[英]Livedata doesn't update data in fragment correctly
我在单独的选项卡中有tablayout
和 2 个fragments
。 Fragment A 有一个被覆盖的方法,当 Activity(从 Fragment A 开始)在其销毁时返回数据时返回数据:
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);
}
}
在同样的方法中,我调用 ViewModel 来观察 TransactionNewItem object:
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 返回数据后,使用新值创建一个新 POJO,并将存储在此 POJO 中的数据发送到 Fragment B,其中基于来自 Fragment 的数据将创建 RecyclerView 的新项目
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,它只会将 1 个项目添加到 RecyclerView 中,并在 Activity 返回新数据时替换它。 如果用户没有至少一次切换到 Fragment B,就会发生这种情况,因为在用户切换到 Fragment B 之前不会调用 onViewCreated。
如果用户之前从未切换到 Fragment B,如何让 ViewModel 观察来自 Fragment A 的数据,并在每次 Activity 返回新数据时在 Fragment B Recyclerview 中创建新的 TransActionItem?
提前致谢
编辑:我设法以另一种方式做我想做的事:
步骤 1. 我使用 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 Fragment AI 中,在 onActivityResult 中添加了具有相同 POJO 类型的 ArrayList。 我现在更改了代码 object 将在 Activity 返回结果后创建并添加,而不是在片段 B 中:
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);
}
}
必须注意我在 TransactionItem 构造函数中添加了 transactionID,这就是我们需要它的原因。
第 3 步我创建了下一个扩展 DiffUtil.Callback 的 TransactionDiffUtilCallback class:
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 的getItemIID()
来通知 ArrayList 中的新项目不同。
第 4 步在 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);
}
So this method uses DiffUtil.CallBack to compare items in ArrayList from Fragment A and ArrayList in Fragment B, then notify adapter that ArrayList from Fragment A is different, and this data should be put in ArrayList in Fragment B, and view should be updated.
步骤 5 在 Fragment B 中的 OnViewCreated() 代码被重写以永远观察 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);
}
而 initObserve() 现在有下一个代码:
private void initObserve(ArrayList<TransactionItem> list){
adapter.updateItemList(list);
}
目前,这个解决方案正在运行,用户不需要切换到 Fragment B 来保持交易记录。 我将继续测试此解决方案。
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