如何从 viewpager2 访问 RecyclerViews？

Question

已创建 tabLayout 和 pager2

public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    myFragment = inflater.inflate(R.layout.fragment_home, container, false);

    tabLayout = myFragment.findViewById(R.id.tab_layout);
    pager2 = myFragment.findViewById(R.id.view_pager2);
    Button calculate = myFragment.findViewById(R.id.calculate_button);

    FragmentManager fm = getFragmentManager();
    adapter = new FragmentAdapterRecycler(fm, getLifecycle());
    pager2.setAdapter(adapter);

位于onCreateView中：如果页面滑动，那么调用fillFragement()时RecyclerView会正常填充如果先点击tab，那么fillFragment()中的RecyclerView会因为java导致程序崩溃.lang.NullPointerException：

pager2.registerOnPageChangeCallback(new ViewPager2.OnPageChangeCallback() {
        @Override
        public void onPageSelected(int position) {

            calc = (TextView) myFragment.findViewById(R.id.numDisplay);
            fillFragment(pager2.getCurrentItem());
            tabLayout.selectTab(tabLayout.getTabAt(position));
        }
    });

    tabLayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
        @Override
        public void onTabSelected(TabLayout.Tab tab) {
            
            pager2.setCurrentItem(tab.getPosition());
        }

        @Override
        public void onTabUnselected(TabLayout.Tab tab) { }

        @Override
        public void onTabReselected(TabLayout.Tab tab) { }
    });

fillFragment() 这会定位 RecyclerView 并根据所选页面填充它。每个页面都有自己的 RecyclerView。

public void fillFragment(int position) {

    if (position == 0 && isChosen) {
        String iterations = "Total Iterations: " + collatz.getIterationTotal();
        calc.setText(iterations);
        if (tabLoaded1 == false) {
            mRecyclerView = (RecyclerView) getView().findViewById(R.id.recycler_view);
            mRecyclerView.setHasFixedSize(false);
            mLayoutManager = new LinearLayoutManager(getActivity());

            mAdapter = new CollatzAdapter(collatz.getCollatzList());
            mRecyclerView.setLayoutManager(mLayoutManager);
            mRecyclerView.setAdapter(mAdapter);
            tabLoaded1 = true;
        }
    }

    if (position == 1 && isChosen) {
        String evenTotal = "Total Even: " + collatz.getEvenTotal();
        calc.setText(evenTotal);

        if (tabLoaded2 == false) {

            mRecyclerView = (RecyclerView) myFragment.findViewById(R.id.recycler_view_even);
            mRecyclerView.setHasFixedSize(false);
            mLayoutManager = new LinearLayoutManager(getActivity());

            mAdapter = new CollatzAdapter(collatz.getEvenList());
            mRecyclerView.setLayoutManager(mLayoutManager);
            mRecyclerView.setAdapter(mAdapter);
            tabLoaded2 = true;
        }
    }

    if (position == 2 && isChosen) {
        String oddTotal = "Total Odd: " + collatz.getOddTotal();
        calc.setText(oddTotal);
        if (tabLoaded3 == false) {
            mRecyclerView = (RecyclerView) myFragment.findViewById(R.id.recycler_view_odd);
            mRecyclerView.setHasFixedSize(false);
            mLayoutManager = new LinearLayoutManager(getActivity());

            mAdapter = new CollatzAdapter(collatz.getOddList());
            mRecyclerView.setLayoutManager(mLayoutManager);
            mRecyclerView.setAdapter(mAdapter);
            tabLoaded3 = true;
        }
    }
}

总而言之，当调用 fillFragment() 时，单击选项卡会导致空对象引用。滑动页面时它工作正常。这是因为单击选项卡时，会在创建 Pager2 项之前首先调用 fillFragment()。滑动时先创建Pager2项，然后调用fillFragment()，即先创建RecyclerView再访问。

