Dart - 从 Stream Subscription 块返回数据

Question

我使用 listen 方法订阅了一个流，因此我可以在数据进入时对其进行处理。如果我找到了我要查找的内容，我想取消该流并从流订阅块中返回数据。

我有一个可行的实现，但我正在使用 Completer 来阻止函数立即返回并且感觉很混乱，所以我想知道是否有更好的方法来实现这一点。这是我的代码 sn-p：

Future<String> _extractInfo(String url) async {
    var data = <int>[];
    var client = http.Client();
    var request = http.Request('GET', Uri.parse(url));
    var response = await client.send(request);

    var process = Completer();
    var interestingData = '';

    StreamSubscription watch;

    watch = response.stream.listen((value) async {
      data.clear(); //clear the previous stale data from the list so searching can be faster
      data.addAll(value);

      if (
        hasInterestingData(data) //search the bytelist for the data I'm looking for
      ){
        interestingData = extractInterestingData(data) //extract the data if it's been found. This is what I want my function to return;
        await watch.cancel(); //cancel the stream subscription if the data has been found. ie: Cancel the download
        process.complete('done'); //complete the future, so the function can return interestingData.
      }
    });

    watch.onDone(() {
      //complete the future if the data was not found...so the function will return an empty string
      if (!process.isCompleted) {
        process.complete('done');
      }
    });

    await process.future; //blocks the sync ops below so the function doesn't return immediately
    client.close();
    return interestingData;
  }

Answer 1

当您在 async 函数中时，您应该使用 await for 来处理此类事情。

  Future<String> _extractInfo(String url) async {
    var client = http.Client();
    var request = http.Request('GET', Uri.parse(url));
    var response = await client.send(request);

    await for (var data in response.stream) {
      if (hasInterestingData(data)) {
        return extractInterestingData(data); // Exiting loop cancels the subscription.
      }
    }
    return "done";
  }

然而，虽然这种方法等同于您的代码，但它可能同样存在缺陷。 除非您正在寻找一个单字节，否则您要冒着被分割为连续数据事件的风险。你喜欢确实需要保留之前数据数组的某些部分，或者一些总结你已经看到的状态，但是多少取决于你实际寻找的内容。