在webview片段中启用后退按钮

Question

我尝试将WebView Fragment设置为启用后退按钮进入我的高中应用程序。但是，由于WebView位于Fragment内，因此插入片段活动的效果不如MainActivity中那样：

@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
    if ((keyCode == KeyEvent.KEYCODE_BACK) && mWeb.canGoBack()) {
        mWeb.goBack();
        return true;
    }
    return super.onKeyDown(keyCode, event);
}

它说cannot resolve method 'onKeyDown'并且我也无法使用@override，然后它说该方法不会覆盖其超类的方法。 然后我尝试使用在这个网站上发布的每一个解决方案，但我不能让它工作，它崩溃或只是关闭整个应用程序。任何帮助，如果可能的话，使用一些示例代码，真的会受到赞赏，因为我还是Android开发的初学者。

这是我的MainActivity.java-Code（来自Android Studio的标签式活动模板代码）：

public class MainActivity extends AppCompatActivity {

        /**
         * The {@link android.support.v4.view.PagerAdapter} that will provide
         * fragments for each of the sections. We use a
         * {@link FragmentPagerAdapter} derivative, which will keep every
         * loaded fragment in memory. If this becomes too memory intensive, it
         * may be best to switch to a
         * {@link android.support.v4.app.FragmentStatePagerAdapter}.
     */
    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_main);

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        // 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);

        TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
        tabLayout.setupWithViewPager(mViewPager);

    }


    @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_main, 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 {@link FragmentPagerAdapter} that returns a fragment corresponding to
     * one of the sections/tabs/pages.
     */
    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            switch (position){
                case 0:
                    Tab1 tab1 = new Tab1();
                    return tab1;
                case 1:
                    Tab2 tab2 = new Tab2();
                    return  tab2;
                case 2:
                    Tab3 tab3 = new Tab3();
                    return  tab3;
            }
            return null;
        }

        @Override
        public int getCount() {
            // Show 3 total pages.
            return 3;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            switch (position) {
                case 0:
                    return "SECTION 1";
                case 1:
                    return "SECTION 2";
                case 2:
                    return "SECTION 3";
            }
            return null;
        }
    }

    /**
     * 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";

        /**
         * 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;
        }

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, 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;
        }
    }
}

这是Tab1.java代码（片段活动）：

public class Tab1 extends Fragment {
    private static WebView WebView;

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

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

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.tab1, container, false);

        //load WebView
        WebView WebView = (WebView) view.findViewById(R.id.webview_news);
        WebView.loadUrl("http://www.hp.rvwg-web.de/webviews/neuigkeiten/");

        //make it possible to run JavaScript
        WebSettings webSettings = WebView.getSettings();
        webSettings.setJavaScriptEnabled(true);

        //make opening links inside the webview possible
        WebView.setWebViewClient(new WebViewClient());

        //clear cache to prevent the webview from not getting the newest content
        WebView.clearCache(true);

        //Display WebView
        return view;

        @Override
        public boolean onKeyDown(int keyCode, KeyEvent event) {
            if ((keyCode == KeyEvent.KEYCODE_BACK) && mWeb.canGoBack()) {
                mWeb.goBack();
                return true;
            }
            return super.onKeyDown(keyCode, event);
        }
    }
}`