Android - 未创建选项卡项

Question

我正在学习如何进行选项卡布局的教程。我不知道我在下面的代码中做了什么，它不输出标签。当应用程序运行时，我看不到选项卡项。其他一切正常。

Activity

public class dashboard extends AppCompatActivity {

    TabLayout tabs;
    ViewPager container;

    private SectionsPagerAdapter mSectionsPagerAdapter;

    /**
     * The {@link ViewPager} that will host the section contents.
     */
    private ViewPager mViewPager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_dashboard);

        // Create the adapter that will return a fragment for each of the three
        // primary sections of the activity.
        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

        // Set up the ViewPager with the sections adapter.
        mViewPager = (ViewPager) findViewById(R.id.container);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        tabs = (TabLayout) findViewById(R.id.tabs);
        tabs.setupWithViewPager(container);
        tabs.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener(){
            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                container.setCurrentItem(tab.getPosition());
            }
            @Override
            public void onTabUnselected(TabLayout.Tab tab) {
                container.setCurrentItem(tab.getPosition());
            }
            @Override
            public void onTabReselected(TabLayout.Tab tab) {
                container.setCurrentItem(tab.getPosition());
            }
        });
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_dashboard, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {
        /**
         * The fragment argument representing the section number for this
         * fragment.
         */
        private static final String ARG_SECTION_NUMBER = "section_number";

        public PlaceholderFragment() {
        }

        /**
         * Returns a new instance of this fragment for the given section
         * number.
         */
        public static PlaceholderFragment newInstance(int sectionNumber) {
            PlaceholderFragment fragment = new PlaceholderFragment();
            Bundle args = new Bundle();
            args.putInt(ARG_SECTION_NUMBER, sectionNumber);
            fragment.setArguments(args);
            return fragment;
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_dashboard, container, false);
            TextView textView = (TextView) rootView.findViewById(R.id.section_label);
            textView.setText(getString(R.string.section_format, getArguments().getInt(ARG_SECTION_NUMBER)));
            return rootView;
        }
    }

    /**
     * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
     * one of the sections/tabs/pages.
     */
    public class SectionsPagerAdapter extends FragmentPagerAdapter {
        private String fragments [] = {"Completed", "In Progress"};
        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            switch(position){
                case 0:
                    return new dashboard_completed();
                case 1:
                    return new dashboard_in_progress();
                default:
                    return null;
            }
        }

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

        @Override
        public CharSequence getPageTitle(int position) {
            return fragments[position];
        }
    }
}

dashboard_in_progress

public class dashboard_in_progress extends Fragment {
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.dashboard_in_progress, container, false);
    }
}

dashboard_completed

public class dashboard_completed extends Fragment {
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.dashboard_completed, container, false);
    }
}

XML

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

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

    <view
        android:layout_height="match_parent"
        class="android.support.v4.view.ViewPager"
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_marginTop="48dp"/>
</android.support.design.widget.CoordinatorLayout>

Answer 1

这是你的问题。

ViewPager container;
...

private ViewPager mViewPager;
...

tabs.setupWithViewPager(container);

您声明了两个 ViewPager，但您只初始化了一个，并尝试使用空值设置 TabLayout。

删除 ViewPager container; 声明，并将设置调用更改为:

tabs.setupWithViewPager(mViewPager);

此外，您不需要自己在 TabLayout 上设置 OnTabSelectedListener。 setupWithViewPager() 方法将为您处理。