java - Firebase 用户界面 : RecyclerView items are not shown

Question

我一直在尝试使用 RecyclerView 在我的应用中实现 Firebase-UI。 我使用实时数据库和 Firebase-UI 在聊天应用程序中创建了一个 recyclerView 用户列表，我想从 ProfileActivity 检索数据并将其显示在我的 AllUsersActivity 中。问题是 RecyclerView 项目未显示，并且我收到此错误:

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.schoolteacher, PID: 19631
java.lang.NullPointerException: Attempt to invoke virtual method 'com.google.firebase.database.ChildEventListener com.google.firebase.database.Query.addChildEventListener(com.google.firebase.database.ChildEventListener)' on a null object reference
    at com.firebase.ui.database.FirebaseArray.onCreate(FirebaseArray.java:54)
    at com.firebase.ui.common.BaseObservableSnapshotArray.addChangeEventListener(BaseObservableSnapshotArray.java:97)
    at com.firebase.ui.database.FirebaseRecyclerAdapter.startListening(FirebaseRecyclerAdapter.java:52)
    at com.example.schoolteacher.AllUsersActivity.onStart(AllUsersActivity.java:112)
    at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1419)
    at android.app.Activity.performStart(Activity.java:7479)
    at android.app.ActivityThread.handleStartActivity(ActivityThread.java:3454)
    at android.app.servertransaction.TransactionExecutor.performLifecycleSequence(TransactionExecutor.java:180)
    at android.app.servertransaction.TransactionExecutor.cycleToPath(TransactionExecutor.java:165)
    at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:142)
    at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:70)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2199)
    at android.os.Handler.dispatchMessage(Handler.java:112)
    at android.os.Looper.loop(Looper.java:216)
    at android.app.ActivityThread.main(ActivityThread.java:7625)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:524)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:987)

这是我的代码:

AllUsersActivity.java

public class AllUsersActivity extends AppCompatActivity {

    private DatabaseReference mUsersDatabase;
    private LinearLayoutManager linearLayoutManager;
    private RecyclerView recyclerView;
    private Query query;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_all_users);

        recyclerView = findViewById(R.id.recycler_view);

        linearLayoutManager = new LinearLayoutManager(this);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        recyclerView.setHasFixedSize(true);

        query = FirebaseDatabase.getInstance().getReference().child("Users");


    }       



    FirebaseRecyclerOptions<Users> options = new FirebaseRecyclerOptions.Builder<Users>()
                    .setQuery(query, Users.class)
                    .build();

    FirebaseRecyclerAdapter adapter = new FirebaseRecyclerAdapter<Users, UsersViewHolder>(options) {

        @Override
        protected void onBindViewHolder(@NonNull UsersViewHolder holder, int position, @NonNull Users model) {

            holder.setName(model.getName());
        }

        @Override
        public UsersViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            // Create a new instance of the ViewHolder, in this case we are using a custom
            // layout called R.layout.message for each item
            View view = LayoutInflater.from(parent.getContext())
                    .inflate(R.layout.activity_all_users, parent, false);

            return new UsersViewHolder(view);
        }
    };

    @Override
    protected void onStart() {
        super.onStart();
        adapter.startListening();
    }

    @Override
    protected void onStop() {
        super.onStop();
        adapter.stopListening();
    }


}


class UsersViewHolder extends RecyclerView.ViewHolder {
    View mView;

    UsersViewHolder(View itemView) {
        super(itemView);
        mView = itemView;
    }

    public void setName(String name) {
        TextView mName = mView.findViewById(R.id.user_single_name);
        mName.setText(name);
    }
}

个人资料信息 Activity

public class ProfileInfoActivity extends AppCompatActivity {

    private DatabaseReference mDatabaseReference;
    private FirebaseUser mCurrentUser;

    private CircleImageView mDisplayImage;
    private TextView mName;
    private TextInputEditText mEditName, mEmail;

    private TextView mStatus;
    private ImageButton mStatusBtn;

    private static final int GALLERY_PICK = 1;

    private StorageReference mImageStorage;

    private ProgressDialog mProgressDialog;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_profile_info);


        mDisplayImage = findViewById(R.id.circle_image);
        mName = findViewById(R.id.name);
        mEditName = findViewById(R.id.et_name);
        mEmail = findViewById(R.id.et_email_address);
        mStatus = findViewById(R.id.status);
        mStatusBtn = findViewById(R.id.status_btn);

        mImageStorage = FirebaseStorage.getInstance().getReference();
        mCurrentUser = FirebaseAuth.getInstance().getCurrentUser();
        String userID = mCurrentUser.getUid();

