Firebase数据库下载聊天数据

Question

我尝试从Firebase数据库下载聊天数据（数据结构如下，建议使用Firebase官方网站）。

"chats": {
      "one": {
        "lastMessage": "ghopper: Relay malfunction found. Cause: moth.",
        "timestamp": 1459361875666
      },
      "two": { ... }

},
"members": {
  "one": {
    "member1": "cxgF3EddyMWiYybXdMsYwu5YQsn1",
    "member2": "jdxBDm5MXBdX4zfGoEp9hgHqscn2"
  };
  "two": {
    "member1": "cxgF3EddyMWiYybXdMsYwu5YQsn1",
    "member2": "yGpTEPDfChVmUeEkzvCCY6IjQnA2"
  };
},
"messages": {
  "one": {
    "m1": {
      "name": "cxgF3EddyMWiYybXdMsYwu5YQsn1",
      "message": "The relay seems to be malfunctioning.",
      "timestamp": 1459361875337
    },
    "m2": {
      "name": "jdxBDm5MXBdX4zfGoEp9hgHqscn2",
      "message": "Yes.",
      "timestamp": 1459361875689
    },
  },
  "two": { ... }
}

现在，我正在尝试检查我（作为登录用户）是否参与了对话并仅下载聊天/特定聊天号码下的最后一条消息。此外，我必须下载此聊天中涉及的另一个用户的个人资料图片，然后将最后一条消息和此图片添加到PrefenceScreen。 这是代码：

final FirebaseDatabase database = FirebaseDatabase.getInstance();
        DatabaseReference myRef = database.getReference("members");
        myRef.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                final ArrayList<String> arrayList = new ArrayList<>();
                for (final DataSnapshot snapshot : dataSnapshot.getChildren()) {
                    GenericTypeIndicator<Map<String, Object>> m = new GenericTypeIndicator<Map<String, Object>>() {
                    };
                    Map<String, Object> map = snapshot.getValue(m);

                    if (map.get("member1").equals(myUID)) {
                        arrayList.add(snapshot.getKey());
                        arrayList.add(map.get("member2").toString());
                    } else if (map.get("member2").equals(myUID)) {
                        arrayList.add(snapshot.getKey());
                        arrayList.add(map.get("member1").toString());
                    }
                }
                mStorage = FirebaseStorage.getInstance().getReference();

                for (int i=0, y=1; y<arrayList.size(); i+=2, y+=2) {
                    Log.i("Chat", arrayList.get(i));
                    StorageReference storageReference = mStorage.child("photos").child(arrayList.get(y));
                    try {
                        final File localFile = File.createTempFile("images", "jpg");
                        final int finalI = i;
                        StorageTask<FileDownloadTask.TaskSnapshot> taskSnapshotStorageTask = storageReference.getFile(localFile).addOnSuccessListener(new OnSuccessListener<FileDownloadTask.TaskSnapshot>() {
                            @Override
                            public void onSuccess(FileDownloadTask.TaskSnapshot taskSnapshot) {
                                Bitmap bitmap = BitmapFactory.decodeFile(localFile.getAbsolutePath());

                                Resources res = getResources();
                                final RoundedBitmapDrawable dr =
                                        RoundedBitmapDrawableFactory.create(res, Bitmap.createScaledBitmap(bitmap, 150, 150, false));
                                dr.setCornerRadius(500);

                                DatabaseReference myRef2 = database.getReference("chats").child(arrayList.get(finalI)).child("lastMessage");
                                myRef2.addValueEventListener(new ValueEventListener() {
                                    @SuppressLint("SetTextI18n")
                                    @Override
                                    public void onDataChange(DataSnapshot dataSnapshot) {
                                        String message = dataSnapshot.getValue(String.class);
                                        Log.d("TAG", "Value is: " + message);

                                        PreferenceScreen screen = getPreferenceManager().createPreferenceScreen(getActivity());

                                        Preference preference = new Preference(getActivity());
                                        preference.setTitle(message);
                                        preference.setKey("Hej celkom v pohode");
                                        preference.setIcon(dr);
                                        preference.setSelectable(true);
                                        preference.setOnPreferenceClickListener(new Preference.OnPreferenceClickListener() {
                                            @Override
                                            public boolean onPreferenceClick(Preference preference) {
                                                Intent intent = new Intent(getActivity(), ScrollingActivity.class);
                                                startActivity(intent);
                                                return false;
                                            }
                                        });
                                        screen.addPreference(preference);
                                        setPreferenceScreen(screen);
                                    }
                                    @Override
                                    public void onCancelled(DatabaseError error) {
                                        // Failed to read value
                                        Log.w("TAG", "Failed to read value.", error.toException());
                                    }
                                });
                            }
                        }).addOnFailureListener(new OnFailureListener() {
                            @Override
                            public void onFailure(@NonNull Exception exception) {
                            }
                        });
                    } catch (IOException e ) {
                    }
                }
            }
            @Override
            public void onCancelled(DatabaseError error) {
                // Failed to read value
                Log.w("TAG", "Failed to read value.", error.toException());
            }
        });

代码几乎正在做它应该做的事情。问题是它始终只添加一个Prefence，因为screen.addPreference（preference）;和setPreferenceScreen（屏幕）;方法在onDataChange中。有没有办法在下载文件后如何使用方法？ 而且我认为这段代码很乱，必须以简单的方式下载所有数据。如果你能帮助我，你会非常感激。感谢。