等待不适用于firebase querySnapshot，执行下一行

Question

我有一个异步方法来返回带有条件的firebase查询的文档长度，并且已经在方法中使用await实现。代码如下。

/////////////////////

getUreadMsgCount(groupChatId, id).then((result) {
    unReadCount = result;
    print("COUNT WITHIN getUreadMsgCount Method : $unReadCount");
});

print("UNREAD OUTSIDE METHOD : $unReadCount");

////////////////////////////////

Future getUreadMsgCount(String groupId, String idFrom) async {
    var respectsQuery = Firestore.instance
        .collection('messages')
        .document(groupId).collection(groupId)
        .where('idFrom', isEqualTo: idFrom).where('isSeen', isEqualTo: 0);
    var querySnapshot = await respectsQuery.getDocuments();
    int totalUnread = querySnapshot.documents.length;
    return totalUnread;
}

////////////OUT PUT/////////////////////

I/flutter (28972): UNREAD OUTSIDE METHOD : 0
I/flutter (28972): UNREAD OUTSIDE METHOD : 0
I/flutter (28972): UNREAD OUTSIDE METHOD : 0
I/flutter (28972): COUNT WITHIN getUreadMsgCount Method : 2
I/flutter (28972): COUNT WITHIN getUreadMsgCount Method : 0
I/flutter (28972): COUNT WITHIN getUreadMsgCount Method : 0

如果你注意到，它总是首先执行 OUTSIDE METHOD 行，因此总是得到值为 0

我需要先从初始化的 getUreadMsgCount 方法中获取值，然后继续下一行。有什么帮助吗？

Answer 1

你需要使用await:

var result = await getUreadMsgCount(groupChatId, id);
    unReadCount = result;
  print("COUNT WITHIN getUreadMsgCount Method : $unReadCount");
  print("UNREAD OUTSIDE METHOD : $unReadCount");

这样，异步方法调用后的代码会在获取数据后执行。

原因：

getUreadMsgCount(groupChatId, id).then((result) {
    unReadCount = result;
    print("COUNT WITHIN getUreadMsgCount Method : $unReadCount");
  });

  print("UNREAD OUTSIDE METHOD : $unReadCount");

没有按你的意愿打印，是因为getUreadMsgCount方法是异步的，所以then()方法后面的代码会先被执行，当数据全部取回后，then()方法里面的代码会被处决。

https://dart.dev/codelabs/async-await#execution-flow-with-async-and-await