使用 LiveData 从网络刷新数据

我正在开发一个应用程序，该应用程序查询 github api 以获取用户列表，并且我正在遵循推荐的 android架构组件指南 https://developer.android.com/jetpack/docs/guide。从网络获取数据后，我使用 Room DB 将其存储在本地，然后使用在 LiveData 对象上观察的 ViewModel 将其显示在 UI 上（这工作正常）。但是，我希望能够有一个按钮，当且仅当存在网络连接时，单击该按钮会触发刷新操作并执行网络请求以从 API 获取新数据。 问题是，当我单击该按钮时，会触发两个网络调用，一个来自刷新用户数据（），另一个来自 onCreate（）期间触发的现有 LiveData。我应该如何最好地处理这种情况，以便我的刷新按钮仅执行一个网络请求，而不是像现在这样执行两个网络请求。 这是我的存储库课程：

public class UserRepository {
private final UserDao mUserDao;
private LiveData<List<GitItem>> mAllUsers;
private LiveData<GitItem> mUser;
private final GithubUserService githubUserService;
private final AppExecutors appExecutors;
private final Application application;
private static String LOG_TAG = UserRepository.class.getSimpleName();
private RateLimiter<String> repoListRateLimit = new RateLimiter<>(1, TimeUnit.MINUTES);

   public UserRepository(Application application, GithubUserService githubUserService, AppExecutors appExecutors) {
        this.application = application;
        UserRoomDatabase db = UserRoomDatabase.getDatabase(application);
        mUserDao = db.userDao();
        this.githubUserService = githubUserService;
        this.appExecutors = appExecutors;
    }

    public LiveData<GitItem> getUser(int userId) {
        LiveData<GitItem> user = mUserDao.loadUser(userId);
        Log.d(LOG_TAG, "retrieved user from database successful");
        return user;
    }

    public LiveData<Resource<List<GitItem>>> getAllUsers() {
        //ResultType, RequestType
        /**
         * List<GitItem> is the [ResultType]
         * GithubUser is the [RequestType]
         */
        return new NetworkBoundResource<List<GitItem>, GithubUser>(appExecutors) {

            @Override
            protected void saveCallResult(@NonNull GithubUser item) {
                Log.d(LOG_TAG, "call to insert results to db");
                mUserDao.insertUsers(item.getItems());
            }

            @Override
            protected boolean shouldFetch(@Nullable List<GitItem> data) {
                Log.d(LOG_TAG, "null?" + (data == null));
                Log.d(LOG_TAG, "empty? " + (data.isEmpty()));
                Log.d(LOG_TAG, "rate? " + (repoListRateLimit.shouldFetch("owner")));
                Log.d(LOG_TAG, "should fetch? " + (data.isEmpty() || repoListRateLimit.shouldFetch("owner")));
                return data.isEmpty() || data == null;

            }

            @NonNull
            @Override
            protected LiveData<List<GitItem>> loadFromDb() {
                Log.d(LOG_TAG, " call to load from db");
                return mUserDao.getAllUsers();
            }

            @NonNull
            @Override
            protected LiveData<ApiResponse<GithubUser>> createCall() {
                Log.d(LOG_TAG, "creating a call to network");
                return githubUserService.getGithubUsers("language:java location:port-harcourt");
            }

            @Override
            protected GithubUser processResponse(ApiResponse<GithubUser> response) {
                return super.processResponse(response);
            }
        }.asLiveData();
    }


    public LiveData<Resource<List<GitItem>>> refreshUserData() {
        //ResultType, RequestType
        /**
         * List<GitItem> is the [ResultType]
         * GithubUser is the [RequestType]
         */
        return new NetworkBoundResource<List<GitItem>, GithubUser>(appExecutors) {

            @Override
            protected void saveCallResult(@NonNull GithubUser item) {
                Log.d(LOG_TAG, "call to insert results to db");
                mUserDao.insertUsers(item.getItems());
            }

            @Override
            protected boolean shouldFetch(@Nullable List<GitItem> data) {
                Log.d(LOG_TAG, "refreshUserData");
                return true;
            }

            @NonNull
            @Override
            protected LiveData<List<GitItem>> loadFromDb() {
                Log.d(LOG_TAG, "refreshUserData");
                Log.d(LOG_TAG, " call to load from db");
                return mUserDao.getAllUsers();
            }

