无法读取或写入firebase实时数据库（android studio）。

Posted 2021-04-11

我想我连接到了火力基地，因为我收到了终端的信息，那就是

I/FirebaseInitProvider: FirebaseApp initialization successful

我想我制定的规则是正确的

{
"rules": {
    ".read": true,
    ".write": true
  }
}

这是我的读写代码。

public class MainActivity extends AppCompatActivity {

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

    // Write a message to the database
    FirebaseDatabase database = FirebaseDatabase.getInstance();
    DatabaseReference myRef = database.getReference("message");

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

}

@Override
protected void onStart() {
    super.onStart();
    // Read from the database
    FirebaseDatabase database = FirebaseDatabase.getInstance();
    DatabaseReference myRef = database.getReference("message");
    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);
            System.out.println(value);
            Log.d("T", "Value is: " + value);
        }

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

还有一件很奇怪的事情，它可以打印数值，但数据在控制台中却没有更新。

打印值

I/System.out: Hello, World!
D/T: Value is: Hello, World!

但在控制台...

答案

你必须推送消息，这样它们就不会每次都被更新。

// Write a message to the database
FirebaseDatabase database = FirebaseDatabase.getInstance();
DatabaseReference myRef = database.getReference("message");

//use a map
Map<String, String> message = new HashMap<>();  
message.put("text", "Hello, World!");

myRef.push().setValue(message);

在这之后，你的消息会是这样的:

message
|
|----------randomID
           |-----text : "Hello, World!"
|
|

要阅读消息的文本:

@Override
protected void onStart() {
    super.onStart();
    // Read from the database
    FirebaseDatabase database = FirebaseDatabase.getInstance();
    DatabaseReference myRef = database.getReference("message");
    myRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            for (DataSnapshot ds: dataSnapshot.getChildren()){
              //this loops through all random ids and grabs the text 
              //you can maybe store the text in some list and do whatever.
              String value = ds.child("text").getValue(String.class);
              System.out.println(value);
              Log.d("T", "Value is: " + value);

             }

        }

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

另一答案

    tableUser.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
         tv.setText(dataSnapshot.getValue().toString());
            Log.e(TAG, "onDataChange: " + dataSnapshot.getValue().toString());
            Toast.makeText(MainActivity.this, "" + dataSnapshot, Toast.LENGTH_SHORT).show();
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
    });