//UserManager
public Observable<User> startSessionForUser(String username) {
    return githubApiService.getUser(username)
            .map(User.UserResponseToUser())
            .doOnNext(new Action1<User>() {
                @Override
                public void call(User user) {
                    userDataStore.createUser(user);
                    startUserSession();
                }
            })
            .subscribeOn(Schedulers.io())
            .observeOn(AndroidSchedulers.mainThread());
}

private boolean startUserSession() {
    User user = userDataStore.getUser();
    if (user != null) {
        Timber.i("Session started, user: %s", user);
        userComponent = userComponentBuilder.sessionModule(new UserModule(user)).build();
        return true;
    }

    return false;
}

//UserManager
public void closeUserSession() {
    Timber.i("Close session for user: %s", userDataStore.getUser());
    userComponent.logoutManager().startLogoutProcess();
    userDataStore.clearUser();
    userComponent = null;
}

// AppComponent.java 

@Singleton
@Component(modules = {
        AppModule.class,
        GithubApiModule.class
})
public interface AppComponent {
    UserComponent.Builder userComponentBuilder();
}


// UserComponent.java 

@UserScope
@Subcomponent(modules = UserModule.class)
public interface UserComponent {
    
    @Subcomponent.Builder
    interface Builder {
        UserComponent.Builder sessionModule(UserModule userModule);
        UserComponent build();
    }

    UserDetailsActivityComponent plus(UserDetailsActivityComponent.UserDetailsActivityModule module);
}

// UserDetailsActivityComponent.java 

@ActivityScope
@Subcomponent(modules = UserDetailsActivityComponent.UserDetailsActivityModule.class)
public interface UserDetailsActivityComponent {
    UserDetailsActivity inject(UserDetailsActivity activity);
}

public abstract class BaseUserActivity extends BaseActivity {

    @Inject
    UserManager userManager;

    @Override
    protected void setupActivityComponent(AppComponent appComponent) {
        appComponent.inject(this);
        setupUserComponent();
    }

    private void setupUserComponent() {
    isUserSessionStarted = userManager.isUserSessionStartedOrStartSessionIfPossible();
    onUserComponentSetup(userManager.getUserComponent());

    //This screen cannot work when user session is not started.
    if (!isUserSessionStarted) {
        finish();
    }
}

    protected abstract void onUserComponentSetup(UserComponent userComponent);

}

public boolean isUserSessionStartedOrStartSessionIfPossible() {
    return userComponent != null || startUserSession();
}

private boolean startUserSession() {
    //This method was described earlier
}

private void setupUserComponent() {
    //...

    //This screen cannot work when user session is not started.
    if (!isUserSessionStarted) {
        finish();
    }
}

//UserDetailsActivity
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_user_details);
    tvUser = (TextView) findViewById(R.id.tvUser);
    tvUserUrl = (TextView) findViewById(R.id.tvUserUrl);

    //This check has to be called. Otherwise, when user is not logged in, presenter won't be injected and this line will cause NPE
    if (isUserSessionStarted()) {
        presenter.onCreate();
    }
}