如何解决首次访问mvvm时从mvvm中获取的数据不会更新
PreAdmissionList.java
public class PreAdmissionList extends Fragment implements View.OnClickListener,AdapterApprovalList.OnItemClickListener {
private BasicInfoViewModel basicInfoViewModel;
private AdapterApprovalList adapterApprovalList;
private RecyclerView rvApprovalList;
public PreAdmissionList() {
// Required empty public constructor
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
@Override
public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState) {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.fragment_pre_admission_list,container,false);
}
@Override
public void onViewCreated(@NonNull View view,@Nullable Bundle savedInstanceState) {
super.onViewCreated(view,savedInstanceState);
view.findViewById(R.id.fab_add_new_admission).setOnClickListener(this);
rvApprovalList = view.findViewById(R.id.rv_approval_list);
basicInfoViewModel = new ViewModelProvider(requireActivity()).get(BasicInfoViewModel.class);
basicInfoViewModel.init();
basicInfoViewModel.getApprovalList().observe(getViewLifecycleOwner(),new Observer<List<ModelBasicInfo>>() {
@Override
public void onChanged(List<ModelBasicInfo> modelBasicInfos) {
adapterApprovalList.notifyDataSetChanged();
}
});
initRecyclerView();
}
private void initRecyclerView() {
adapterApprovalList = new AdapterApprovalList(this,basicInfoViewModel.getApprovalList().getValue());
rvApprovalList.setHasFixedSize(true);
rvApprovalList.setLayoutManager(new LinearLayoutManager(getContext()));
rvApprovalList.setAdapter(adapterApprovalList);
}
}
AdapterApprovalList.java
public class AdapterApprovalList extends RecyclerView.Adapter<AdapterApprovalList.ALViewHolder>{
private Context context;
private OnItemClickListener onItemClickListener;
private List<ModelBasicInfo> modelBasicInfoList;
public AdapterApprovalList(OnItemClickListener onItemClickListener,List<ModelBasicInfo> modelBasicInfoList) {
this.onItemClickListener = onItemClickListener;
this.modelBasicInfoList=modelBasicInfoList;
}
@NonNull
@Override
public ALViewHolder onCreateViewHolder(@NonNull ViewGroup parent,int viewType) {
context = parent.getContext();
View view = LayoutInflater.from(context).inflate(R.layout.template_approval_list_item,parent,false);
return new ALViewHolder(view,onItemClickListener);
}
@Override
public void onBindViewHolder(@NonNull ALViewHolder holder,int position) {
ModelBasicInfo basicInfo = modelBasicInfoList.get(position);
StringBuilder fullName = new StringBuilder();
fullName.append(basicInfo.getFirstName()).append(" ");
fullName.append(basicInfo.getMiddleName()).append(" ");
fullName.append(basicInfo.getLastName()).append(" ");
holder.fullName.setText(fullName);
holder.id.setText("RKC00"+String.valueOf(basicInfo.getId()));
}
@Override
public int getItemCount() {
return modelBasicInfoList.size();
}
static class ALViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
OnItemClickListener mOnItemClickListener;
TextView fullName,id;
public ALViewHolder(@NonNull View itemView,OnItemClickListener mOnItemClickListener) {
super(itemView);
this.mOnItemClickListener = mOnItemClickListener;
fullName = itemView.findViewById(R.id.tv_text_full_name);
id = itemView.findViewById(R.id.tv_text_approval_id);
itemView.setOnClickListener(this);
}
@Override
public void onClick(View v) {
mOnItemClickListener.onApprovalItemClick(getAbsoluteAdapterPosition());
}
}
public interface OnItemClickListener{
void onApprovalItemClick(int position);
}
}
BasicInfoViewModel.java
public class BasicInfoViewModel extends ViewModel {
private BasicInfoRepo basicInfoRepo;
private MutableLiveData<List<ModelBasicInfo>> approvalList;
public void init(){
if(approvalList != null){
return;
}
basicInfoRepo = BasicInfoRepo.getInstance();
approvalList = basicInfoRepo.getApprovalList();
}
public LiveData<List<ModelBasicInfo>> getApprovalList(){
return approvalList;
}
public void insertBasicInfo(ModelBasicInfo modelBasicInfo){
basicInfoRepo.insertData(modelBasicInfo);
}
public void updateApprovalStatus(int id){
basicInfoRepo.updateStatus(id);
}
}
BasicInfoRepo.java
public class BasicInfoRepo {
private static BasicInfoRepo instance;
static ConnectionClass connectionClass = new ConnectionClass();
