Question

在我的应用中，我使用了Firebase数据库。在单独的节点中存储有问题和相应的注释。现在，我尝试使用一个侦听器获取问题，并使用第二个侦听器访问评论。不幸的是，我对他们的行为感到困惑：recyclerView总是得到一个空的questionsList，就像跳过了第二个侦听器一样。但是在recyclerView获得列表并设置了适配器之后，我的LogCat开始打印问题和评论信息。但是，为什么recyclerView在处理数据的for循环完成之前被填充和使用？

获取信息的方法：

private void getQuestionsFromDatabase() {

    mQuestions.addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            questionList = new ArrayList<>();
            for (DataSnapshot dataSnapshot1 : dataSnapshot.getChildren()) {

                final String title = dataSnapshot1.child("title").getValue().toString();
                final String question = dataSnapshot1.child("question").getValue().toString();
                final String commentId = dataSnapshot1.child("commentId").getValue().toString();

                mComments.child(commentId).addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                        count = dataSnapshot.getChildrenCount();

                        QuestionModel questionModel = new QuestionModel(title, question, commentId, String.valueOf(count));
                        questionList.add(questionModel);

                    }

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

                    }
                });

            }

            Log.d("questionList length: ", String.valueOf(questionList.size()));
            recyclerViewAdapter = new RecyclerViewQuestionAdapter(questionList, getActivity());
            recyclerViewlayoutManager = new LinearLayoutManager(getActivity());
            recyclerView.setLayoutManager(recyclerViewlayoutManager);
            recyclerView.setAdapter(recyclerViewAdapter);

        }

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

        }

    });

}

Answer 1

之所以使用它，是因为onDataChange是异步的，这意味着编译器不会等到从数据库中获取数据，而是会在侦听器之后执行代码。因此，要解决您的问题，您应该执行以下操作：

                mComments.child(commentId).addListenerForSingleValueEvent(new ValueEventListener() {
                @Override
                public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                    count = dataSnapshot.getChildrenCount();

                    QuestionModel questionModel = new QuestionModel(title, question, commentId, String.valueOf(count));
                    questionList.add(questionModel);
                    Log.d("questionList length: ", String.valueOf(questionList.size()));
                    recyclerViewAdapter = new RecyclerViewQuestionAdapter(questionList, getActivity());
                    recyclerViewlayoutManager = new LinearLayoutManager(getActivity());
                    recyclerView.setLayoutManager(recyclerViewlayoutManager);
                    recyclerView.setAdapter(recyclerViewAdapter);

                }

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

                }
            });

        }

侦听器中的Firebase侦听器已跳过

1 个答案: