无法集成将一个片段之间的数据传递到另一个片段的视图模型，然后将其最终发送到recycleviewadapter

Question

该应用程序的结构很简单。 我有一个片段，它调用对话片段，在接收到数据之后，我想通过使用viewmodel（我发现使用最简单）将其发送到另一个片段。 在第二个片段中，数据被接收，然后最终传递给recycleviewadapter，以显示数据。 我收到错误消息。

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.newvibe, PID: 10141
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.newvibe.Book.getTitle()' on a null object reference
        at com.example.newvibe.RecyclerViewAdapter.onBindViewHolder(RecyclerViewAdapter.java:45)
        at com.example.newvibe.RecyclerViewAdapter.onBindViewHolder(RecyclerViewAdapter.java:15)
        at androidx.recyclerview.widget.RecyclerView$Adapter.onBindViewHolder(RecyclerView.java:6781)
        at androidx.recyclerview.widget.RecyclerView$Adapter.bindViewHolder(RecyclerView.java:6823)
        at androidx.recyclerview.widget.RecyclerView$Recycler.tryBindViewHolderByDeadline(RecyclerView.java:5752)
        at androidx.recyclerview.widget.RecyclerView$Recycler.tryGetViewHolderForPositionByDeadline(RecyclerView.java:6019)
        at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:5858)
        at androidx.recyclerview.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:5854)
        at androidx.recyclerview.widget.LinearLayoutManager$LayoutState.next(LinearLayoutManager.java:2230)
        at androidx.recyclerview.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1557)
        at androidx.recyclerview.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1517)
        at androidx.recyclerview.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:612)
        at androidx.recyclerview.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:3924)
        at androidx.recyclerview.widget.RecyclerView.onMeasure(RecyclerView.java:3336)
        at android.view.View.measure(View.java:23169)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6749)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1535)
        at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1187)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:706)
        at android.view.View.measure(View.java:23169)
        at androidx.viewpager.widget.ViewPager.onMeasure(ViewPager.java:1638)
        at android.view.View.measure(View.java:23169)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:978)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:704)
        at android.view.View.measure(View.java:23169)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6749)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
        at androidx.appcompat.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:143)
        at android.view.View.measure(View.java:23169)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6749)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1535)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:825)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:704)
        at android.view.View.measure(View.java:23169)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6749)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
        at android.view.View.measure(View.java:23169)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6749)
        at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1535)
        at android.widget.LinearLayout.measureVertical(LinearLayout.java:825)
        at android.widget.LinearLayout.onMeasure(LinearLayout.java:704)
        at android.view.View.measure(View.java:23169)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6749)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
        at com.android.internal.policy.DecorView.onMeasure(DecorView.java:716)
        at android.view.View.measure(View.java:23169)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2718)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1572)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1855)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1460)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7183)
E/AndroidRuntime:     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:949)
        at android.view.Choreographer.doCallbacks(Choreographer.java:761)
        at android.view.Choreographer.doFrame(Choreographer.java:696)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:935)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6669)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
Process 10141 terminated.

我正在传递一个自定义书对象，该对象具有getTitle方法，由于空对象错误，我认为没有书被传递。 浏览完网络后，我发现一个可能的错误可能是我通过了这本书的OnCreateView而不是OnActivityCreated，但是据我所知，我无法实例化整个recyclerview和layoutmanager等。onActivityCreated。

这是第一个片段

   import android.os.Bundle;
    import android.view.LayoutInflater;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.Button;
    import android.widget.Toast;


    import androidx.annotation.Nullable;
    import androidx.fragment.app.Fragment;
    import androidx.lifecycle.Observer;
    import androidx.lifecycle.ViewModelProviders;


    import java.util.Objects;

    public class FragmentHome extends Fragment implements AddBookDialogue.AddBookDialogueListener, View.OnClickListener {

     private BookViewModel bookPass;
     public Book passedBook;
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){
    View w = inflater.inflate(R.layout.fragment_home, container, false);
    Button b = (Button) w.findViewById(R.id.button);
    b.setOnClickListener(this);
    return w;
    }
        @Override
        public void onStart() {
            super.onStart();
        }

