尽管获取了正确的数据，为什么RecyclerView适配器仍未正确更新（仅第一次）？

Question

This is very strange. For some reason, despite getting the correct data from the REST call, the adapter is not updated correctly in the RecyclerView.

Here are the two methods:

public Observable<Response<GetTopicsByCreatorResponseBody>> getTopicsByCreator(GetTopicsByCreatorRequest request) {
    return getApi()
            .getTopics(request)
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread());
}

public void getTopicsByCreator(GetTopicsByCreatorRequest request) {
    mBinding.swipeRefreshContainer.setRefreshing(true);
    Log.d(TAG, "getTopicsByCreator: in request" + request.getUserId());
    Disposable disposable = mViewModel.getTopicsByCreator(request)
            .subscribe((Response<GetTopicsByCreatorResponseBody> response) -> {
                if (response.body() != null) {
                    GetTopicsByCreatorResponseBody body = response.body();
                    if (body.getTopics() != null) {
                        Log.d(TAG, "getTopicsByCreator: inside" + body.getTopics().get(0).getCreator().getUserId());
                        List<Topic> topics = body.getTopics();
                        Log.d(TAG, "getTopicsByCreator: " + topics.size());
                        RecyclerView.Adapter adapter = mBinding.recyclerForumTopicsForUser.getAdapter();
                        if (adapter instanceof GetTopicsByCreatorAdapter) {
                            ((GetTopicsByCreatorAdapter) adapter).setTopics(topics);
                            Log.d(TAG, "getTopicsByCreator: instanceof" + ((GetTopicsByCreatorAdapter) adapter).getItemCount());
                        } else {
                            adapter = GetTopicsByCreatorAdapter.getInstance(topics);
                            Log.d(TAG, "getTopicsByCreator: new instance" + ((GetTopicsByCreatorAdapter) adapter).getItemCount());
                            mBinding.recyclerForumTopicsForUser.setAdapter(adapter);
                        }
                    }
                }
            }, (Throwable ex) -> {
                Log.e(TAG, "getTopicsByCreator: " + ex.getClass().getSimpleName() + ": " + ex.getMessage());
            });
    mViewModel.addDisposable(disposable);
    mBinding.swipeRefreshContainer.setRefreshing(false);
}

Here are the logs. Let me first clarify that the first half is correct. First line, "in request" user id is ABC and the first number 2 represents topics.size() and the "new instance" number 2 represents itemCount (also topics.size() in the adapter) after the adapter has been initiated. The topic size of 2 is what it supposed to be for user ABC.

I sign out user ABC and sign in a different user with id DEF. So naturally, it also first logs "in request" with the new user id. Good. "inside" is also logging correctly. The topics.size() of 1 is logging correctly.

Now here comes the strange part. "new instance", which is again the itemCount for the topics.size() in the adapter after it had been initiated, somehow is equal to 2, not 1. As you can see, the log with "new instance" comes after adapter has been initiated to getInstance(topics), a new adapter instance with topics set changed.

Also weird, after I close the app, and open the app again, the data for user DEF seem to go back to normal.

Also, as a note, all this is happening in a fragment, not an activity

D/GetTopicsByCreatorFragment: getTopicsByCreator: in requestABC
D/GetTopicsByCreatorFragment: getTopicsByCreator: insideABC
D/GetTopicsByCreatorFragment: getTopicsByCreator: 2
D/GetTopicsByCreatorFragment: getTopicsByCreator: new instance2
D/GetTopicsByCreatorFragment: getTopicsByCreator: in requestDEF
D/GetTopicsByCreatorFragment: getTopicsByCreator: insideDEF
D/GetTopicsByCreatorFragment: getTopicsByCreator: 1
D/GetTopicsByCreatorFragment: getTopicsByCreator: new instance2

The getInstance(topics) code is as follows:

public static GetForumTopicsByCreatorAdapter getInstance(List<Topic> topics) {
    if (sInstance == null) {
        synchronized (GetForumTopicsByCreatorAdapter.class) {
            if (sInstance == null) {
                sInstance = new GetForumTopicsByCreatorAdapter(topics);
                sInstance.notifyDataSetChanged();
            } else {
                sInstance.setTopics(topics);
            }
        }
    }
    return sInstance;
}

public void setTopics(List<Topic> topics) {
    mTopics = topics;
    notifyDataSetChanged();
}

Answer 1

public static GetForumTopicsByCreatorAdapter getInstance(List<Topic> topics) {

   // two checks for null 
    if (sInstance == null) { //one here
        synchronized (GetForumTopicsByCreatorAdapter.class) {
            if (sInstance == null) {  //and one here
                sInstance = new GetForumTopicsByCreatorAdapter(topics);
                sInstance.notifyDataSetChanged();
            }  else  {
                sInstance.setTopics(topics);
            }
        }
    }
    /// at this point, if instance was not null, nothing is done on it and is returned as is.
     /// So new items are not added.
    return sInstance;
}

This code is incorrect. Look at it and think about what happens when instance is not null.

public static GetForumTopicsByCreatorAdapter getInstance(List<Topic> topics) {

    // this is what heppens when instance is not null. topics variable is not used
    return sInstance;
}