如果 pageView 总是在我填充它导致空对象引用后创建，我如何在单击选项卡时填充 RecyclerView？

StackTrace

Answer 1

您可以通过对当前解决方案稍作改动来解决此问题。但我建议你重新考虑一下什么是对什么负责。

由于片段遵循自己的生命周期，因此您无法保证在任何给定时间附加片段。

因此，当您将 RecyclerViews 从可能“存在”或可能不“存在”的片段中提取出来时，您将得到 null。

Fragments 应该对自己的观点负全部责任，任何其他实体都不应触及它们。

最好的解决方案是拥有一个所有 4 个片段都可以访问的 SharedViewModel。

    // in evenFragment
    CollatzViewModel collatzViewModel;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
         collatzViewModel = new ViewModelProviders(getActivity()).get(CollatzViewModel.class);
         evenFragment = inflater.inflate(R.layout.fragment_even, container, false);
         mRecyclerView = (RecyclerView) evenFragment .findViewById(R.id.recycler_view_even);
         mRecyclerView.setHasFixedSize(false);
         mLayoutManager = new LinearLayoutManager(getActivity());
         mAdapter = new CollatzAdapter();
         mRecyclerView.setLayoutManager(mLayoutManager);
         mRecyclerView.setAdapter(mAdapter);
         collatzViewModel.getCollatz().observe(getViewLifeCycle(),(list)->{
             mAdapter.setList(list);
             mAdapter.notifyDataSetChanged();
         });
         return evenFragment;
    } 
    //in odd fragment
    CollatzViewModel collatzViewModel;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
         collatzViewModel = new ViewModelProviders(getActivity()).get(CollatzViewModel.class);
         oddFragment = inflater.inflate(R.layout.fragment_odd, container, false);
         mRecyclerView = (RecyclerView) oddFragment.findViewById(R.id.recycler_view_odd);
         mRecyclerView.setHasFixedSize(false);
         mLayoutManager = new LinearLayoutManager(getActivity());
         mAdapter = new CollatzAdapter();
         mRecyclerView.setLayoutManager(mLayoutManager);
         mRecyclerView.setAdapter(mAdapter);
         collatzViewModel.getCollatz().observe(getViewLifeCycle(),(list)->{
             mAdapter.setList(list);
             mAdapter.notifyDataSetChanged();
         });
         return oddFragment;
    }
    //in iteration fragment
    CollatzViewModel collatzViewModel;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        iterationFragment = inflater.inflate(R.layout.fragment_iteration, container, false);
         collatzViewModel = new ViewModelProviders(getActivity()).get(CollatzViewModel.class);
        mRecyclerView = (RecyclerView) iterationFragment.findViewById(R.id.recycler_view);
        mRecyclerView.setHasFixedSize(false);
        mLayoutManager = new LinearLayoutManager(getActivity());
        mAdapter = new CollatzAdapter();
        mRecyclerView.setLayoutManager(mLayoutManager);
        mRecyclerView.setAdapter(mAdapter);
        collatzViewModel.getCollatz().observe(getViewLifeCycle(),(list)->{
             mAdapter.setList(list);
             mAdapter.notifyDataSetChanged();
         });
        return iterationFragment;
    }
    //ViewModel class
    public class CollatzViewModel extends ViewModel{
        private MutableLiveData<List<BigInteger>> collatz = new MutableLiveData();
        public MutableLiveData<List<BigInteger>> getCollatz(){
             return collatz;
        }
    }
    //home fragment
    CollatzViewModel collatzViewModel;

    public void fillFragment(int position) {

        if (position == 0 && isChosen) {
            String iterations = "Total Iterations: " + collatz.getIterationTotal();
            calc.setText(iterations);
            if (tabLoaded1 == false) {
                collatzViewModel.getCollatz().setValue(collatz.getCollatzList())
                tabLoaded1 = true;
            }
        }
        if (position == 1 && isChosen) {
            String evenTotal = "Total Even: " + collatz.getEvenTotal();
            calc.setText(evenTotal);
            if (tabLoaded2 == false) {
                collatzViewModel.getCollatz().setValue(collatz.getEvenList())
                tabLoaded2 = true;
            }
        }
        if (position == 2 && isChosen) {
            String oddTotal = "Total Odd: " + collatz.getOddTotal();
            calc.setText(oddTotal);
            if (tabLoaded3 == false) {
                collatzViewModel.getCollatz().setValue(collatz.getOddList())
                tabLoaded3 = true;
            }
        }
    }