            @NonNull
            @Override
            protected LiveData<ApiResponse<GithubUser>> createCall() {
                Log.d(LOG_TAG, "refreshUserData");
                Log.d(LOG_TAG, "creating a call to network");
                return githubUserService.getGithubUsers("language:java location:port-harcourt");
            }

            @Override
            protected GithubUser processResponse(ApiResponse<GithubUser> response) {
                return super.processResponse(response);
            }
        }.asLiveData();
    }

    public Application getApplication() {
        return application;
    }
}

我的 ViewModel 类是：

public class UserProfileViewModel extends AndroidViewModel {
    private UserRepository mRepository;


    public UserProfileViewModel(UserRepository mRepository) {
        super(mRepository.getApplication());
        this.mRepository = mRepository;
      }

    public LiveData<Resource<List<GitItem>>> getmAllUsers() {
        return mRepository.getAllUsers();
      }

    public LiveData<Resource<List<GitItem>>> refreshUserData() {
        return mRepository.refreshUserData();
      }

    public LiveData<GitItem> getUser(int userId) {
        return mRepository.getUser(userId);
      }
    }

我的 MainActivity 类：

    public class MainActivity extends AppCompatActivity implements GithubAdapter.ListItemClickListener {
    private RecyclerView mRecyclerView;
    private UserProfileViewModel mUserViewModel;
    public static final String USER_ID = "userId";
    private ConnectivityManager cm;
    private boolean isConnected;
    private UserRepository mRepository;
    private  GithubUserService mGithubUserService;

    private NetworkInfo activeNetwork;
    private Picasso mPicasso;



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        mGithubUserService = GithubApplication.get(MainActivity.this).getGithubUserService();
        mPicasso = GithubApplication.get(MainActivity.this).getPicasso();

        mRepository = new UserRepository(getApplication(), mGithubUserService, new AppExecutors());


        // the factory and its dependencies instead should be injected with DI framework like Dagger
        ViewModelFactory factory = new ViewModelFactory(mRepository);

        mUserViewModel = ViewModelProviders.of(this, factory).get(UserProfileViewModel.class);

        //  initViews();
        mRecyclerView = findViewById(R.id.users_recycler);
        final GithubAdapter mAdapter = new GithubAdapter(this, this, mPicasso);
        mRecyclerView.setAdapter(mAdapter);
        mRecyclerView.setHasFixedSize(true);
        mRecyclerView.setLayoutManager(new LinearLayoutManager(this));

        cm = (ConnectivityManager) this.getSystemService(Context.CONNECTIVITY_SERVICE);

        getUsers(mAdapter);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //check if there is a network connection
                // if there is a network connection the LoaderManager is called but
                //  displays a message if there's no network connection
                activeNetwork = cm.getActiveNetworkInfo();
                isConnected = activeNetwork != null && activeNetwork.isConnectedOrConnecting();
                if (isConnected) {
                    mUserViewModel.refreshUserData().observe(MainActivity.this, new Observer<Resource<List<GitItem>>>() {
                        @Override
                        public void onChanged(@Nullable Resource<List<GitItem>> listResource) {
//                            Toast.makeText(MainActivity.this, "second" + listResource.status, Toast.LENGTH_LONG).show();
                            Snackbar.make(view, "refresh:" + listResource.status, Snackbar.LENGTH_LONG)
                                    .setAction("Action", null).show();
                            mAdapter.setUsers(listResource.data);
                        }
                    });

                } else {
                    Snackbar.make(view, "no connection", Snackbar.LENGTH_LONG)
                            .setAction("Action", null).show();

                }

            }
        });


    }



 private void getUsers(GithubAdapter mAdapter) {
        mUserViewModel.getmAllUsers().observe(this, new Observer<Resource<List<GitItem>>>() {
            @Override
            public void onChanged(@Nullable Resource<List<GitItem>> listResource) {
                Toast.makeText(MainActivity.this, "" + listResource.status, Toast.LENGTH_LONG).show();
                mAdapter.setUsers(listResource.data);
            }
        });
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onListItemClick(int userId) {
        Intent detailIntent = new Intent(MainActivity.this, 
        DetailActivity.class);
        detailIntent.putExtra(USER_ID, userId);
        startActivity(detailIntent);
      }
    }

你可以找到完整的代码here https://github.com/Ikhiloya/GithubDevs/tree/constructor-DI

最好的方法是在 onCreate 方法的 MainActivity 中注册您的 LiveData 并像您一样监听数据库更改。在 FAB onClick 中，只需发出网络请求并将其保存到数据库，无需 LiveData。 onCreate方法中的LiveData会被触发。