我正在开发一个应用程序，该应用程序查询github api以获得用户列表，并且我遵循推荐的android architecture component guide。从网络中获取数据后，我将使用Room DB将其存储在本地，然后使用在LiveData对象上观察到的ViewModel将其显示在UI上（这很好）。但是，我希望能够有一个按钮，当且仅当存在网络连接时，单击该按钮将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，将触发两个网络调用，一个来自refreshUserData（），另一个来自已存在的LiveData（在onCreate（）期间触发）。我应如何最好地处理这种情况，以使刷新按钮仅执行一个网络请求，而不执行两个网络请求。 这是我的存储库类：
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);
}
}
您可以找到完整的代码here
答案 0 :(得分：3)
最好的方法是在LiveCreate方法中的MainActivity中注册LiveData并像您一样侦听数据库更改。在FAB onClick中，只需发出网络请求并将其保存到没有LiveData的数据库中即可。 onCreate方法中的LiveData将被触发。