        mDatabaseReference = FirebaseDatabase.getInstance().getReference().child("Users").child(userID);
        mDatabaseReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                String name = dataSnapshot.child("name").getValue().toString();
                String et_name = dataSnapshot.child("name").getValue().toString();
                String et_email_adress = dataSnapshot.child("email").getValue().toString();
                String image = dataSnapshot.child("image").getValue().toString();
                String status = dataSnapshot.child("status").getValue().toString();
                String thumb_image = dataSnapshot.child("thumb_image").getValue().toString();

                mName.setText(name);
                mEditName.setText(et_name);
                mEmail.setText(et_email_adress);
                mStatus.setText("Teacher at "+ status);


                Picasso.get().load(image).into(mDisplayImage);

            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });

        mDisplayImage.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent galleryIntent = new Intent();
                galleryIntent.setType("image/*");
                galleryIntent.setAction(Intent.ACTION_GET_CONTENT);

                startActivityForResult(Intent.createChooser(galleryIntent, "SELECT IMAGE"), GALLERY_PICK);
            }
        });


        mStatusBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //send status data to changeSchoolActivity to retrieve it
                String status_value = mStatus.getText().toString();
                Intent status_intent = new Intent(ProfileInfoActivity.this, ChangeSchoolActivity.class);
                status_intent.putExtra("status_value", status_value);
                startActivity(status_intent);
            }
        });


    }


    @Override
    protected void onActivityResult(int requestCode, int resultCode, @Nullable Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if(requestCode == GALLERY_PICK && resultCode == RESULT_OK ) {
            Uri imageUri = data.getData();

            CropImage.activity(imageUri)
                    .setAspectRatio(1, 1)
                    .start(this);

            //Toast.makeText(ProfileInfoActivity.this, imageUri, Toast.LENGTH_SHORT).show();
        }

        if (requestCode == CropImage.CROP_IMAGE_ACTIVITY_REQUEST_CODE) {
            CropImage.ActivityResult result = CropImage.getActivityResult(data);
            if (resultCode == RESULT_OK) {

                mProgressDialog = new ProgressDialog(ProfileInfoActivity.this);
                mProgressDialog.setTitle("Uploading Image...");
                mProgressDialog.setMessage("Please wait while we upload and process the image");
                mProgressDialog.setCanceledOnTouchOutside(false);
                mProgressDialog.show();


                Uri resultUri = result.getUri();
                String current_user_id = mCurrentUser.getUid();


                final StorageReference filepath = mImageStorage.child("profile_images").child(current_user_id + ".jpg");

                filepath.putFile(resultUri).addOnSuccessListener(new OnSuccessListener<UploadTask.TaskSnapshot>() {
                    @Override
                    public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {
                        filepath.getDownloadUrl().addOnSuccessListener(new OnSuccessListener<Uri>() {
                            @Override
                            public void onSuccess(Uri uri) {
                                String download_url = uri.toString();
                                mDatabaseReference.child("image").setValue(download_url).addOnSuccessListener(new OnSuccessListener<Void>() {
                                    @Override
                                    public void onSuccess(Void aVoid) {
                                        mProgressDialog.dismiss();
                                        Toast.makeText(ProfileInfoActivity.this,"Success Uploading",Toast.LENGTH_SHORT).show();
                                    }
                                });

                            }
                        });
                    }
                });


            } else if (resultCode == CropImage.CROP_IMAGE_ACTIVITY_RESULT_ERROR_CODE) {
                Exception error = result.getError();
            }
        }
    }
}

Answer 1

根据您的最新评论:

protected void onStart() { super.onStart(); firebaseRecyclerAdapter.startListening(); }

是发生错误的行。要解决此问题，请通过添加以下代码行将 firebaseRecyclerAdapter 变量创建为全局变量:

private FirebaseRecyclerAdapter firebaseRecyclerAdapter;

紧接着:

private Query query;

并从代码中删除类型声明:

//FirebaseRecyclerAdapter does not exist anymore
firebaseRecyclerAdapter = new FirebaseRecyclerAdapter<Users, UsersViewHolder>(firebaseRecyclerOptions) {
    @NonNull
    @Override
    public UsersViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View mView = LayoutInflater.from(parent.getContext()).inflate(R.layout.users_single_layout, parent, false);
        return new UsersViewHolder(mView);
    }

    @Override
    protected void onBindViewHolder(@NonNull UsersViewHolder holder, int position, @NonNull Users model) {
        holder.setName(model.getName());

    }
};