java - 在 Android 和 Java 上复制 Swift 完成处理程序

Question

多年后，我尝试使用 Firebase Firestore 开发一款 Android 应用。我基本上是在尝试复制这个 Swift 函数:

func getCategories(onCompletion completionBlock: @escaping (_ categories: [Category]?, _ error: Error?) -> Void) {

  firestore.collection("cats").getDocuments { (snap, error) in
    guard let snap = snap else {
      completionBlock(nil, error ?? anUnknownError)
      return
    }

    var categories: [Category] = []
    for document in snap.documents {
        let cat = Category.init(data: document.data())
        categories.append(cat)
    }
    completionBlock(categories, nil)
  }
}

但是我不知道swift的 block 相当于什么，甚至不知道它是否存在。

我检查了 Firebase 源代码。 Query.get()返回 Task<QuerySnapshot>所以我试图返回 Task<List<Category>>没有运气。

有什么帮助吗？谢谢。

编辑:添加 Android 代码以阐明我正在尝试做的事情。

public class FirestoreService {

    private static volatile FirestoreService singleton = new FirestoreService();

    public static FirestoreService getInstance() {
        return singleton;
    }

    private FirebaseFirestore firestore() {
        // default firestore instance
        FirebaseFirestore db = FirebaseFirestore.getInstance();

        // default firestore settings
        FirebaseFirestoreSettings settings = db.getFirestoreSettings();

        // firestore settings builder
        FirebaseFirestoreSettings.Builder builder = new FirebaseFirestoreSettings.Builder(settings);

        // enable timstamps
        builder.setTimestampsInSnapshotsEnabled(true);

        // set new settings to db instance
        db.setFirestoreSettings(builder.build());


        // return db with new settings.
        return db;
    }



    public void getProductCategories(Handler? handler) {

       Task<QuerySnapshot> task = firestore().collection("coll").get();
       task.addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
        @Override
        public void onComplete(@NonNull Task<QuerySnapshot> task) {
            try {
                if (task.isSuccessful()) {
                    List<Category> cats = new ArrayList<>();
                    for (QueryDocumentSnapshot doc : task.getResult()) {
                        String id = doc.getId();
                        Map<String, Object> data = doc.getData();
                        Category cat = new Category(id, data);
                        cats.add(cat);
                    }
                    // now I need completion handler
                } else {
                    Log.w("ERROR", "Error getting categories", task.getException());
                }
            } catch (Exception e) {
                Log.e("ERROR", e.getMessage());
            }
        }
    });
    }

}



public class MainActivity extends AppCompatActivity {

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

        FirestoreService.getInstance().getCategories().addCompletionListener(
            // handle List<Category> and respresent in UI
        );
    }
}

Answer 1

非常感谢四位的帮助和领导@Daniel-b。

我现在已经解决了我的问题。

首先，我创建了一个用于处理结果的接口(interface)；正如你所建议的。

public interface ResultHandler<T> {
    void onSuccess(T data);
    void onFailure(Exception e);
}

然后在服务类中，我将 ResultHandler 添加到函数的输入参数中:

public void getUserInfo(String id, ResultHandler<UserInfo> handler) {
    firestore().collection("userInfo").document(id).get().addOnCompleteListener(snap -> {
        if (snap.isSuccessful()) {
           try {
               // failable constructor. use try-catch
               UserInfo info = new UserInfo(snap.getResult().getId(), snap.getResult().getData());
               handler.onSuccess(info);
           } catch (Exception e) {
                handler.onFailure(e);
           }
        } else {
            handler.onFailure(snap.getException())
        }
    });
}

并在Activity中调用服务

FirestoreService.getInstance().getUserInfo("ZsrAdsG5HVYLTZDBeZtkGDlIBW42", new ResultHandler<UserInfo>() {
    @Override
    public void onSuccess(UserInfo data) {
        Log.i("UserInfo", data.id);                    
    }

    @Override
    public void onFailure(Exception e) {
        // getting data failed for some reason
    }
});