java - 值事件监听器返回 Null

Question

我的数据结构

我正在尝试从我的数据库中使用与当前登录教师相同的电子邮件从教师那里获取 TeacherId。然后将其存储到一个字符串变量中，然后将其传递给一个适配器类以供再次查询。但每次我尝试使用 ValueEventListener 检索它时，它都会返回 null。

我的代码

public class SchedList extends AppCompatActivity {

    //member vars. ui objects
    public ListView SubSchedListView;
    public TextView schedList;
    public TextView tIDTextView;

    //data vars
    private String email;
    private String teacherid;
    private ArrayList<DataSnapshot> snapshot;

    //classes
    private TeacherAccount ta;
    private subjSchedAdapter mAdapter;

    //
    private DatabaseReference mDatabaseReference;
    private SharedPreferences prefs;

    //listener

    private String x;
    private String y;
    private String z;

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

        mDatabaseReference = FirebaseDatabase.getInstance().getReference().child("teacheraccount").child("TeacherId");

        //ui objects
        SubSchedListView = (ListView) findViewById(R.id.SubSchedListView);
        schedList = (TextView) findViewById(R.id.schedTextView);
        tIDTextView = (TextView) findViewById(R.id.tIDTextView);

        //data vars
        Intent intent = new Intent();
        email = intent.getStringExtra("email");
        snapshot = new ArrayList<>();

    }

    @Override
    public void onStart() {
        super.onStart();

        Intent intent = new Intent();
        Query query = mDatabaseReference.orderByChild("Email").equalTo(intent.getStringExtra("email"));

        query.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                ta = dataSnapshot.getValue(TeacherAccount.class);
                x = ta.getTeacherId(); //the part where it says it returns null
                y = dataSnapshot.getValue(String.class);
                if (dataSnapshot.getKey().equals("TeacherId")) {
                    z = dataSnapshot.getValue(String.class);
                }

                Log.d("WHICH_HAS", x + y + z);
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {

            }
        });

        // Setting the adapter
        Log.d("WHICH_HAS", x + y + z);

        //mAdapter = new subjSchedAdapter(this, mDatabaseReference,teacherid);

        SubSchedListView.setAdapter(mAdapter);

    }

    @Override
    public void onStop() {
        super.onStop();

        mAdapter.cleanup();
    }
}

哪里有问题？ valueListener 应该在 Onstart 中吗？ OnCreate ?还是靠它自己的功能？如果是这样，我在哪一部分调用函数？我还有哪些地方做错了？

如何将字符串数据传递给基础适配器类？

我的基础适配器类

public class subjSchedAdapter extends BaseAdapter{

private Activity mActivity;
private DatabaseReference mDatabaseReference;
private String mTeacherId;
private ArrayList<DataSnapshot> mSnapshotList;
private Query query;
private Context mContext;
// child event listener

private ChildEventListener mListener = new ChildEventListener() {
    @Override
    public void onChildAdded(DataSnapshot dataSnapshot, String s) {

        mSnapshotList.add(dataSnapshot);
        notifyDataSetChanged();

    }

    @Override
    public void onChildChanged(DataSnapshot dataSnapshot, String s) {
        mSnapshotList.add(dataSnapshot);
        notifyDataSetChanged();
    }

    @Override
    public void onChildRemoved(DataSnapshot dataSnapshot) {

    }

    @Override
    public void onChildMoved(DataSnapshot dataSnapshot, String s) {

    }

    @Override
    public void onCancelled(DatabaseError databaseError) {

    }
};


//constructor

public subjSchedAdapter(Activity activity, DatabaseReference ref, String id){

    mActivity = activity;
    //mTeacherId = subjteacherid;
    mDatabaseReference = ref.child("subjects");

    //this is where id = the teacherId I retrieved
    query = mDatabaseReference.orderByChild("teacherid").equalTo(id).limitToFirst(1);

    query.addChildEventListener(mListener);

    mSnapshotList = new ArrayList<>();

}

//view holder

static class ViewHolder{
    TextView subjectCodeAndName;
    TextView time;
    TextView sectionName;
    TextView teacherId;
    LinearLayout.LayoutParams params;
}

@Override
public int getCount() {
    return mSnapshotList.size();
}

@Override
public SubjectScheds getItem(int position) {

    DataSnapshot snapshot = mSnapshotList.get(position);

    return snapshot.getValue(SubjectScheds.class);
}

@Override
public long getItemId(int position) {
    return 0;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {

    if(convertView == null){
        LayoutInflater inflater = (LayoutInflater) mActivity.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        convertView = inflater.inflate(R.layout.subject_sched_list, parent, false);

        final ViewHolder holder = new ViewHolder();
        holder.subjectCodeAndName = (TextView) convertView.findViewById(R.id.subjectCodeAndName);
        holder.time = (TextView) convertView.findViewById(R.id.time);
        holder.sectionName = (TextView) convertView.findViewById(R.id.sectionName);
        holder.teacherId = (TextView) convertView.findViewById(R.id.teacherid);
        holder.params = (LinearLayout.LayoutParams) holder.subjectCodeAndName.getLayoutParams();
        convertView.setTag(holder);
    }

    final SubjectScheds subjectsched = getItem(position);
    final ViewHolder holder = (ViewHolder) convertView.getTag();

    String subjcodeandname = subjectsched.getSubjectCode() + " - " + subjectsched.getSubjectName();
    String time = "time : " + subjectsched.getTime();
    String sectionName = "section : " + subjectsched.getSectionname();
    String teacherId = "teacher : " + subjectsched.getTeacherid();

    holder.subjectCodeAndName.setText(subjcodeandname);
    holder.time.setText(time);
    holder.sectionName.setText(sectionName);
    holder.teacherId.setText(teacherId);

    return convertView;
}

public void cleanup(){

    mDatabaseReference.removeEventListener(mListener);

}

我设置适配器的代码部分

方法调用

@Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for(DataSnapshot finalSnap :dataSnapshot.getChildren()){
                ta = finalSnap.getValue(TeacherAccount.class);
                x = ta.getTeacherId(); //the part where it says it returns null

                tIDTextView.setText(x);

                Display(x);

            }
        }

方法

private void Display(String x){

    mAdapter = new subjSchedAdapter(this,mDatabaseReference,x);
    SubSchedListView.setAdapter(mAdapter);

}

Answer 1

如果您想通过电子邮件订购 OrderbyChiled，那么您得到了错误的数据库引用，然后您想要获取对该特定字段的父项的引用。

根据您的情况，您需要按照以下方式找到对教师帐户的引用:

mDatabaseReference = FirebaseDatabase.getInstance().getReference().child("teacheraccount");