在页面切换一次之前，不会出现对片段子视图的更改

Question

我已经在Android Studio中开发了一个基本的Android应用程序，其中包含一个包含两个片段/页面的ViewPager。第一页包含多个元素，其中只有一个按钮和一个ProgressBar与该问题有关。

当应用从零开始（没有待机状态）时，代码确实到达了隐藏ProgressBar的地步，但并没有走得更远-代码此时就中断了。我可以按需要多次单击“ Heute”按钮或其他按钮-ProgressBar将出现并消失，但是隐藏它之后的代码未执行，除非我一次切换到第二页并切换回主窗口页。 奇怪的是，一切正常之后。

注意：当我注释掉隐藏ProgressBar的部分时，应用程序卡在CoverPlanAdapter中的notifyDataSetChanged（）处。

我感谢所有可能的解决方案！ 问候

主要活动（ViewPager的初始化）：

private SectionsPagerAdapter sectionsPagerAdapter;
private CoverPlanFragment coverPlanFragment;
private SettingsFragment settingsFragment;

private ViewPager viewPager;

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

    CoverPlanFetcher.setContext(getApplicationContext());

    // Create the adapter that will return a fragment for each of the sections
    sectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

    // Set up the ViewPager with the sections adapter.
    viewPager = findViewById(R.id.pager);
    viewPager.setAdapter(sectionsPagerAdapter);
    ViewPager.OnPageChangeListener pageChangeListener = new ViewPager.SimpleOnPageChangeListener() {
        @Override
        public void onPageSelected(int position) {
            if (position == 0) {
                coverPlanFragment.refreshCurrent();
            }
        }
    };
    viewPager.setOnPageChangeListener(pageChangeListener);
    viewPager.post(() -> pageChangeListener.onPageSelected(0));

    // Add Tabs for easier identifying
    TabLayout tabLayout = findViewById(R.id.sliding_tabs);
    tabLayout.setupWithViewPager(viewPager);
}

public class SectionsPagerAdapter extends FragmentPagerAdapter {

    private final String[] tabTitles = {"Vertretungsplan", "Einstellungen"};

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

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        Object fragment = super.instantiateItem(container, position);
        if(position == 0) {
            coverPlanFragment = (CoverPlanFragment) fragment;
        } else if(position == 1) {
            settingsFragment = (SettingsFragment) fragment;
        }
        return fragment;
    }

    @Override
    public Fragment getItem(int position) {
        if (position == 0) {
            if (coverPlanFragment == null) {
                coverPlanFragment = new CoverPlanFragment();
            }
            return coverPlanFragment;
        } else if (position == 1) {
            if (settingsFragment == null) {
                settingsFragment = new SettingsFragment();
            }
            return settingsFragment;
        }
        return null;
    }

    @Override
    public int getCount() {
        return 2;
    }

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

}

第一个片段：

private ExpandedListView listVertretung;
private ProgressBar loading;

public CoverPlanFragment() {
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_cover_plan, container, false);

    listVertretung = view.findViewById(R.id.list_vertretung);
    listVertretung.setAdapter(new CoverPlanAdapter(getContext(), new ArrayList<>()));
    loading = view.findViewById(R.id.loading);

    view.findViewById(R.id.today).setOnClickListener(v -> today());

    return view;
}

public void today() {
    loading.setVisibility(View.VISIBLE);
    CoverPlanFetcher.getInstance().getTodayCoverPlan(plan -> {
        update(plan);
    });
}

public void refreshCurrent() {
    loading.setVisibility(View.VISIBLE);
    CoverPlanFetcher.getInstance().getCoverPlan(null, plan -> {
        update(plan);
    });
}

private void update(CoverPlanFetcher.CoverPlan plan) {
    // code goes until here
    loading.setVisibility(View.INVISIBLE);
    // ProgressBar is hidden, but following function is never executed
    ((CoverPlanAdapter) listVertretung.getAdapter()).update(plan.getVertretung());
}

第二个片段：

public SettingsFragment() {
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_settings, container, false);
    return view;
}

CoverPlanAdapter（ListView的适配器）：

private Context context;
private List<CoverPlanFetcher.CoverPlanRow> rows;

public CoverPlanAdapter(Context context, List<CoverPlanFetcher.CoverPlanRow> rows) {
    this.context = context;
    this.rows = rows;
}

@Override
public int getCount() {
    return rows.size();
}

@Override
public CoverPlanFetcher.CoverPlanRow getItem(int position) {
    return rows.get(position);
}

@Override
public long getItemId(int position) {
    return position;
}

@Override
public View getView(int position, View view, ViewGroup parent) {
    CoverPlanRowViewHolder holder;

    if (view == null) {
        LayoutInflater layoutInflater = (LayoutInflater) context.getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
        view = layoutInflater.inflate(R.layout.cover_plan_row, null);
        holder = new CoverPlanRowViewHolder(view);
        view.setTag(holder);
    } else {
        holder = (CoverPlanRowViewHolder) view.getTag();
    }

    if (position % 2 == 0) {
        view.setBackgroundColor(Color.parseColor("#e6e6ff"));
    } else {
        view.setBackgroundColor(Color.parseColor("#ffffff"));
    }

    CoverPlanFetcher.CoverPlanRow row = getItem(position);
    holder.first.setText(row.getHour() + "\n" + row.getSubject() + "\n" + row.getGrade());
    holder.second.setText(row.getNotice());
    holder.third.setText(row.getTeacher() + "\n\n" + row.getRoom());

    return view;
}

public void update(List<CoverPlanFetcher.CoverPlanRow> rows) {
    this.rows.clear();
    this.rows.addAll(rows);
    // when hiding the ProgressBar is commented out, this doesnt even do anything -> the ListView shows no content
    notifyDataSetChanged();
}

public class CoverPlanRowViewHolder {
    private TextView first;
    private TextView second;
    private TextView third;

    public CoverPlanRowViewHolder(View view) {
        first = view.findViewById(R.id.row_first);
        second = view.findViewById(R.id.row_second);
        third = view.findViewById(R.id.row_third);
    }
}