private List<ModelBasicInfo> approvalList = new ArrayList<>();
public static BasicInfoRepo getInstance(){
if(instance== null){
instance = new BasicInfoRepo();
}
return instance;
}
public MutableLiveData<List<ModelBasicInfo>> getApprovalList(){
loadApprovalList();
MutableLiveData<List<ModelBasicInfo>> mList = new MutableLiveData<>();
mList.setValue(approvalList);
return mList;
}
private void loadApprovalList() {
LoadApprovalList loadApprovalList = new LoadApprovalList();
loadApprovalList.execute();
}
public void insertData(ModelBasicInfo modelBasicInfo){
InsertBasicInfo insertBasicInfo = new InsertBasicInfo();
insertBasicInfo.execute(modelBasicInfo);
}
public void updateStatus(int id){
UpdateBasicInfo updateBasicInfo = new UpdateBasicInfo();
updateBasicInfo.execute(id);
}
private static class InsertBasicInfo extends AsyncTask<ModelBasicInfo,Integer,String>{
@Override
protected void onPreExecute() {
super.onPreExecute();
}
@Override
protected String doInBackground(ModelBasicInfo... model) {
String result = null;
// Log.i("Testing db",lists[0].get(0).getFirstName());
try{
Connection connection = connectionClass.CONN();
if(connection==null){
result = "Error in connection !!!";
}else{
//Date object
Date date= new Date();
//getTime() returns current time in milliseconds
long time = date.getTime();
//Passed the milliseconds to constructor of Timestamp class
Timestamp ts = new Timestamp(time);
PreparedStatement ps = connection.prepareStatement("insert into PreAdmissionDetails values(?,?,?)");
ps.setString(1,model[0].getFirstName());
ps.setString(2,model[0].getMiddleName());
ps.setString(3,model[0].getLastName());
ps.setString(4,model[0].getMotherName());
ps.setDate(5,java.sql.Date.valueOf(model[0].getDateOfBirth()));
ps.setString(6,model[0].getMobileNo());
ps.setInt(7,0);
ps.setInt(8,0);
ps.setBoolean(9,false);
ps.setTimestamp(10,ts);
ps.executeUpdate();
result = "Submitted Successfully !!!";
}
}catch (Exception ex){
Log.e("sqlerror",ex.toString());
result=ex.getMessage();
}
Log.e("sqlerror","result : "+result);
return result;
}
@Override
protected void onPostExecute(String s) {
super.onPostExecute(s);
}
}
private static class UpdateBasicInfo extends AsyncTask<Integer,String>{
@Override
protected String doInBackground(Integer... integers) {
String result = null;
try{
Connection connection = connectionClass.CONN();
if(connection==null){
result = "Error in connection !!!";
}else{
PreparedStatement ps = connection.prepareStatement("UPDATE PreAdmissionDetails SET STATUS=? WHERE id=?");
ps.setInt(1,0);
ps.setInt(2,integers[0]);
ps.executeUpdate();
result = "Updated Successfully !!!";
}
}catch (Exception ex){
Log.e("sqlerror","result : "+result.toString());
return result;
}
}
private class LoadApprovalList extends AsyncTask<Void,Void,Void>{
@Override
protected Void doInBackground(Void... voids) {
String result = null;
try{
Connection connection = connectionClass.CONN();
if(connection==null){
result = "Error in connection !!!";
}else{
PreparedStatement ps = connection.prepareStatement("select * from preadmissiondetails");
ResultSet rs = ps.executeQuery();
approvalList.clear();
while (rs.next()) {
approvalList.add(new ModelBasicInfo(rs.getInt(1),rs.getString(2),rs.getString(3),rs.getString(4),rs.getString(5),rs.getString(6),rs.getString(7),rs.getInt(8),rs.getInt(9),rs.getBoolean(10)));}
result = "Fetched Successfully !!!";
}
}catch (Exception ex){
Log.e("sqlerror","result : "+result.toString());
return null;
}
}
}
问题出在这里，当我打开应用程序时，在回收站视图中没有任何项目，首先，我认为互联网速度可能很慢，它会在一段时间后获取数据，但不会显示数据。但是，当我导航到其他片段并返回到预录入列表时，它会显示数据。
解决方法
您的问题在这里：
display:none public MutableLiveData<List<ModelBasicInfo>> getApprovalList(){
loadApprovalList();
MutableLiveData<List<ModelBasicInfo>> mList = new MutableLiveData<>();
mList.setValue(approvalList);
return mList;
}
启动一个AsyncTask，这是一个异步操作（换句话说，要花一些时间才能产生结果）。 loadApprovalList()方法不仅会停在那里并等待getApprovalList()完成。它会一直继续，并在第一次执行时返回一个空列表。但是到第二次执行时，loadApprovalList()现在有了一个值，因为AsyncTask已完成。因此，它将在第二次执行时返回正确的数据。当您返回到Fragment时，它会第二次执行，因为ViewModel中的approvalList块此时正在第二次执行。
解决方案是使init成为LiveData。这样，当AsyncTask更新ApprovalList时，您的ViewModel可以观察到更改。您的ViewModel应该在存储库中观察approvalList，就像您的片段在ViewModel中观察approvalList方法一样。
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