当我使用 recyclerview 和适配器启动片段时出错

Question

当我以 fragment、recyclerview 和 adapter 启动 activity 时，应用程序正在停止。我不知道问题出在哪里。我尝试了不同的方法。

我的日志猫：

java.lang.NullPointerException: 尝试调用虚方法 'void android.support.v7.widget.RecyclerView.setAdapter(android.support.v7.widget.RecyclerView$Adapter)' 在空对象引用上 在 com.orthodox.FeedActivity.onStart(FeedActivity.java:133) 在 android.support.v4.app.Fragment.performStart(Fragment.java:2287)

我的代码：

public class FeedActivity extends Fragment{
private RecyclerView mBlogList;
private Query mQuery;
private Query mQueryck;
private DatabaseReference mDatabaseUsers;

private FirebaseUser mCurrent;
private FirebaseAuth mAuth;
private FirebaseAuth.AuthStateListener mAuthListener;

FirebaseRecyclerAdapter mAdapter;
public FeedActivity() {
    // Required empty public constructor
}
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    final FragmentActivity c = getActivity();
    View layout = inflater.inflate(R.layout.fragment_feed, container, false);
    return layout;
}
@Override
public void onViewCreated(View view, Bundle savedInstanceState) {
    mBlogList = (RecyclerView) view.findViewById(R.id.blog_list);
    mBlogList.setHasFixedSize(true);
    LinearLayoutManager layoutManager = new LinearLayoutManager(getActivity());
    mBlogList.setLayoutManager(layoutManager);
    mAuth = FirebaseAuth.getInstance();
    mAuthListener = new FirebaseAuth.AuthStateListener() {
        @Override
        public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
            if(firebaseAuth.getCurrentUser() == null){
                Intent loginIntent = new Intent(getActivity(), SignUpActivity.class);
                loginIntent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                startActivity(loginIntent);
            }
        }
    };
    mQuery = FirebaseDatabase.getInstance().getReference().child("Blog").limitToLast(20);
    mQuery.keepSynced(true);
    mQueryck = FirebaseDatabase.getInstance().getReference().child("Block").limitToLast(20);
    mQueryck.keepSynced(true);
    mDatabaseUsers = FirebaseDatabase.getInstance().getReference().child("users");
    mDatabaseUsers.keepSynced(true);
    mCurrent = mAuth.getCurrentUser();
}
@Override
public void onStart() {
    super.onStart();
    mAuth.addAuthStateListener(mAuthListener);
    FirebaseRecyclerOptions<User> options = new FirebaseRecyclerOptions.Builder<User>().setQuery(mQuery, User.class).build();
    FirebaseRecyclerOptions<User> optionss = new FirebaseRecyclerOptions.Builder<User>().setQuery(mQueryck, User.class).build();
    mAdapter = new FirebaseRecyclerAdapter<User, BlogViewHolder>(options) {
        @Override
        public BlogViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            // Create a new instance of the ViewHolder
            View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.blog_row, parent, false);
            return new BlogViewHolder(view);
        }

        @Override
        protected void onBindViewHolder(BlogViewHolder viewHolder, int position, User model) {
            final String post_key = getRef(position).getKey();
            viewHolder.setTitle(model.getTitle());
            viewHolder.setDesc(model.getDesc());
            viewHolder.setTime(model.getFirstname());
            viewHolder.setImage(getApplicationContext(), model.getImage());
        }
    };
    mBlogList.setAdapter(mAdapter);
    mAdapter.startListening();
    mAdapter = new FirebaseRecyclerAdapter<User, BlogViewHolder>(optionss) {
        @Override
        protected void onBindViewHolder(BlogViewHolder viewHolder, int position, User model) {
            final String post_key = getRef(position).getKey();
            viewHolder.setTitle(model.getTitle());
            viewHolder.setDesc(model.getDesc());
            viewHolder.setTime(model.getFirstname());
            viewHolder.setImage(getApplicationContext(), model.getImage());
            viewHolder.mView.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    Intent bsingle = new Intent(getActivity(), SinglePostActivity.class);
                    bsingle.putExtra("post", post_key);
                    startActivity(bsingle);
                }
            });
        }
        @Override
        public BlogViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.blog_row, parent, false);
            return new BlogViewHolder(view);
        }
    };
}
public static class BlogViewHolder extends RecyclerView.ViewHolder {

    View mView;
    public BlogViewHolder(View itemView) {
        super(itemView);
        mView = itemView;
    }

    public void setTitle(String title){
        TextView post_title = (TextView) mView.findViewById(R.id.post_title);
        post_title.setText(title);
    }

    public void setDesc(String desc){
        TextView post_desc = (TextView) mView.findViewById(R.id.post_desc);
        post_desc.setText(desc);
    }

    public void setImage(Context ctx, String image){
        if (image != null) {

            ImageView post_image = (ImageView) mView.findViewById(R.id.post_image);
            Picasso.with(ctx).load(image).into(post_image);
        }
        else {
            ImageView post_image = (ImageView) mView.findViewById(R.id.post_image);
            post_image.setVisibility(View.GONE);
        }
    }
    public void setTime(String time){
        TextView post_time = (TextView) mView.findViewById(R.id.post_username);
        post_time.setText(time);
    }

Answer 1

在onCreateView 方法中初始化所有视图。

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                     Bundle savedInstanceState) {
  final FragmentActivity c = getActivity();
  View layout = inflater.inflate(R.layout.fragment_feed, container, false);
  mBlogList = (RecyclerView) layout.findViewById(R.id.blog_list);
  return layout;
}