android 中的 ViewModel 和可变实时数据无法正常工作

Question

我只是想获取地震列表并从中制作一个数组列表。为了避免在屏幕旋转时多次 API 调用，我使用 ViewModel 实现了可变实时数据 class，但是通过登录我发出网络请求的 QueryUtils class，我发现即使我旋转设备，网络请求仍在进行中。我不明白为什么。这是我的相关 classes:

public class EarthquakeActivity extends AppCompatActivity {

    private static final String LOG_TAG = "MainActivity";
    private MyModel mMyModel;
    private ViewModelProvider mViewModelProvider;
    private RecyclerView mRecyclerView;
    private TextView mEmptyView;
    private EarthquakeAdapter mEarthquakeAdapter;
    private ProgressBar mProgressBar;
    private ConnectivityManager mConnectivityManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.earthquake_activity);
        mEmptyView = findViewById(R.id.empty_view);
        // get the progress bar to indicate loading
        mProgressBar = findViewById(R.id.loading_bar);
        // set a view model provider for the current activity
        mViewModelProvider = new ViewModelProvider(this);
        // get the view model for the class
        mMyModel = mViewModelProvider.get(MyModel.class);
        // find a reference to the {@link RecyclerView} in the layout
        mRecyclerView = findViewById(R.id.earthquakes);
        mConnectivityManager = getSystemService(ConnectivityManager.class);
        mEmptyView.setText("No internet available");
        mEmptyView.setVisibility(View.VISIBLE);
        Handler handler = new Handler(Looper.getMainLooper());
        mConnectivityManager.registerDefaultNetworkCallback(new ConnectivityManager.NetworkCallback(){
            @Override
            public void onAvailable(Network network) {
                handler.post(new Runnable() {
                    @Override
                    public void run() {
                        mEmptyView.setVisibility(View.GONE);
                        mProgressBar.setVisibility(View.VISIBLE);
                        fetchData();
                    }
                });
            }
        });
        Log.e(LOG_TAG,"Visibility changed");
    }

    private void fetchData(){
        // fetch the list of earthquakes
        mMyModel.getEarthquakes().observe(EarthquakeActivity.this, new Observer<ArrayList<Earthquake>>() {
            @Override
            public void onChanged(ArrayList<Earthquake> earthquakes) {
                Log.v(LOG_TAG,"fetching the data");
                // set up recycler view with this data, this will work even if you rotate the device
                setUpRecyclerView(earthquakes);
            }
        });
    }

    private void setUpRecyclerView(ArrayList<Earthquake> earthquakes) {
        if(earthquakes == null || earthquakes.size() == 0) {
            mRecyclerView.setVisibility(View.GONE);
            mProgressBar.setVisibility(View.GONE);
            mEmptyView.setText(R.string.no_data_available);
            mEmptyView.setVisibility(View.VISIBLE);
        }
        else {
            mRecyclerView.setVisibility(View.VISIBLE);
            mEmptyView.setVisibility(View.GONE);
            mProgressBar.setVisibility(View.GONE);
            // create adapter passing in the earthquake data
            mEarthquakeAdapter = new EarthquakeAdapter(EarthquakeActivity.this, earthquakes);
            // attach the adapter to the recyclerView to populate the items
            mRecyclerView.setAdapter(mEarthquakeAdapter);
            // set the layout manager to position the items
            mRecyclerView.setLayoutManager(new LinearLayoutManager(EarthquakeActivity.this));
            Log.e(LOG_TAG,"Recycler view about to be setup");
            // click listener for when an item is clicked
            mEarthquakeAdapter.setClickListener((view, position) -> searchWeb(earthquakes.get(position).getUrl()));
        }
    }
}

我的视图模型class

public class MyModel extends ViewModel {

    private static final String LOG_TAG = "MyModel";
    private MutableLiveData<ArrayList<Earthquake>> mMutableLiveData;

    public MutableLiveData<ArrayList<Earthquake>> getEarthquakes() {
        mMutableLiveData = new MutableLiveData<>();
        // call the API
        init();
        return mMutableLiveData;
    }

    public void init() {
        // perform the network request on separate thread
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        executorService.execute(new Runnable() {
            @Override
            public void run() {
                // create array list of earthquakes
                mMutableLiveData.postValue(QueryUtils.fetchEarthquakeData("https://earthquake.usgs.gov/fdsnws/event/1/query?format=geojson&starttime=2021-06-01&limit=300"));
            }
        });
        executorService.shutdown();

    }


}

和我的 Query Utils class 相关方法只是因为 class 很大

private static String makeHttpRequest(URL url) throws IOException {
        String jsonResponse = "";

        // if url null, return
        if(url == null) {
            return jsonResponse;
        }
        HttpURLConnection urlConnection = null;
        InputStream inputStream = null;
        try {
            urlConnection= (HttpURLConnection)url.openConnection();
            urlConnection.setReadTimeout(10000/*milliseconds*/);
            urlConnection.setConnectTimeout(15000/*milliseconds*/);
            urlConnection.setRequestMethod("GET");
            urlConnection.connect();
            // this is being logged again even after screen rotation
            Log.v(LOG_TAG,"Network request made");

            // if the request was successful(response code 200)
            //then read the input stream and parse the output
            if(urlConnection.getResponseCode() == 200) {
                inputStream = urlConnection.getInputStream();
                jsonResponse = readFromStream(inputStream);
            }
            else{
                Log.e(LOG_TAG,"Error Response code: " + urlConnection.getResponseCode());
            }
        }
        catch (IOException e) {
            Log.e(LOG_TAG,"Problem retrieving the earthquake JSON results",e);
        }
        finally {
            if(urlConnection != null) {
                urlConnection.disconnect();
            }
            if(inputStream != null) {
                inputStream.close();
            }
        }
        return jsonResponse;

请帮我看看哪里出错了。这让我现在很困惑。

Answer 1

看看这里，也许能帮到你 Activity restart on rotation Android

可能在屏幕旋转时您再次调用 onCreate，ViewModel 也是如此

编辑：尝试以这种方式调用 API

 public MutableLiveData<ArrayList<Earthquake>> getEarthquakes() {
        mMutableLiveData = new MutableLiveData<>();
        //call the API
        mMutableLiveData.init();

Answer 2

配置更改时，您的 activity 会重新创建并调用 fetchData() 调用 getEarthquakes() 和 init()。

步骤应该是：

在 viewModel 中创建一个私有的 MutableLiveData
为该 MutableLiveData
在 onCreate() 方法中，您应该在不进行任何调用的情况下注册观察者
然后您应该调用 public 方法来获取数据并将值发布到 MutableLiveData。
这样，您在 UI 中的观察器应该被触发。

您应该通过调用 getEarthquakes() 而不是调用 init() 来注册观察者。如果数据最初为空，则可以在观察者方法中进行空检查。

创建 activity 之后，您应该调用在 viewModel 中创建的 public 方法来获取数据，在您的例子中是 init()。

android 中的 ViewModel 和可变实时数据无法正常工作

ViewModel and Mutable Live Data in android not working properly

java

android

multithreading

android-livedata

android-viewmodel