation="vertical"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                app:layout_constraintEnd_toEndOf="parent"
                app:layout_constraintStart_toStartOf="parent"
                app:layout_constraintTop_toTopOf="parent">

                <ImageView
                    android:layout_width="40dp"
                    android:layout_height="40dp"
                    android:id="@+id/close_login_screen"
                    android:layout_gravity="left"
                    android:src="@drawable/icondelete"/>

                <ImageView
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:layout_gravity="center"
                    android:src="@drawable/ic_launcher_background"
                    android:layout_marginTop="60dp"
                    android:layout_marginBottom="40dp"/>

                <androidx.viewpager.widget.ViewPager
                    android:id="@+id/login_view_pager"
                    android:layout_width="match_parent"
                    android:layout_height="match_parent">

                    <com.google.android.material.tabs.TabLayout
                        android:id="@+id/tab_layout"
                        android:layout_width="match_parent"
                        android:layout_height="wrap_content"
                        app:tabIndicatorColor="#121212"
                        app:tabTextColor="@android:color/darker_gray"
                        app:tabSelectedTextColor="#121212"
                        app:tabIndicatorFullWidth="false"
                        app:tabTextAppearance="@android:style/TextAppearance.Widget.TabWidget"/>

                </androidx.viewpager.widget.ViewPager>

            </LinearLayout>

        </androidx.constraintlayout.widget.ConstraintLayout>

    </ScrollView>

</RelativeLayout>

现在是更不吸引人的部分。定义FirstFragment并SecondFragment使用它的视图。这将从ViewPageras TabLayout、您的电话和电子邮件页面保留。

FirstFragment.java：

public class FirstFragment extends Fragment {



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


        TextView textView = view.findViewById(R.id.textView_phone);

        return view;
    }
}

fragment_first.xml：

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:tools="http://schemas.android.com/tools"
    tools:context=".FirstFragment">
    
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Phone"
        android:textSize="28sp"
        android:id="@+id/textView_phone"
        android:layout_centerInParent="true"/>

</ScrollView>

SecondFragment.java：

public class SecondFragment extends Fragment {


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


        TextView textView = view.findViewById(R.id.textView_email);

        return view;
    }
}

fragment_second.xml：

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:tools="http://schemas.android.com/tools"
    tools:context=".SecondFragment">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Email"
        android:textSize="28sp"
        android:id="@+id/textView_email"
        android:layout_centerInParent="true"/>

</ScrollView>

结果

现在你看到Viewpager它工作正常。如果你想片段滚动您需要添加ScrollView的fragment_first.xml和fragment_second.xml。干杯!