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从 firebase 数据库中检索数据并存储在数组列表中

分类: 技术问答 • 2022-04-16 13:22:02
问题描述：

我正在尝试以 Android 应用程序的形式创建一个小型课程聚合器程序.

I am attempting to create a small course aggregator program in the form of an android application.

我的课程都存储在 Firebase 实时数据库中，可从 Firebase 控制台查看，一切正常.

My courses are all stored in a Firebase Realtime Database, which is viewable from the firebase console and everything appears fine.

问题是我编写了一个 java 方法来连接到数据库，从数据库中检索数据，将数据转换为自定义 Java 对象 Course，将其附加到另一个自定义 Java 对象CourseCardModel，然后将CourseCardModel对象保存到一个ArrayList中.

The issue is that I have written a java method to connect to the DB, retrieve the data from the DB, cast the data as a Custom Java object Course, attach it to another custom Java object CourseCardModel, and then save the CourseCardModel object into an ArrayList.

与数据库的连接成功建立，它生成的快照包含正确的信息，我通过循环遍历快照并打印每个课程的变量课程名称来验证这一点 对象输出成功.问题是，当方法完成时，由于某种原因，arrayList 返回的是空的，即使在整个快照迭代过程中检查 ArrayList 的大小时，我也可以看到 CardModel 对象被添加到其中.

The connection to the database is made successfully, and the snapshot it generates contains the correct information, which I have verified by looping through the snapshot and printing a variable Course Name of each Course object out successfully. The problem is that when the method completes, for some reason the arrayList is returns is empty, even though upon checking the size of the ArrayList throughout the snapshot iteration I can see the CardModel objects being added to it.

如果有人在解决此问题上有任何帮助，我将不胜感激，我对 Firebase 还很陌生.

If anybody has any help in solving this it would be hugely appreciated, I am fairly new to Firebase.

GenerateCourses() 方法

private ArrayList<CourseCardModel> generateCourseCards() {

    courseCardModelList = new ArrayList<CourseCardModel>();
    cardModel = new CourseCardModel();

    dbref =  FirebaseDatabase.getInstance().getReference().child("courses");


    // Retrieve the course data from firebase db and cast as Course object
    dbref.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot snapshot) {
            Log.e("Count " ,"" + snapshot.getChildrenCount());
            for (DataSnapshot postSnapshot: snapshot.getChildren()) {

                c = postSnapshot.getValue(Course.class);
                System.out.println("COURSE INFO: " + c.getCourseName());

                cardModel.setCourse(c);

                courseCardModelList.add(cardModel);

                System.out.println("COURSE CARD MODEL LIST SIZE: " + courseCardModelList.size());

            }
        }
        @Override
        public void onCancelled(DatabaseError databaseError) {
            Log.e("The read failed: ", databaseError.getMessage());
        }

    });

    System.out.print("END OF METHOD ARRAY SIZE CHECK: " + courseCardModelList.size());

    return courseCardModelList;

