java - fragment View 寻呼机显示为空

Question

我尝试了很多这个问题的答案，但它们都不适合我。我有一个 View 寻呼机，其中包含三个 fragment ，但是当我在手机上运行该应用程序时， fragment 不会出现，并且 View 寻呼机显示为空。

这是主要 Activity 代码:

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        ActionBar actionBar = getSupportActionBar();
        assert actionBar != null;
        actionBar.setDisplayShowTitleEnabled(false);


        mAuth = FirebaseAuth.getInstance();
        database = FirebaseDatabase.getInstance();
        myRef = database.getReference("posts");

        mViewPager = findViewById(R.id.viewpager);
        adapter = new SamplePagerAdapter(getSupportFragmentManager(),this);
        tabLayout = findViewById(R.id.tablayout);
        mViewPager.setAdapter(adapter);
        tabLayout.setupWithViewPager(mViewPager);

        mViewPager.setCurrentItem(1);
        TextView tabOne = (TextView) LayoutInflater.from(this).inflate(R.layout.custom_tab, null);
        tabOne.setText("تعلم");
        tabOne.setCompoundDrawablesWithIntrinsicBounds(0, R.drawable.learn, 0, 0);
        tabLayout.getTabAt(0).setCustomView(tabOne);

        TextView tabTwo = (TextView) LayoutInflater.from(this).inflate(R.layout.custom_tab, null);
        tabTwo.setText("الصفحة الرئيسية");
        tabTwo.setCompoundDrawablesWithIntrinsicBounds(0, R.drawable.home, 0, 0);
        tabLayout.getTabAt(1).setCustomView(tabTwo);

        TextView tabThree = (TextView) LayoutInflater.from(this).inflate(R.layout.custom_tab, null);
        tabThree.setText("التحديات");
        tabThree.setCompoundDrawablesWithIntrinsicBounds(0, R.drawable.challenges, 0, 0);
        tabLayout.getTabAt(2).setCustomView(tabThree);
        tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                tabPosition = tab.getPosition();
                Toast.makeText(MainActivity.this,
                        "Selected page position: " + tab.getPosition(), Toast.LENGTH_SHORT).show();
            }

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

            }

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

            }
        });

    }

and this is the view pager adapter code : 

public class SamplePagerAdapter extends FragmentPagerAdapter {
    Context context;

    public SamplePagerAdapter(FragmentManager fragmentManager, Context context) {
        super(fragmentManager);
        this.context = context;
    }

    /**
     * @return the number of pages to display
     */
    @Override
    public int getCount() {
        return 3;
    }

    /**
     * @return true if the value returned from {@link #instantiateItem(ViewGroup, int)} is the
     */
    @Override
    public boolean isViewFromObject(View view, Object o) {
        return o == view;
    }

    // BEGIN_INCLUDE (pageradapter_getpagetitle)

    /**
     * Return the title of the item at {@code position}. This is important as what this method
     * <p>
     * Here we construct one using the position value, but for real application the title should
     * refer to the item's contents.
     */
    @Override
    public CharSequence getPageTitle(int position) {
        String title = "";
        switch (position) {
            case 0:
                title = "تعلم";
                break;
            case 1:
                title = "الصفحة الرئيسية";
                break;
            case 2:
                title = "التحديات";
                break;
        }
        return title;
    }
    // END_INCLUDE (pageradapter_getpagetitle)

    @Override
    public Fragment getItem(int position) {
        switch (position) {
            case 0:
                Toast.makeText(context, "position 0", Toast.LENGTH_SHORT).show();
                return new LearnFragment();
            case 1:
                Toast.makeText(context, "position 1", Toast.LENGTH_SHORT).show();
                return new HomeFragment();
            case 2:
                Toast.makeText(context, "position 2", Toast.LENGTH_SHORT).show();
                return new ChallengesFragment();
        }

        return null;
    }

}

这是 Activity 主要 xml 代码:

  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:id="@+id/sample_main_layout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <android.support.design.widget.AppBarLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content">

            <android.support.v7.widget.Toolbar
                android:id="@+id/toolbar"
                android:layout_width="match_parent"
                android:layout_height="?attr/actionBarSize"
                android:background="@color/color1">

                <TextView
                    android:id="@+id/toolbar_title"
                    android:layout_width="wrap_content"
                    android:layout_height="wrap_content"
                    android:layout_gravity="center"
                    android:text="@string/app_name"
                    android:textColor="@color/white"
                    android:textSize="@dimen/text_size_in_toolbar" />
            </android.support.v7.widget.Toolbar>

            <android.support.design.widget.TabLayout
                android:id="@+id/tablayout"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:background="@color/color1">

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


        <android.support.v4.view.ViewPager
            android:id="@+id/viewpager"
            android:layout_width="match_parent"
            android:layout_height="0px"
            android:layout_weight="1"
            android:background="@android:color/white" />

    </LinearLayout>

Answer 1

@Override
public boolean isViewFromObject(View view, Object o) {
    return o == view;
}

删除此代码