tabLayout 和 pager2 創建
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：如果頁面滑動，則RecyclerView在呼叫fillFragement()時正常填充如果先點擊tab，則fillFragment()中的RecyclerView會因為java.lang.NullPointerException導致程式崩潰 ：
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()定位 RecyclerView 并根據選擇的頁面填充它。每個頁面都有自己的 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;
}
}
}
總之，在呼叫 fillFragment() 時，單擊選項卡會導致空物件參考。滑動頁面時它作業正常。這是因為單擊選項卡時，會在創建 Pager2 項之前首先呼叫 fillFragment()。滑動時，首先創建 Pager2 項，然后呼叫 fillFragment() 這意味著在訪問之前首先創建 RecyclerView。
How can I fill the RecyclerView when clicking on a tab if the pageView is always created after I fill it causing a Null Object reference?
StackTrace
uj5u.com熱心網友回復：
您可以通過對當前解決方案進行小的更改來解決此問題。但我建議你重新考慮什么負責什么。
因為片段遵循它們自己的生命周期，所以您不能保證在任何給定時間都附加了片段。
因此，當您從可能“存在”或可能不“存在”的片段中提取 RecyclerViews 時，您將獲得空值。
Fragment 應該對它們自己的觀點完全負責，其他物體不應觸及它們。
最好的解決方案是擁有一個所有 4 個片段都可以訪問的 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;
}
}
}
轉載請註明出處，本文鏈接：https://www.uj5u.com/caozuo/345958.html
標籤：android android-studio android-layout android-fragments android-viewpager2
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