等待API调用完成RxJava和Retrofit

dependencies {
    implementation 'com.squareup.retrofit2:retrofit:2.9.0'
    implementation 'com.squareup.retrofit2:converter-gson:2.9.0'
    implementation 'com.squareup.retrofit2:adapter-rxjava3:2.9.0'
    implementation 'io.reactivex.rxjava3:rxjava:3.1.3'
    implementation 'io.reactivex.rxjava3:rxandroid:3.0.0'
}

import retrofit2.Retrofit;
import retrofit2.adapter.rxjava3.RxJava3CallAdapterFactory;
import retrofit2.converter.gson.GsonConverterFactory;

public class ApiClient {
    private static final String BASE_URL = "https://api.example.com/";
    private static Retrofit retrofit = null;

    public static Retrofit getClient() {
        if (retrofit == null) {
            retrofit = new Retrofit.Builder()
                    .baseUrl(BASE_URL)
                    .addConverterFactory(GsonConverterFactory.create())
                    .addCallAdapterFactory(RxJava3CallAdapterFactory.create())
                    .build();
        }
        return retrofit;
    }
}

import io.reactivex.rxjava3.core.Observable;
import retrofit2.http.GET;
import retrofit2.http.Path;

public interface ApiService {
    @GET("users/{user}")
    Observable<User> getUser(@Path("user") String user);
}

import io.reactivex.rxjava3.android.schedulers.AndroidSchedulers;
import io.reactivex.rxjava3.core.Observer;
import io.reactivex.rxjava3.disposables.Disposable;
import io.reactivex.rxjava3.schedulers.Schedulers;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ApiService apiService = ApiClient.getClient().create(ApiService.class);

        apiService.getUser("username")
                .subscribeOn(Schedulers.io())
                .observeOn(AndroidSchedulers.mainThread())
                .subscribe(new Observer<User>() {
                    @Override
                    public void onSubscribe(Disposable d) {
                        // 订阅开始时的操作
                    }

                    @Override
                    public void onNext(User user) {
                        // 处理成功的响应
                        Log.d("API Response", user.toString());
                    }

                    @Override
                    public void onError(Throwable e) {
                        // 处理错误
                        Log.e("API Error", e.getMessage());
                    }

                    @Override
                    public void onComplete() {
                        // 订阅完成时的操作
                    }
                });
    }
}