}

LOGCAT 输出

01-11 09:57:57.105 28709-28780/coursematch.coursematchuk D/FA: Connected to remote service
01-11 09:57:57.105 28709-28780/coursematch.coursematchuk V/FA: Processing queued up service tasks: 4
01-11 09:57:59.625 28709-28709/coursematch.coursematchuk E/Count: 200
01-11 09:57:59.650 28709-28709/coursematch.coursematchuk I/System.out: END OF METHOD ARRAY SIZE CHECK: 0COURSE INFO: Physiotherapy BSc (Hons)
01-11 09:57:59.650 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 1
01-11 09:57:59.650 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: History and Archaeology BA (Hons)
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 2
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Arabic and French MA (Hons)
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 3
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Social Policy and Spanish BA (Hons)
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 4
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Modern Languages (French and Spanish) and Greek (with year abroad) MA (Hons)
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 5
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Politics BA (Hons)
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 6
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Nursing (Child) BSc (Hons)
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 7
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Electrical and Electronic Engineering MEng (Hons)
01-11 09:57:59.655 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 8
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: English and Russian MA (Hons)
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 9
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: French and Medieval History MA (Hons)
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 10
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: French and Modern History MA (Hons)
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 11
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Physics with Sports Science BSc (Hons)
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 12
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Electronic Engineering with Management BEng (Hons)
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 13
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Optometry BSc (Hons)
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 14
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Social Policy and Sociology BA (Hons)
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 15
01-11 09:57:59.660 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Comparative Literature and French and Russian (with year abroad) MA (Hons)
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 16
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Statistics Economics and Finance BSc (Hons)
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 17
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Mechanical Engineering BEng (Hons)
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 18
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: International Business Management (Spain) BBA (Hons)
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 19
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Ancient History and Archaeology BA (Hons)
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 20
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Electronic Engineering MEng (Hons)
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 21
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Drama and Music BA (Hons)
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 22
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Chemistry BSc (Hons)
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 23
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Medicine MB
01-11 09:57:59.665 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 24
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Linguistics MA (Hons)
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 25
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Aeronautical Engineering BEng (Hons)
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 26
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: International Business and Marketing BSc (Hons)
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 27
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: German and Theatre and Performance BA (Hons)
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 28
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Social Anthropology BA (Hons)
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 29
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Social Policy and Spanish BA (Hons)
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 30
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Modern Languages (French and Spanish) and Greek MA (Hons)
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 31
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Accounting and Mathematics and Statistics BA (Hons)
01-11 09:57:59.670 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 32
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Art History and French MA (Hons)
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 33
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Accounting and Business Analysis and Technology BA (Hons)
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 34
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Modern Languages (Russian and Spanish) and English (with integrated year abroad) MA (Hons)
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 35
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Social Policy and Crime BA (Hons)
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 36
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: International Business and Marketing BSc (Hons)
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 37
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Statistics Economics and a Language BSc (Hons)
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 38
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Social Work BA (Hons)
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 39
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Combined Honours in Social Sciences BA (Hons)
01-11 09:57:59.675 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 40
01-11 09:57:59.680 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Comparative Literature and Russian and Spanish (with year abroad) MA (Hons)
01-11 09:57:59.680 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 41
01-11 09:57:59.680 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Automotive Engineering BEng (Hons)
01-11 09:57:59.680 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 42
01-11 09:57:59.680 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Electrical Engineering MEng (Hons)
01-11 09:57:59.680 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 43
01-11 09:57:59.680 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Comparative Literature and Russian and Spanish MA (Hons)
01-11 09:57:59.680 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 44
01-11 09:57:59.685 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Economics and Finance (with European study) BA (Hons)
01-11 09:57:59.685 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 45
01-11 09:57:59.685 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Mathematics Statistics and Finance BSc (Hons)
01-11 09:57:59.685 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 46
01-11 09:57:59.685 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: French and German Studies BA (Hons)
01-11 09:57:59.685 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 47
01-11 09:57:59.685 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Sociology BA (Hons)
01-11 09:57:59.685 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 48
01-11 09:57:59.685 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Electronic Engineering with Management MEng (Hons)
01-11 09:57:59.685 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 49
01-11 09:57:59.690 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Mechanical Engineering BEng (Hons)
01-11 09:57:59.690 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 50
01-11 09:57:59.690 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Medicine (graduate entry) BMBS
01-11 09:57:59.690 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 51
01-11 09:57:59.690 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Electrical Engineering BEng (Hons)
01-11 09:57:59.690 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 52
01-11 09:57:59.690 28709-28709/coursematch.coursematchuk I/System.out: COURSE INFO: Automotive Engineering MEng (Hons)
01-11 09:57:59.690 28709-28709/coursematch.coursematchuk I/System.out: COURSE CARD MODEL LIST SIZE: 53

答

调用 dbref.addValueEventListener() 的结果在 onDataChange 回调中异步返回...这很可能在您从该方法返回时没有发生，这就是 courseCardModelList 为空的原因.您需要相应地构建您的逻辑.

The results from call to dbref.addValueEventListener() are returned asynchronously in onDataChange callback...which more than likely hasn't occurred at point that you return from that method which is why courseCardModelList is empty. You'll need to structure your logic accordingly.

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