        @Override
        public void getTexts(String bookAuthor, String bookTitle, String bookBarcode, String bookCourse, String selectedBookshelf) {
                    Book a = new Book(bookAuthor, bookTitle, bookBarcode, bookCourse, selectedBookshelf);
                    Toast.makeText(getActivity(), a.toString(), Toast.LENGTH_LONG).show();
                    bookPass.setBook(a);

    }

        @Override
        public void onClick(View w) {
            if (w.getId() == R.id.button) {
                AddBookDialogue addBookDialogue = new AddBookDialogue();
                assert getFragmentManager() != null;
                addBookDialogue.setTargetFragment(FragmentHome.this, 1 );
                addBookDialogue.show(getFragmentManager(), "add book dialogue");

            }
        }

        @Override
        public void onActivityCreated(@Nullable Bundle savedInstanceState) {
            super.onActivityCreated(savedInstanceState);
            bookPass = ViewModelProviders.of(Objects.requireNonNull(getActivity())).get(BookViewModel.class);
            bookPass.getBook().observe(getViewLifecycleOwner(), new Observer<Book>() {
                @Override
                public void onChanged(@Nullable Book book) {
                     passedBook = book;
                }
            });
        }
    }

这是接收片段

import android.os.Bundle;

import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;


import java.util.ArrayList;
import java.util.Objects;

public class Fragment_Withdrawn extends Fragment {

private RecyclerView mRecyclerview;
private RecyclerView.LayoutManager myLayoutManager;
private RecyclerViewAdapter withDrawnAdapter;
public ArrayList<Book> bookSet;
private BookViewModel bookPass;
public Book passedBook;
    View withdrawnView;
    public Fragment_Withdrawn() {
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        bookSet = new ArrayList<>();
        bookSet.add(passedBook);
        withdrawnView = inflater.inflate(R.layout.fragment_withdrawn, container, false);
        mRecyclerview = (RecyclerView) withdrawnView.findViewById(R.id.WithdrawnRecycler);
        myLayoutManager= new LinearLayoutManager(getActivity());
        Log.d("debugMode", "mayTest");
        mRecyclerview.setLayoutManager(myLayoutManager);
        withDrawnAdapter = new RecyclerViewAdapter(getContext(  ), bookSet);
        mRecyclerview.setAdapter(withDrawnAdapter);
        return withdrawnView;
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        bookPass = ViewModelProviders.of(Objects.requireNonNull(getActivity())).get(BookViewModel.class);
        bookPass.getBook().observe(getViewLifecycleOwner(), new Observer<Book>() {
            @Override
            public void onChanged(@Nullable Book book) {
                passedBook = book;
            }
        });
    }
}

这是recycleviewadapter

import android.content.Context;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import java.util.ArrayList;

public class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerViewAdapter.MyViewHolder> {
    public ArrayList<Book> bookSet;
    public Context newContext;


    public static class MyViewHolder extends RecyclerView.ViewHolder{
        public TextView textView;
        public MyViewHolder(View vv) {
            super(vv);
            textView = vv.findViewById(R.id.RecyclerViewTextView);
        }
    }

    public RecyclerViewAdapter(Context newContext , ArrayList<Book> bookSet) {
        this.newContext = newContext; this.bookSet = bookSet;
    }

    @NonNull
    @Override
    public RecyclerViewAdapter.MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View view;
        view = LayoutInflater.from(newContext).inflate(R.layout.recycler_view_text_view, parent, false);
        MyViewHolder viewHolder = new MyViewHolder(view);
        return viewHolder;

    }

    @Override
    public void onBindViewHolder(@NonNull RecyclerViewAdapter.MyViewHolder holder, int position) {
        if(!bookSet.isEmpty())
holder.textView.setText(bookSet.get(position).getTitle());
    }

