如何为recycleview生成一个适配器类，以便在片段类中使用它

Question

我正在尝试构建一个有片段的应用程序。在这些碎片中，我使用的是RecyclerView。此外，在我的layouts目录下有一个Customer实体类的CardView小部件，名为card_view_customer.xml。但是，当我在CustomerFragment.java类下为当前适配器创建一个实例时，由于一些我不知道的原因，应用程序正在崩溃。我相信问题是我的适配器实现错误，但我不知道如何修复它；

CustomerFragment.java下的onCreateView()函数

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_customer, container, false);
// Inflate the layout for this fragment
RecyclerView recyclerView = (RecyclerView) view.findViewById(R.id.recyclerView);
final SalesTrackerAdapter adapter = new SalesTrackerAdapter(getActivity());
recyclerView.setAdapter(adapter);
recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
mSalesTrackerViewModel = new ViewModelProvider(getActivity()).get(SalesTrackerViewModel.class);
mSalesTrackerViewModel.getAllCustomers().observe(getActivity(), new Observer<List<Customer>>() {
@Override
public void onChanged(List<Customer> customers) {
adapter.setCustomers(customers);
}
});
return view;
}

card_view_customer.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:padding="10dp"
android:background="#00D8D8">
<androidx.cardview.widget.CardView
android:id="@+id/card_view_customer"
android:layout_width="match_parent"
android:layout_height="wrap_content"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:cardCornerRadius="4dp"
app:contentPadding="10dp">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/customerTitle"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
style="@style/customer_title"
android:layout_alignParentStart="true" />
</RelativeLayout>
</androidx.cardview.widget.CardView>

fragment_customer.xml，其中我使用RecyclerView

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".CustomerFragment">
<androidx.recyclerview.widget.RecyclerView
android:id="@+id/recyclerView"
android:layout_width="match_parent"
android:layout_height="match_parent"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="1.0" />
</FrameLayout>

问题最有可能出现在这段代码中，我曾尝试实现适配器，但当我在CustomerFragment.java中的onCreateView()函数下使用它时崩溃了

SalesTrackerAdapter

public class SalesTrackerAdapter extends RecyclerView.Adapter<SalesTrackerAdapter.SalesTrackerViewHolder> {
class SalesTrackerViewHolder extends RecyclerView.ViewHolder{
private final TextView customerTitle;
private SalesTrackerViewHolder(View itemView){
super(itemView);
customerTitle = itemView.findViewById(R.id.customerTitle);
}
}
private final LayoutInflater myInflater;
private List<Customer> myCustomers;
SalesTrackerAdapter(Context context){myInflater = LayoutInflater.from(context);}
@Override
public SalesTrackerViewHolder onCreateViewHolder(ViewGroup parent, int viewType){
View itemView = myInflater.inflate(R.layout.card_view_customer, parent, false);
return new SalesTrackerViewHolder(itemView);
}
@Override
public void onBindViewHolder(SalesTrackerViewHolder holder, int position){
if (myCustomers != null) {
Customer current = myCustomers.get(position);
holder.customerTitle.setText(current.getName());
} else {
holder.customerTitle.setText("No Customers");
}
}
void setCustomers(List<Customer> customers){
myCustomers = customers;
notifyDataSetChanged();
}
@Override
public int getItemCount(){
if(myCustomers != null){
return myCustomers.size();
}
return 0;
}
}

我认为这个适配器有问题，因为每当在onCreateView()模拟器中引用它时，都会说应用程序"一直在停止"。当我评论出这些台词时，到目前为止效果还不错。我想要的是从数据库中获取客户，这是我通过实现存储库和视图模型类来完成的，并将它们显示在片段下的cardview中。以下是我的存储库和ViewModel:的代码

public class SalesTrackerRepository {
//Dao instances.
private CustomerDao mCustomerDao;
//Listing items.
private LiveData<List<Customer>> mAllCustomers;

SalesTrackerRepository(Application application){
SalesTrackerDatabase db = SalesTrackerDatabase.getDatabase(application);
mCustomerDao = db.customerDao();
mAllCustomers = mCustomerDao.getCustomers();
}
LiveData<List<Customer>> getAllCustomers(){
return mAllCustomers;
}
void insert(Customer customer){
SalesTrackerDatabase.databaseWriteExecutor.execute(() -> mCustomerDao.insert(customer));
}
}

ViewModel

public class SalesTrackerViewModel extends AndroidViewModel {
private SalesTrackerRepository mRepository;
private LiveData<List<Customer>> mAllCustomers;
private SalesTrackerViewModel(Application application){
super(application);
mRepository = new SalesTrackerRepository(application);
mAllCustomers = mRepository.getAllCustomers();
}
LiveData<List<Customer>> getAllCustomers(){return mAllCustomers;}
public void insert(Customer customer){mRepository.insert(customer);}
}

