连接firebase Realtime Database 并且在recyclerview中显示读取数

写在之前
一开始连接firebase碰到了几个问题。

首先是按照官网步骤把firebase连接到android app上一直显示连接不上。但是如果看android studio的tool->firebase->realtime database里面显示的是已经connect上了。所以可以skip 掉connect 的那一步
使用realdatabase的时候，按照官网
FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference myRef = database.getReference("message");

myRef.setValue("Hello, World!");

myRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
// This method is called once with the initial value and again
// whenever data at this location is updated.
String value = dataSnapshot.getValue(String.class);
Log.d(TAG, "key is: " + dataSnapshot.getKey());
Log.d(TAG, "Value is: " + value);
}

@Override
public void onCancelled(DatabaseError error) {
// Failed to read value
Log.w(TAG, "Failed to read value.", error.toException());
}
});

log里面打印出来了key和value，但是database中仍然未显示。在网上查询，有说要重启的，有说要关掉adblock的，都试了一下没有用。结果解决办法是…我在windows上重新连接，就成功了…

Realdatabase Asynchronous
存在firebase里面的data格式是：

想要监听数据，则要使用addValueEventListener 一开始的错误代码如下：

public class FolderActivity extends AppCompatActivity {

private ActivityFolderBinding mBinding;
private RecyclerView recyclerView;
private RecyclerView.Adapter mAdapter;
FirebaseDatabase database;
final String TAG = "Database";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mBinding= DataBindingUtil.setContentView(this, R.layout.activity_folder);

final List<Event> eventsList = new ArrayList<>();
database= FirebaseDatabase.getInstance();
//定位到Event
final DatabaseReference eventRef = database.getReference().child("Event");
Log.w(TAG,"Completed connection");
eventRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(@NonNull DataSnapshot datasnapshot) {
// 提取数据
String usernamePublished = datasnapshot.child("usernamePublished").getValue().toString();
String datePublished = datasnapshot.child("datePublished").getValue().toString();
String eventDate =datasnapshot.child("eventDate").getValue().toString();
String eventTime =datasnapshot.child("eventTime").getValue().toString();
String address = datasnapshot.child("address").getValue().toString();
//加入到eventsList里面
eventsList.add(new Event(usernamePublished,datePublished,eventDate,eventTime,address));
Log.w(TAG,"Completed saving data");
}
@Override
public void onCancelled(@NonNull DatabaseError error) {

}
});
eventsList.add(new Event("旋转木马的悲伤1","2020-02-19","2020-02-20","18:00pm","University of Melbourne"));
Log.w(TAG,"second"+eventsList.get(0).getusernamePublished ());
final FolderCellAdapter adapter = new FolderCellAdapter(eventsList);
mBinding.eventRecycler.setLayoutManager(new GridLayoutManager(this,1));
mBinding.eventRecycler.setAdapter(adapter);
}
}

但是当查询log的时候，他的打印顺序是：
“Completed connection”
“旋转木马的悲伤1” （此时eventslist只有一个）
“Completed saving data"（此时eventslist中有两个）

但是app会报错，或者只显示我下面添加的”旋转木马的悲伤1“。查询以后，发现是因为onDataChange是一个asynchronous的方式，当数据从realdatabase加载过来的时候，程序会继续执行。所以其实adapter带进的eventlist是只有一个事件的list。

解决办法
所以解决办法是首先adapter和list都要放在addValueEventListener之前，这样我们可以在onDataChange的时候用到他。
在加载数据之后，告知我们的adapter数据变了。这样他就可以显示出来了。具体代码如下

public class FolderActivity extends AppCompatActivity {

private ActivityFolderBinding mBinding;
private RecyclerView recyclerView;
private RecyclerView.Adapter mAdapter;
FirebaseDatabase database;
final String TAG = "Database";


@Override

protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mBinding= DataBindingUtil.setContentView(this, R.layout.activity_folder);



final List<Event> eventsList = new ArrayList<>();
eventsList.add(new Event("旋转木马的悲伤1","2020-02-19","2020-02-20","18:00pm","University of Melbourne"));
// 这里提前创建
final FolderCellAdapter adapter = new FolderCellAdapter(eventsList);
mBinding.eventRecycler.setLayoutManager(new GridLayoutManager(this,1));
mBinding.eventRecycler.setAdapter(adapter);

database= FirebaseDatabase.getInstance();
final DatabaseReference eventRef = database.getReference().child("Event");
eventRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(@NonNull DataSnapshot datasnapshot) {
String usernamePublished = datasnapshot.child("usernamePublished").getValue().toString();
String datePublished = datasnapshot.child("datePublished").getValue().toString();
String eventDate =datasnapshot.child("eventDate").getValue().toString();
String eventTime =datasnapshot.child("eventTime").getValue().toString();
String address = datasnapshot.child("address").getValue().toString();
eventsList.add(new Event(usernamePublished,datePublished,eventDate,eventTime,address));
//记得告知adapter
adapter.notifyDataSetChanged();

}

@Override
public void onCancelled(@NonNull DatabaseError error) {
}
});

Log.w(TAG,"second"+eventsList.get(0).getDatePublished());
}
```