在片段中的后退按钮而不是汉堡包

Question

我已实现此功能，使后退按钮出现并在我想要的某些时间消失。这有效，但按下它不会做任何事情。当我按下它时，我也无法找到被调用的函数，有人知道吗？

这是我的onCreate：

public class Homescreen extends AppCompatActivity
        implements NavigationView.OnNavigationItemSelectedListener {

    ActionBarDrawerToggle toggle;

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


        setContentView(R.layout.activity_homescreen);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setDisplayShowTitleEnabled(false);


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

            @Override
            public void onDrawerClosed(View view) {
                super.onDrawerClosed(view);
            }

            @Override
            public void onDrawerOpened(View drawerView) {
                super.onDrawerOpened(drawerView);
                super.onDrawerSlide(drawerView, 0);
            }

            @Override
            public void onDrawerSlide(View drawerView, float slideOffset) {
                super.onDrawerSlide(drawerView, 0); // this disables the animation
            }
        };


        drawer.setDrawerListener(toggle);
        toggle.syncState();
        drawer.setScrimColor(Color.TRANSPARENT);

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



        if (savedInstanceState != null) {
            return;
        }

        getSupportFragmentManager().beginTransaction()
                .add(R.id.FragmentSpace, new GeneralFragment())
                .commit();

    }
 public void BackNav(boolean y) {
        if(y) {toggle.setDrawerIndicatorEnabled(false); getSupportActionBar().setDisplayHomeAsUpEnabled(true);} else {getSupportActionBar().setDisplayHomeAsUpEnabled(false);toggle.setDrawerIndicatorEnabled(true);}
    }}

这也是我的工具栏：

 <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">



        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay">

            <TextView
                android:id="@+id/actionbar_title"
                android:layout_width="wrap_content"
                android:layout_height="match_parent"
                android:layout_gravity="top|center_horizontal"
                android:layout_weight="1"
                android:gravity="center"
                android:text="my Title"
                android:textColor="@color/barTextColor"
                android:textSize="18sp"
                app:layout_anchor="@+id/include"
                app:layout_anchorGravity="top|center_horizontal" />

        </android.support.v7.widget.Toolbar>

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