java - Firebase ValueEventListener onDataChange() 未被调用

Question

我已经追踪我的代码找到崩溃的根本原因，显然这个构造函数无法更新类的变量。当我试图从此类获取数据时出现空指针异常。您可以安全地假设记录已经在数据库中，它所要做的只是获取数据并将其放入类/对象中。我现在只想获取名称，因为我正在测试对象是否仍为 null。

class Saver{
    private String name;
// constructor of Saver class
public Saver(final String uid) {
        Log.d("Saver.java","Reached here"); // this works
        FirebaseDatabase.getInstance().getReference().addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(final DataSnapshot dataSnapshot) {
                Log.d("Saver.java", "OnDataChange"); // does not work
                if(dataSnapshot.hasChild("users/" + uid)){
                    LoadRecord(dataSnapshot, uid);
                }
                else{
                    // set a new record into the database
                    FirebaseDatabase.getInstance().getReference().child("users/" + uid).setValue(CreateNewRecord(FirebaseAuth.getInstance().getCurrentUser())).addOnCompleteListener(new OnCompleteListener<Void>() {
                        @Override
                        public void onComplete(@NonNull Task<Void> task) {
                            if(task.isSuccessful()){
                                LoadRecord(dataSnapshot, uid);
                            }
                            else{
                                Log.e("Saver.java","Failed to set record in database");
                            }
                        }
                    });
                }
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                  Log.e("LifeSaver.java","Error: " + databaseError.getMessage());
            }
        });
    }

// This function loads the data into the object
private void LoadRecord(DataSnapshot dataSnapshot, String uid){
        Log.d("LifeSaver.java","Uid:"+uid);
        // load the existing record in the database
        Saver temp = dataSnapshot.child("users/" + uid).getValue(Saver.class);
        setName(temp.getName());
    }
}
public void setName(String name) {
    this.name = name;
}
private Saver CreateNewRecord(FirebaseUser firebaseUser){
    Saver saver = new Saver ();
    saver.setName(firebaseUser.getDisplayName());
    Log.d("saver","saver name: " + saver.getName());
    return continuLifeUser;
}

很明显，onDataChange 函数只有在数据库发生变化后才会运行。

1. 如果可能，如何手动触发此功能？
2. 我应该有一个包含该节点子节点的 DataSnapshot 吗？如果是这样，如何？ (请显示代码，以便我可以想象您正在解释的内容)
3. 有更好的方法吗？

---

编辑 1:以下是本应调用 FirebaseDatabase 的构造函数的 logcat 语句:

D/Saver.java: Reached here with hA4hZrBieISwMOZaMYe7m6K5tpI3
I/DynamiteModule: Considering local module com.google.android.gms.firebase_database:4 and remote module com.google.android.gms.firebase_database:6
I/DynamiteModule: Selected remote version of com.google.android.gms.firebase_database, version >= 6
I/art: DexFile_isDexOptNeeded failed to open oat file '/data/dalvik-cache/x86_64/data@data@com.google.android.gms@app_chimera@m@00000004@DynamiteModulesC_GmsCore_prodlmp_alldpi_release.apk@classes.dex' for file location '/data/data/com.google.android.gms/app_chimera/m/00000004/DynamiteModulesC_GmsCore_prodlmp_alldpi_release.apk': Failed to open oat filename for reading: No such file or directory
D/Saver.java: Ended here
D/LoginAct.java: Name: null

Answer 1

要获取users节点下所有用户的所有姓名，请使用以下代码:

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference usersRef = rootRef.child("users");
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        for(DataSnapshot ds : dataSnapshot.getChildren()) {
            String name = ds.child("name").getValue(String.class);
            Log.d("TAG", name);
        }
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
usersRef.addListenerForSingleValueEvent(eventListener);

输出将是:

Michael Ong
//other names

如果只想获取特定用户的名称，请使用以下代码:

FirebaseUser firebaseUser = firebaseAuth.getCurrentUser();
String uid = firebaseUser.getUid();

DatabaseReference rootRef = FirebaseDatabase.getInstance().getReference();
DatabaseReference uidRef = rootRef.child("users").child(uid);
ValueEventListener eventListener = new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        String name = dataSnapshot.child("name").getValue(String.class);
        Log.d("TAG", name);
    }

    @Override
    public void onCancelled(DatabaseError databaseError) {}
};
uidRef.addListenerForSingleValueEvent(eventListener);

输出将只有一条记录:

Michael Ong