Flutter：没有为类 DataSnapshot 定义方法 forEach

Question

我需要DatabaseReference在 firebase 中迭代一个节点。但是连图书馆里没有那个forEach功能！DataSnapshotfirebase_database

我还尝试使用库中的DataSnapshot对象firebase（其中包含 forEach 函数），但出现错误：

[dart] The argument type '(DataSnapshot) ? List<dynamic>' can't be assigned to the parameter type '(DataSnapshot) ? FutureOr<dynamic>'.

这是我的代码：

getAccountsList() {
  return firebaseDbService.getAccounts().once().then((DataSnapshot snapshot) {
    var list = [];
    snapshot.forEach((DataSnapshot account) => list.add({
      'id': snapshot.key,
      'name': snapshot.child('name').val(),
    }));
    return list;
  });
}

Answer 1

目前尚不清楚您要在代码中做什么，child(String path)并且val()在类中都不存在DataSnapshot，您可以在此处查看：

https://github.com/flutter/plugins/blob/master/packages/firebase_database/lib/src/event.dart#L27

你也不能像这样循环：

for( var values in snapshot.value){
 print("Connected to second database and read ${values}");
}

因为你会得到以下错误：

which means also you cannot use forEach() on the snapshot to iterate.

Let's say you have this database, and you want to get the names:

user
  randomId
     name: John
  randomId
     name: Peter

You need to do the following:

_db=FirebaseDatabase.instance.reference().child("user");
_db.once().then((DataSnapshot snapshot){
   Map<dynamic, dynamic> values=snapshot.value;
   print(values.toString());
     values.forEach((k,v) {
        print(k);
        print(v["name"]);
     });
 });

Here the reference points toward the node users, since snapshot.value is of type Map<dynamic,dynamic> then you are able to do this Map<dynamic, dynamic> values=snapshot.value;.

Then you loop inside the map using forEach() to be able to get the keys and values, you will get the following output:

This line I/flutter ( 2799): {-LItvfNi19tptjdCbHc3: {name: peter}, -LItvfNi19tptjdCbHc1: {name: john}} is the output of print(values.toString());

Both the following lines:

I/flutter ( 2799): -LItvfNi19tptjdCbHc3
I/flutter ( 2799): -LItvfNi19tptjdCbHc1

are the output of print(k);

The other two lines are the output of print(v["name"]);

To add the names into a list do the following inside the forEach():

list.add(v["name"]);
    print(list);