android - Livedata无法正确更新 fragment 中的数据

Question

我在单独的选项卡中有tablayout和2 fragments。
片段A有一个重写的方法，当Activity(从片段A开始)销毁数据时，该方法将返回数据:

public class Fragment A extends Fragment {
    ...
    @Override
    public void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        if(resultCode != RESULT_CANCELED) {
            assert data != null;
            String accountTransaction = data.getStringExtra("Account");
            String categoryTransaction = data.getStringExtra("Category");
            Double getDouble = data.getDoubleExtra("Value", 0);
            
            TransactionNewItem item = new TransactionNewItem(String.valueOf(getDouble),accountTransaction,categoryTransaction);
            model.setSelected(item);
  
        }
           super.onActivityResult(requestCode, resultCode, data);
    }
    }

在相同的方法中，我使用了对ViewModel的调用，该调用应遵守TransactionNewItem对象:

public class TransactionViewModel extends ViewModel {
    private final MutableLiveData<TransactionNewItem> selected = new MutableLiveData<>();
    public void setSelected (TransactionNewItem item){
        selected.setValue(item);
    }

    public LiveData<TransactionNewItem> getSelected() {
        return selected;
    }
}

从Activity返回具有新值的数据后，它将创建一个新的POJO并将此POJO中存储的数据发送到Fragment B，在Fragment B的基础上，将为RecyclerView创建一个新项

public class Fragment B extends Fragment {
    ...
    @Override
        public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
            initObserve();
            initRecView();
           super.onViewCreated(view, savedInstanceState);
        }
        //init RecyclerView
        private void initRecView(){
            binding.transactionView.setLayoutManager(new LinearLayoutManager(requireContext()));
            adapter = new TransactionRecViewAdapter(listContentArr);
            adapter.setListContent(listContentArr);
            binding.transactionView.setAdapter(adapter);
        }
        
        //observe data from Fragment A and create object based on it
        private void initObserve(){
            model = new ViewModelProvider(requireActivity()).get(TransactionViewModel.class);
            model.getSelected().observe(getViewLifecycleOwner(), item -> {
                TransactionItem newAccountItem = new TransactionItem() ;
                newAccountItem.setTransactionValue(item.getTransactionValue());
                newAccountItem.setTransactionCategory(item.getTransactionCategory());
                newAccountItem.setTransactionAccount(item.getTransactionAccount());
                listContentArr.add(0,newAccountItem);
                adapter.notifyDataSetChanged();
            });
            }
}

但是，它将仅向RecyclerView中添加一项，并在Activity返回新数据时将其替换为。如果用户至少一次没有切换到片段B，则会发生这种情况，因为在用户切换到片段B之前，不会调用onViewCreated。
如果用户以前从未切换到片段B，那么如何使ViewModel观察片段A的数据，并在Activity每次返回新数据时在片段B Recyclerview中创建新的TransActionItem？
提前致谢

Answer 1

编辑:我设法做我想要的下一步:
步骤1.使用POJO-ArrayList将ViewModel从POJO更改为Arraylist:

public class TransactionViewModel extends ViewModel {

        private final MutableLiveData<ArrayList<TransactionItem>> selected = new MutableLiveData<>();
  
        public void setSelected (ArrayList<TransactionItem> arrayList){
            selected.setValue(arrayList);
        }
    
        public LiveData<ArrayList<TransactionItem>> getSelected() {
            return selected;
        }
    }

步骤2。在片段A中，我在onActivityResult中添加了具有相同POJO类型的ArrayList。我更改了代码，现在将在Activity返回结果(而不是在片段B中)之后创建并添加对象:

public class Fragment A extends Fragment {
    ArrayList<TransactionItem> listTransactions = new ArrayList<>();
    …
@Override
    public void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        if(resultCode != RESULT_CANCELED) {
            ... 
        //Create TransactionItem and use setSelected method from ViewModel
            TransactionItem item = new TransactionItem(accountTransaction,
                    String.valueOf(getDouble),categoryAccount),transactionID);
            listTransactions.add(0,item);
            model.setSelected(listTransactions);            
        }
           super.onActivityResult(requestCode, resultCode, data);
    }
    }

必须注意，我已将transactionID添加到TransactionItem构造函数中，这就是我们需要它的原因。
步骤3我创建了下一个TransactionDiffUtilCallback类，该类扩展了DiffUtil.Callback:

public class TransactionDiffUtilCallback extends `DiffUtil.Callback` {
   
     public TransactionDiffUtilCallback(ArrayList<TransactionItem> oldList, ArrayList<TransactionItem> newList) {
            this.oldList = oldList;
            this.newList = newList;
        }
    
        ArrayList<TransactionItem> newList;
    
        @Override
        public int getOldListSize() {
            return oldList.size();
        }
    
        @Override
        public int getNewListSize() {
            return newList.size();
        }
    
        @Override
        public boolean areItemsTheSame(int oldItemPosition, int newItemPosition) {
            return oldList.get(oldItemPosition).getItemIID() == newList.get(newItemPosition).getItemIID();
        }
    
        @Override
        public boolean areContentsTheSame(int oldItemPosition, int newItemPosition) {
            return oldList.get(oldItemPosition).equals(newList.get(newItemPosition));
        }
    
        @Nullable
        @Override
        public Object getChangePayload(int oldItemPosition, int newItemPosition) {
           
            return super.getChangePayload(oldItemPosition, newItemPosition);
        }
    }

我使用POJO中的getItemIID()通知ArrayList中的新项目不同。
步骤4在recyclerview适配器中，我创建了updateItemList(list):

public void updateItemList(ArrayList<TransactionItem> items){
    final TransactionDiffUtilCallback diffCallback = new TransactionDiffUtilCallback(this.pad_list, items);
    final DiffUtil.DiffResult diffResult = DiffUtil.calculateDiff(diffCallback);

    this.pad_list.clear();
    this.pad_list.addAll(items);
    diffResult.dispatchUpdatesTo(this);
}

因此，此方法使用DiffUtil.CallBack比较Fragment A中的ArrayList和Fragment B中的ArrayList中的项目，然后通知适配器Fragment A中的ArrayList不同，并且此数据应放在Fragment B中的ArrayList中，并应更新 View 。
步骤5在片段B中，重写了OnViewCreated()代码以永远观察Arraylist:

 @Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    initRecView();
    model = new ViewModelProvider(requireActivity()).get(TransactionViewModel.class);
    Observer<ArrayList<TransactionItem>> observer = (Observer<ArrayList<TransactionItem>>) this::initObserve;
    model.getSelected().observeForever(observer);
    super.onViewCreated(view, savedInstanceState);
}

并且initObserve()现在具有下一个代码:

private void initObserve(ArrayList<TransactionItem> list){
    adapter.updateItemList(list);
}

目前，此解决方案正在运行，用户无需切换到片段B即可保留交易记录。我将继续测试该解决方案。