android - 架构组件 : Observer keep observing even after removing it on onDestroy

Question

我正在开发一个应用程序，我需要每 30 秒进行一次网络调用，然后删除以前的数据并插入新数据。每次插入新数据时，我都会在 RecyclerView 中显示它。我使用 Handler 进行网络调用，使用 LiveData 观察数据变化。一切正常，只是 Live data observer 触发了多次，因此数据被多次删除和插入，导致 RecyclerView 频繁刷新，导致它每 30 秒闪烁多次。

下面是我试过的代码:

在我的 fragment 中我这样做:

private LiveData<List<RestaurantTablesModel>> mData;
private Observer<List<RestaurantTablesModel>> mObserver;
private TablesViewModel mViewModel;

 @Override
public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View mView = inflater.inflate(R.layout.fragment_tables, container, false);
    ButterKnife.bind(this, mView);

    TablesViewModelFactory factory = InjectorUtils.provideTablesFactory(getActivity());
    mViewModel = ViewModelProviders.of(this, factory).get(TablesViewModel.class);

    setUpUserRecyclerView();

    return mView;

}

private void setUpRecyclerView() {

  mData = mViewModel.getTablesData(mLocationID);

    mObserver = tablesModels -> {
        if (tablesModels != null) {
            mTablesRecyclerAdapter.addTables(tablesModels);
            Log.e(LOG_TAG, "setUpUserRecyclerView: tablesModels");
        }
    };

  mData.observe(this, mObserver);
}

移除 onDestroy 的观察者:

@Override
public void onDestroy() {
   mData.removeObserver(mObserver);
   super.onDestroy();
}

以下是我在 ViewModel 中的方法:

public LiveData<List<TablesModel>> getTablesData(int mLocationID){
    return mRepository.getTablesData(mLocationID);
}

存储库:

public LiveData<List<TablesModel>> getTablesData(int mLocationID){

    LiveData<TablesModel[]> mTablesData = mDataSource.getTablesData();

    mTablesData.observeForever(tablesModels -> {
        mExecutors.diskIO().execute(() -> {

            //Completed: delete old table data if there are conflicts.

            if (tablesModels != null) {
                mDatabaseDao.deleteTables();

                mDatabaseDao.insertTablesData(tablesModels);
            }else {
                Log.e(LOG_TAG, "Nothing: ");
            }
        });
        Log.e("Handlers", "repository getTablesData");
    });

    return mDatabaseDao.getTablesData(mLocationID);
}

数据源:

private MutableLiveData<RestaurantTablesModel[]> mDownloadedTablesModel;

public LiveData<RestaurantTablesModel[]> getTablesData() {
    Log.e("Handlers", "getTablesData");
    fetchTablesData();
    return mDownloadedTablesModel;
}

public void fetchTablesData() {

    if (Utils.isNetworkAvailable(mContext)) {
        NetworkUtils.NetworkInterface mInterface = this;

        handler = new Handler();

        runnableCode = new Runnable() {

            @Override
            public void run() {
                // Do something here on the main thread
                Log.e("Handlers", "Called on network thread");

                URL getTablesURL = NetworkUtils.getAllTableUrl(mContext);

                NetworkUtils.getResponseFromAPI(mContext, getTablesURL, mInterface);

                // Repeat this the same runnable code block again another 30 seconds
                // 'this' is referencing the Runnable object
                handler.postDelayed(this, 30000);
            }
        };

        handler.post(runnableCode);

    } else {
        Log.d(LOG_TAG, "fetchTablesData: No network!");
    }
}

现在的问题是当我的 fragment 被销毁并重新创建时，Observer 被多次触发，这是日志:

09-05 10:28:29.853 3666-3666/? E/TablesFragment: setUpRecyclerView: tablesModels
09-05 10:28:30.039 3666-3666/? E/TablesFragment: setUpRecyclerView: tablesModels
09-05 10:28:30.607 3666-3666/? E/TablesFragment: setUpRecyclerView: tablesModels
09-05 10:28:30.657 3666-3666/? E/TablesFragment: setUpRecyclerView: tablesModels
09-05 10:28:30.669 3666-3666/? E/TablesFragment: setUpRecyclerView: tablesModels
09-05 10:28:30.704 3666-3666/? E/TablesFragment: setUpRecyclerView: tablesModels

而且它触发的次数比以前更多，每次重新创建 fragment 时，我认为观察者被要求重新创建 fragment ，而观察者的前一个实例仍在播放中。

但是如果我要在 OnDestroy 中移除观察者，为什么会这样呢？任何帮助将不胜感激。

编辑:

我更改了代码以检查 LiveData 和 Observer 是否为空，然后仅对其进行初始化。但这无济于事，它仍然被多次调用。

if (mTablesData == null){
        mData = mViewModel.getTablesData(mLocationID);

        if (mObserver == null){
            mObserver = tablesModels -> {
                if (tablesModels != null) {
                    mTablesRecyclerAdapter.addTables(tablesModels);
                    Log.e(LOG_TAG, "setUpUserRecyclerView: tablesModels");
                }
            };

            mData.observe(this, mObserver);
        }

    }

编辑 2:

也试过这个，但效果不佳:

   mTablesData = mViewModel.getTablesData(mLocationID);

    mObserver = tablesModels -> {
        if (tablesModels != null) {
            mTablesRecyclerAdapter.addTables(tablesModels);
            Log.e(LOG_TAG, "setUpRecyclerView: tablesModels");
        }
    };

    if (!mTablesData.hasObservers()) {
        mTablesData.observe(this, mObserver);
    }

Answer 1

所以我们从评论中的实验中学到的，你需要在观察之前检查mTablesData是否已经被观察到，只有没有被观察到时才观察，比如

if (!mTablesData.hasObservers()) {
mTablesData.observeForever(tablesModels -> {
...