从firebase获取数据并将其返回到主线程[重复]

Question

目前遇到我从 firebase 获取数据的问题。我知道这是因为 Firebase 是异步的，所以当我进行 firebase 调用时，它会在自己的线程上执行，并且调用它的当前线程会继续执行。我正在使用来自 firebase 的数据填充对象列表，然后返回该对象列表。问题是，列表总是返回 null，因为 firebase 代码的执行没有及时完成。

我创建了一些从 SQLite db 中获取的异步代码，效果很好，但这种方法似乎不适用于 firebase（我相信这是因为 firebases API 是异步的）这是我从 firebase 返回对象列表的方法.

/** Method to get activity data from firebase.
 * @param userInput the user query to select the data
 * @return a list of activity models based on the query
 * */
public List<ActivityModel> retrieveActivityData(String userInput) {
    Log.d(TAG, "retrieveActivityData: starts");
    List<ActivityModel> models = new ArrayList<ActivityModel>();
    // execute the query in firebase
    CollectionReference activfitCollection = db.collection("activity");
    activfitCollection.orderBy("isoTimestamp")
            .startAt(userInput)
            .endAt(DateHelper.getDayEndingDate(userInput))
            .get()
            .addOnCompleteListener(task -> {
                if (task.isSuccessful()) {
                    Log.d(TAG, "onComplete: Getting data successful!");
                    // check to see if it exists
                    if (!task.getResult().isEmpty()) {
                        for (DocumentSnapshot documentSnapshot : task.getResult().getDocuments()) {
                            Log.d(TAG, "retrieveActivityData: document = " + documentSnapshot.getId());
                            // cast the document to the activity model
                            Log.d(TAG, "retrieveActivityData: document data " + documentSnapshot.getData());
                            ActivityModel model = mapToActivityModel(documentSnapshot);
                            models.add(model);
                            Log.d(TAG, "retrieveActivityData: array size" + models.size());
                        }
                    }

                } else {
                    Log.e(TAG, "onComplete: Error getting documents: ", task.getException());
                }
            });
    Log.d(TAG, "retrieveActivityData: array size outside " + models.size());
    return models;
}

Answer 1

选项 - 1：您可以使用LiveData 来实现此目的。操作完成后将值发布到 LiveData 并观察您的活动或片段中的内容

MutableLiveData<List<ActivityModel>> listMutableLiveData = new MutableLiveData<>();

public MutableLiveData<List<ActivityModel>> retrieveActivityData(String userInput) {

    List<ActivityModel> models = new ArrayList<ActivityModel>();

    ....
        if (task.isSuccessful()) {
            for (DocumentSnapshot documentSnapshot : task.getResult().getDocuments()) {
                ....
                models.add(model);
            }

            //Post value to live data from here
            listMutableLiveData.postValue(models);
        }

    ....

    return listMutableLiveData;
}

然后像这样observe

retrieveActivityData(userInput).observe(this, new Observer<List<ActivityModel>>() {
    @Override
    public void onChanged(List<ActivityModel> activityModels) {
        //you can use list here
    }
});

选项 - 2：您可以在firebase操作完成时使用回调函数获取结果。

• 为回调创建interface

interface FirebaseResultListener {
    void onComplete(List<ActivityModel> activityModels);
}

• 配置您的 retrieveActivityData 以处理此回调

public void retrieveActivityData(String userInput, FirebaseResultListener callback) {

    List<ActivityModel> models = new ArrayList<ActivityModel>();

    ....
    if (task.isSuccessful()) {
        for (DocumentSnapshot documentSnapshot : task.getResult().getDocuments()) {
                ....
            models.add(model);
        }

        //Invoke callback with result from here
        callback.onComplete(models);
    }

    ....
}

• 在您的活动或片段中实现此接口

retrieveActivityData(userInput, new FirebaseResultListener() {
    @Override
    public void onComplete(List<ActivityModel> activityModels) {
        //you can use list here
    }
});

Answer 2

由于数据是异步加载的，无法返回数据，需要在retrieveActivityData方法中传入callback(Interface)，并使用interface的callback来加载数据，查看以下代码

  @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);

        MyFirebaseCallback myFirebaseCallback = new MyFirebaseCallback() {
            @Override
            public void dataLoaded(List<ActivityModel> activityModels) {
                //set the data in recycler view
            }
        };

        retrieveActivityData("myInput",myFirebaseCallback);
    }

    public void retrieveActivityData(String userInput, final MyFirebaseCallback callback) {
        Log.d(TAG, "retrieveActivityData: starts");
        // execute the query in firebase
        CollectionReference activfitCollection = db.collection("activity");
        activfitCollection.orderBy("isoTimestamp")
                .startAt(userInput)
                .endAt(DateHelper.getDayEndingDate(userInput))
                .get()
                .addOnCompleteListener(task -> {
                    if (task.isSuccessful()) {
                        List<ActivityModel> models = new ArrayList<ActivityModel>();
                        Log.d(TAG, "onComplete: Getting data successful!");
                        // check to see if it exists
                        if (!task.getResult().isEmpty()) {
                            for (DocumentSnapshot documentSnapshot : task.getResult().getDocuments()) {
                                Log.d(TAG, "retrieveActivityData: document = " + documentSnapshot.getId());
                                // cast the document to the activity model
                                Log.d(TAG, "retrieveActivityData: document data " + documentSnapshot.getData());
                                ActivityModel model = mapToActivityModel(documentSnapshot);
                                models.add(model);
                                Log.d(TAG, "retrieveActivityData: array size" + models.size());
                            }
                        }

                        callback.dataLoaded(models);

                    } else {
                        Log.e(TAG, "onComplete: Error getting documents: ", task.getException());
                    }
                });
        Log.d(TAG, "retrieveActivityData: array size outside " + models.size());
    }

    interface MyFirebaseCallback{
        void dataLoaded(List<ActivityModel> activityModels);
    }

Answer 3

你猜对了！该查询是异步的，因此retrieveActivityData() 不应返回List<ActivityModel>，否则它将始终为空。一旦您的List 在onComplete() 中编译，您就必须使用事件总线来触发事件，或者使用LiveData 并观察它。

LiveData

ViewModel

EventBus