导航抽屉backstack，如何让actionbar标题随点击后的片段而改变

Question

因此，我设法让一个股票导航抽屉工作，并使动作栏标题随着所选片段的变化而改变。我还设法让backstack很容易地工作。

我不知道该怎么做才能让动作栏的标题在后退的时候变回来。谷歌文档说要添加一个onBackStackChangedListener：

getSupportFragmentManager().addOnBackStackChangedListener(
        new FragmentManager.OnBackStackChangedListener() {
            public void onBackStackChanged() {
                // Update your UI here.
            }
        });

但是我不知道该把它放在哪里？他们说when I .commit to changes，所以我认为它被放在

if (id == R.id.nav_spatial_awareness) {
            setTitle("Spatial Awareness");
            SpatialAwareness spatialAwarenessFragment = new SpatialAwareness();
            android.support.v4.app.FragmentManager spatialAwarenessManager = getSupportFragmentManager();
            spatialAwarenessManager.beginTransaction()
                    .addToBackStack(null)
                    .replace(R.id.main_content_layout, spatialAwarenessFragment, spatialAwarenessFragment.getTag())
                    .commit();

但这并不管用，这就是我所尝试的，我得到的都是红色的曲折

if (id == R.id.nav_spatial_awareness) {
            setTitle("Spatial Awareness");
            SpatialAwareness spatialAwarenessFragment = new SpatialAwareness();
            android.support.v4.app.FragmentManager spatialAwarenessManager = getSupportFragmentManager();
            spatialAwarenessManager.beginTransaction()
                    .addToBackStack(null)
                    .replace(R.id.main_content_layout, spatialAwarenessFragment, spatialAwarenessFragment.getTag())
                    .commit();

            getSupportFragmentManager().addOnBackStackChangedListener(
                    new FragmentManager.OnBackStackChangedListener() {
                        public void onBackStackChanged() {
                            setTitle("Spatial Awareness");
                        }
                    });

请帮帮我菜鸟

所以我试了一下

if (id == R.id.nav_spatial_awareness) {
            setTitle("Spatial Awareness");
            final SpatialAwareness spatialAwarenessFragment = new SpatialAwareness();
            android.support.v4.app.FragmentManager spatialAwarenessManager = getSupportFragmentManager();
            spatialAwarenessManager.beginTransaction()
                    .addToBackStack(null)
                    .replace(R.id.main_content_layout, spatialAwarenessFragment, "spatialAwarenessFragmentTag")
                    .commit();

            getSupportFragmentManager().addOnBackStackChangedListener(
                    new FragmentManager.OnBackStackChangedListener() {
                        public void onBackStackChanged() {
                            android.app.Fragment currentBackStackFragment = getFragmentManager().findFragmentByTag("spatialAwarenessFragmentTag");
                            if(currentBackStackFragment instanceof SpatialAwareness){
                                setTitle("Spatial");
                            }
                        }
                    });

我给了我的片段一个标签，然后尝试匹配实例，然后更改标题，仍然不行:(

Answer 1

是的，前面提到的here解决方案是将FragmentManager.OnBackStackChangedListener添加到您的活动的FragmentManager中。

以下是我参与的一个项目的示例：

(我有一个包含7个片段的导航抽屉，当MainActivity打开时，OverviewFragment是最初打开的那个)

我的MainActivity：

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener, FragmentManager.OnBackStackChangedListener {

    ...

    @Override
    protected void onCreate(Bundle savedInstanceState) {

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

        if (savedInstanceState == null) {
            // open the default fragment
            OverviewFragment fragment = new OverviewFragment();
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.fragment_container, fragment).commit();
            setTitle(R.string.title_fragment_overview);
        }

        getSupportFragmentManager().addOnBackStackChangedListener(this);
    }

    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.

        Fragment fragment = null;
        switch (item.getItemId()) {
            case R.id.nav_overview:
                fragment = new OverviewFragment();
                break;
            case R.id.nav_schedule:
                fragment = new ScheduleFragment();
                break;
            case R.id.nav_all_tasks:
                fragment = new AllTasksFragment();
                break;
            case R.id.nav_announcements:
                fragment = new AnnouncementFragment();
                    break;
            case R.id.nav_my_courses:
                fragment = new MyCoursesFragment();
                break;
            case R.id.nav_map:
                fragment = new MapFragment();
                break;
            case R.id.nav_settings:
                fragment = new SettingsFragment();
                break;
            default:
                fragment = new OverviewFragment();
        }

        getSupportFragmentManager().beginTransaction()
                .replace(R.id.fragment_container, fragment)
                .addToBackStack(null)
                .commit();

        drawer.closeDrawer(GravityCompat.START);
        return true;
    }

    @Override
    public void onBackStackChanged() {
        FragmentManager fragmentManager = getSupportFragmentManager();
        Fragment currentFragment = 
        fragmentManager.findFragmentById(R.id.fragment_container);
        if (currentFragment instanceof  OverviewFragment) {
            setTitle(R.string.title_fragment_overview);
            navigationView.getMenu().findItem(R.id.nav_overview).setChecked(true);
        }
        else if (currentFragment instanceof ScheduleFragment) {
            setTitle(R.string.title_fragment_schedule);
            navigationView.getMenu().findItem(R.id.nav_schedule).setChecked(true);
        }
        else if (currentFragment instanceof AllTasksFragment) {
            setTitle(R.string.title_fragment_all_tasks);
            navigationView.getMenu().findItem(R.id.nav_all_tasks).setChecked(true);
        }
        else if (currentFragment instanceof MyCoursesFragment) {
            setTitle(R.string.title_fragment_my_courses);
            navigationView.getMenu().findItem(R.id.nav_my_courses).setChecked(true);
        }
        else if (currentFragment instanceof AnnouncementFragment) {
            setTitle(R.string.title_fragment_announcements);
            navigationView.getMenu().findItem(R.id.nav_announcements).setChecked(true);
        }
        else if (currentFragment instanceof MapFragment) {
            setTitle(R.string.title_fragment_map);
            navigationView.getMenu().findItem(R.id.nav_map).setChecked(true);
        }
        else if (currentFragment instanceof SettingsFragment) {
            setTitle(R.string.title_fragment_settings);
            navigationView.getMenu().findItem(R.id.nav_settings).setChecked(true);
        }
    }
}

下面是整个项目：https://github.com/FCI-E-campus/fci-e-campus-android