我刚刚开始使用安卓系统，并试图通过这个项目学习。如果问题中有任何没有意义的地方，我很抱歉。提前谢谢。

编辑：问题可能也出现在ViewModel类中。我认为崩溃是由于以下阻塞造成的：

mSalesTrackerViewModel = new ViewModelProvider(getActivity()).get(SalesTrackerViewModel.class);
mSalesTrackerViewModel.getAllCustomers().observe(getActivity(), new Observer<List<Customer>>() {
@Override
public void onChanged(List<Customer> customers) {
adapter.setCustomers(customers);
}
});

编辑2：这是我的logcat

2020-04-19 23:17:09.786 11114-11114/com.example.salestrackertest3 E/lestrackertest: Unknown bits set in runtime_flags: 0x8000
2020-04-19 23:17:12.011 11114-11114/com.example.salestrackertest3 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.salestrackertest3, PID: 11114
java.lang.RuntimeException: Cannot create an instance of class com.example.salestrackertest3.SalesTrackerViewModel
at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:269)
at androidx.lifecycle.SavedStateViewModelFactory.create(SavedStateViewModelFactory.java:106)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:185)
at androidx.lifecycle.ViewModelProvider.get(ViewModelProvider.java:150)
at com.example.salestrackertest3.CustomerFragment.onCreateView(CustomerFragment.java:90)
at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2698)
at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:310)
at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1185)
at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1354)
at androidx.fragment.app.FragmentManager.moveFragmentToExpectedState(FragmentManager.java:1432)
at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1495)
at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:447)
at androidx.fragment.app.FragmentManager.executeOps(FragmentManager.java:2167)
at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:1990)
at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1945)
at androidx.fragment.app.FragmentManager.execSingleAction(FragmentManager.java:1816)
at androidx.fragment.app.BackStackRecord.commitNowAllowingStateLoss(BackStackRecord.java:303)
at androidx.fragment.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:238)
at androidx.viewpager.widget.ViewPager.populate(ViewPager.java:1244)
at androidx.viewpager.widget.ViewPager.populate(ViewPager.java:1092)
at androidx.viewpager.widget.ViewPager.onMeasure(ViewPager.java:1622)
at android.view.View.measure(View.java:24530)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at androidx.coordinatorlayout.widget.CoordinatorLayout.onMeasureChild(CoordinatorLayout.java:760)
at com.google.android.material.appbar.HeaderScrollingViewBehavior.onMeasureChild(HeaderScrollingViewBehavior.java:99)
at com.google.android.material.appbar.AppBarLayout$ScrollingViewBehavior.onMeasureChild(AppBarLayout.java:1892)
at androidx.coordinatorlayout.widget.CoordinatorLayout.onMeasure(CoordinatorLayout.java:831)
at android.view.View.measure(View.java:24530)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at androidx.appcompat.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:143)
at android.view.View.measure(View.java:24530)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1552)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:842)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:721)
at android.view.View.measure(View.java:24530)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at android.view.View.measure(View.java:24530)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1552)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:842)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:721)
at android.view.View.measure(View.java:24530)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6828)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:194)
at com.android.internal.policy.DecorView.onMeasure(DecorView.java:742)
at android.view.View.measure(View.java:24530)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:3006)
2020-04-19 23:17:12.012 11114-11114/com.example.salestrackertest3 E/AndroidRuntime:     at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1833)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2122)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1721)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7598)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:966)
at android.view.Choreographer.doCallbacks(Choreographer.java:790)
at android.view.Choreographer.doFrame(Choreographer.java:725)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:951)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
Caused by: java.lang.NoSuchMethodException: com.example.salestrackertest3.SalesTrackerViewModel.<init> [class android.app.Application]
at java.lang.Class.getConstructor0(Class.java:2332)
at java.lang.Class.getConstructor(Class.java:1728)
at androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:267)
... 64 more

Answer 1

崩溃日志的以下部分表明运行时找不到ViewModel的构造函数

由java.lang.NoSuchMethodException引起：com.example.salestrackertest3.SalesTrackerViewModel。<init>[类别android.app.Application]位于java.lang.Class.getConstructor0(Class.java：2332(位于java.lang.Class.getConstructor(Class.java：1728(在androidx.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:267(

。。。事实上，构造函数是存在的，但它是private:

private SalesTrackerViewModel(Application application){
super(application);
mRepository = new SalesTrackerRepository(application);
mAllCustomers = mRepository.getAllCustomers();
}

因此，解决方案是使构造函数public