Android 片段不'；不显示

我正在尝试使用导航栏并用它打开新的片段。 我浏览了所有关于它的问题，看了很多YouTube视频。。。但我不明白为什么片段内容没有出现。 下面是一些代码：

main活动

class:

public class MainActivity extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener, MainFragment.OnFragmentInteractionListener, OtherFragment.OnFragmentInteractionListener {

    NavigationView navigationView;
    Toolbar toolbar;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
         toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        MainFragment frag = new MainFragment();
        FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
        ft.replace(R.id.fragment_container, frag);
        ft.commit();

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
                this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
        drawer.setDrawerListener(toggle);
        toggle.syncState();

        navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);
    }

    @SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        int id = item.getItemId();

        if (id == R.id.nav_camera) {
            MainFragment frag = new MainFragment();
            FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
            ft.replace(R.id.fragment_container, frag).commit();
        } else if (id == R.id.nav_gallery) {
            OtherFragment frag = new OtherFragment();
            FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
            ft.replace(R.id.fragment_container, frag);
            ft.commit();
        } else if (id == R.id.nav_slideshow) {

        } else if (id == R.id.nav_manage) {

        } else if (id == R.id.nav_share) {

        } else if (id == R.id.nav_send) {

        }

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        if(drawer!=null)
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }

    @Override
    public void onFragmentInteraction(Uri uri) {

    }

}

public class MainFragment extends Fragment {
    // TODO: Rename parameter arguments, choose names that match
    // the fragment initialization parameters, e.g. ARG_ITEM_NUMBER
    private static final String ARG_PARAM1 = "param1";
    private static final String ARG_PARAM2 = "param2";

    // TODO: Rename and change types of parameters
    private String mParam1;
    private String mParam2;

    private OnFragmentInteractionListener mListener;

    public MainFragment() {
        // Required empty public constructor
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_main, container, false);
    }

    // TODO: Rename method, update argument and hook method into UI event

    /**
     * This interface must be implemented by activities that contain this
     * fragment to allow an interaction in this fragment to be communicated
     * to the activity and potentially other fragments contained in that
     * activity.
     * <p>
     * See the Android Training lesson <a href=
     * "http://developer.android.com/training/basics/fragments/communicating.html"
     * >Communicating with Other Fragments</a> for more information.
     */
    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        void onFragmentInteraction(Uri uri);
    }
}

MainFragment

class（非常等同于

OtherFragment

class）：

fragment_main.xml：

<FrameLayout 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"
    tools:context="de.package.test.MainFragment">

        <TextView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:text="@string/hello_blank_fragment" />

</FrameLayout>

我没有收到任何错误代码。

---

如果你能帮助我，我将非常高兴

您是否为坐标布局中的碎片容器提供了背景色，以查看它是否出现

---

另外，您应该创建一个片段切换器方法，该方法接受一个片段，而不是重复事务代码的if-else情况

你曾经一步一步地调试过你的代码吗？这应该马上给你答案。你到底是什么意思？哦，是的。。。但它告诉我的唯一一件事是片段类被正确初始化，并且onCreateView方法被调用。这没有多大帮助…实际上它确实帮助很大-因为您知道

片段

会附加到

视图

层次结构。因此，现在您需要弄清楚它为什么不可见，例如使用新的视图层次检查器工具查看您的视图层次，并查看

片段

的隐藏位置。是的，我刚刚测试了它，应用程序栏中的片段容器显示了，但没有显示其内容。if-else的东西也肯定不是原因。我是说，最好有一个片段切换器，这样就不会有多余的代码。

<FrameLayout 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"
    tools:context="de.package.test.MainFragment">

        <TextView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:text="@string/hello_blank_fragment" />

</FrameLayout>