Firebase 侦听器代码未触发

Question

我有一个 RideList 类，它从一个从 Firebase 数据库中检索数据的 Activity 类中调用。但是，当我调试我的程序时，我的 addValueEventListener 中的代码永远不会被访问。

public class RideList {
        private ArrayList<Ride> listofRides;
        public Firebase myFirebase = new Firebase("https://luminous-torch-1510.firebaseio.com/rides");
        Context context;
    public RideList(Context context) {
        this.context = context;
        this.listofRides = new ArrayList <Ride>();
    }

    public ArrayList<Ride> getRides() {
        Firebase.setAndroidContext(context);
        // Attach an listener to read the data at our rides reference
        Query queryRef = myFirebase.orderByChild("timePosted");
        try {
            queryRef.addValueEventListener(new ValueEventListener() {
                @Override
                public void onDataChange(DataSnapshot snapshot) {
                    System.out.println("There are " + snapshot.getChildrenCount() + " rides");
                    for (DataSnapshot postSnapshot : snapshot.getChildren()) {
                        String rideString = postSnapshot.getValue().toString();
                        String[] rideA = rideString.split(" ");
                        String value;
                        for (int i = 0; i < rideA.length - 1; i++) {
                            rideA[i] = rideA[i].substring(rideA[i].indexOf("=") + 1);
                            rideA[i] = rideA[i].substring(0, rideA[i].indexOf(","));
                        }
                        rideA[rideA.length - 1] = rideA[rideA.length - 1].substring(rideA[rideA.length - 1].indexOf("=") + 1);
                        rideA[rideA.length - 1] = rideA[rideA.length - 1].substring(0, rideA[rideA.length - 1].indexOf("}"));


                        double numOfPassengers = Double.valueOf(rideA[6]);
                        double fare = Double.valueOf(rideA[4]);
                        double distance = Double.valueOf(rideA[3]);
                        String origin = rideA[7];
                        String destination = rideA[2];
                        double maxPassengers = Double.valueOf(rideA[5]);
                        String departTime = rideA[1];
                        String arrivalTime = rideA[0];
                        String timePosted = rideA[8];
                        String title = rideA[9];
                        String type1 = rideA[10];
                        boolean type;
                        if (type1.equals("offer"))
                            type = false;
                        else
                            type = true;


                        Ride ride = new Ride(numOfPassengers, fare, distance, origin, destination, maxPassengers, departTime, arrivalTime,
                                timePosted, title, type);
                        listofRides.add(ride);
                    }
                }

                @Override
                public void onCancelled(FirebaseError firebaseError) {
                    System.out.println("The read failed: " + firebaseError.getMessage());
                }
            });
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            return null;
        }
        return listofRides;
    }

    }

从 Activity 类的 OnCreate 函数调用此代码。关于为什么永远不会输入/执行侦听器代码的任何想法？

编辑：这是关于如何在活动类中调用此函数的代码。

list = (ListView) findViewById(R.id.showrides_listView);
Firebase.setAndroidContext(this);
RideList rl = new RideList(this);
ArrayList arrayList = rl.getRides();
// Adapter: You need three parameters 'the context, id of the layout (it will be where the data is shown),
// and the array that contains the data
ArrayAdapter adapter = new ArrayAdapter<Ride>(getApplicationContext(), android.R.layout.simple_spinner_item, arrayList){
    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        View view = super.getView(position, convertView, parent);
        TextView text = (TextView) view.findViewById(android.R.id.text1);
        text.setTextColor(Color.BLACK);
        return view;
    }
};

// Here, you set the data in your ListView
list.setAdapter(adapter);
adapter.notifyDataSetChanged();

FloatingActionButton myFab = (FloatingActionButton)  findViewById(R.id.showrides_fab);
myFab.setOnClickListener(new View.OnClickListener() {
    public void onClick(View v) {
       startCreateRideActivity();
    }
});

Answer 1

您的数据正在从 Firebase 异步加载（然后持续同步）。将Thread.sleep() 放在那里不会改变这一事实。

如果添加一些日志语句，您可以很容易地看到会发生什么：

public ArrayList<Ride> getRides() {
    Query queryRef = myFirebase.orderByChild("timePosted");
    try {
        System.out.println("Adding listener");
        queryRef.addValueEventListener(new ValueEventListener() {
            public void onDataChange(DataSnapshot snapshot) {
                // THIS CODE IS CALLED ASYNCHRONOUSLY
                System.out.println("Got data from Firebase");
            }
            public void onCancelled(FirebaseError firebaseError) {
            }
        });
        System.out.println("Starting sleep");
        Thread.sleep(2000);
    } catch (InterruptedException e) {
        return null;
    }
    System.out.println("Returning rides");
    return listofRides;
}

输出很可能：

添加监听器

开始睡觉

回程

从 Firebase 获取数据

您正在尝试使异步流程同步，这会导致头痛和糟糕的用户体验。我不会在这里写出解决方案，而是链接到我 15 分钟前针对同一问题写的答案：Retrieving ArrayList<Object> from FireBase inner class