    @Override
    public int getItemCount() {
        return bookSet.size();
    }
}

ViewModel

   package com.example.newvibe;
import androidx.lifecycle.LiveData;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;

public class BookViewModel extends ViewModel {

    private MutableLiveData<Book> book = new MutableLiveData<>();

    public void setBook(Book input){
        book.setValue(input);
    }

    public LiveData<Book> getBook(){
        return book;
    }
}

最后，以防万一，这里是AddDiaogue，但是它成功地将数据传递到了调用它的片段

package com.example.newvibe;

import android.app.Dialog;
import android.content.Context;
import android.content.DialogInterface;
import android.os.Bundle;

import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.Toast;

import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatDialogFragment;

public class AddBookDialogue extends AppCompatDialogFragment {
    private EditText editTextBookAuthor;
    private EditText editTextBookTitle;
    private EditText editTextBookCourse;
    private EditText editTextBookBarcode;
    private Spinner spinnerBookshelf;
    private AddBookDialogueListener listener;

    @Override
    public void onStart() {
        super.onStart();
        AlertDialog d = (AlertDialog)getDialog();
        if(d !=null){
            Button positiveButton = (Button) d.getButton(Dialog.BUTTON_POSITIVE);
            positiveButton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    String bookAuthor =  editTextBookAuthor.getText().toString();
                    String bookTitle =  editTextBookTitle.getText().toString();
                    String bookBarcode =  editTextBookBarcode.getText().toString();
                    String bookCourse =  editTextBookCourse.getText().toString();
                    String selectedBookshelf =  spinnerBookshelf.getSelectedItem().toString();
if(bookAuthor.isEmpty() || bookTitle.isEmpty() || bookBarcode.isEmpty() || bookCourse.isEmpty() ){
    Toast toast = Toast.makeText(getActivity(), "You didn't enter all the neccesary data", Toast.LENGTH_SHORT);
    toast.setGravity(Gravity.CENTER| Gravity.CENTER_HORIZONTAL, 0, 0);
    toast.show();

}else{
    if(bookBarcode.length()!=8){
        Toast toas =Toast.makeText(getActivity(), "Barcodes should be eight digits long", Toast.LENGTH_LONG);
        toas.setGravity(Gravity.CENTER| Gravity.CENTER_HORIZONTAL, 0, 0 );
        toas.show();
    }else{
        listener.getTexts(bookAuthor, bookTitle,bookBarcode,bookCourse,selectedBookshelf );
        dismiss();
    }
}
                }
            });
        }
        }


    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {
        AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
        LayoutInflater inflater = getActivity().getLayoutInflater();
        View view = inflater.inflate(R.layout.dialogue_addbook, null);
        builder.setView(view)
                .setNegativeButton("cancel", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {

                    }
                })
                .setPositiveButton("Add", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialogInterface, int i) {

                    }
                });


        editTextBookAuthor = view.findViewById(R.id.author);
        editTextBookTitle= view.findViewById(R.id.bookTitle);
        editTextBookBarcode= view.findViewById(R.id.bookBarcode);
        editTextBookCourse= view.findViewById(R.id.bookCourse);
        spinnerBookshelf= view.findViewById(R.id.spinnerBookshelf);
        ArrayAdapter<CharSequence> spinnerAdapter = ArrayAdapter.createFromResource(getActivity(), R.array.addBookBookshelfsAvailable, android.R.layout.simple_spinner_item);
        spinnerAdapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        spinnerBookshelf.setAdapter(spinnerAdapter);
        return builder.create();
    }

    public void onAttach(Context context) {
        super.onAttach(context);
        try {
            listener = (AddBookDialogueListener) getTargetFragment();
        } catch(ClassCastException e){
            throw new ClassCastException(context.toString() + "no listener");
        }
    }
    public interface AddBookDialogueListener{
        void getTexts(String bookAuthor, String bookTitle, String bookBarcode, String bookCourse, String selectedBookshelf);

    }

}

帮助非常感谢:)