firebase - 如何在第二个流中使用流值？

Question

我尝试了2个流：

  Stream stream1() =>

... //return data from Firestore



  Stream stream2 {

    List listFromStream1;
    stream1().listen((event) {
      listFromStream1 = event.key;
    });

但问题是 listFromStream1 = event.key; 调用太晚了，所以 listFromStream1 为空

怎么解决？