有没有flutter高手帮看下这段isolate代码为什么没有输出啊？

Question

最近在学习 dart 的 isolate ，自己写了一段，没有输出…… 麻烦大佬们帮看下问题出在哪里？代码如下：

import 'dart:isolate';

/// isolate06_my_test.dart
/// 抽象一个类来存储自己的SendPort和要传递的消息，每次发送消息时都使用这个类来包装消息
///
///


/// [MessageWrap] 包装了消息和端口
class MessageWrap<T> {
  final SendPort sendPort;
  final T message;
  final int count;
  MessageWrap(this.sendPort, this.message, this.count);
}

/// main
main() async {
  int num = 1;
  ReceivePort mainReceivePort = ReceivePort();
  await Isolate.spawn(topNewIsolateFun, mainReceivePort.sendPort);
  mainReceivePort.listen((wrap){
    if (!wrap is MessageWrap) throw Exception("消息类型传输错误！");
    wrap = wrap as MessageWrap;
    final msg = wrap.message.toString();
    print("Main Isolate 接收到的消息： ${msg}");
    final count = wrap.count;
    if (num == 5) {
      mainReceivePort.close();
    } else {
      print(wrap.message.toString());
      wrap.sendPort.send(MessageWrap(mainReceivePort.sendPort,
          "${count+1} : Main Isolate 向您问好！", count+1));
    }
  });
}

topNewIsolateFun(SendPort sendPort) async {
  ReceivePort localReceivePort = ReceivePort();
  localReceivePort.listen((wrap){
    if (!wrap is MessageWrap) throw Exception("消息类型传输错误！");
    wrap = wrap as MessageWrap;
    final msg = wrap.message.toString();
    print("New Isolate 接收到的消息： ${msg}");
    final count = wrap.count;
    if (msg == 'q') {
      localReceivePort.close();
    } else {
      print(wrap.message.toString());
      wrap.sendPort.send(MessageWrap(localReceivePort.sendPort,
          "${count+1} : New Isolate 向您问好！", count+1));
    }
  });
}

=== add at 2022年1月24日 22:52:01 ===

再次打开电脑审视代码，发现 topNewIsolateFun 方法中的参数 sendPort 没有被使用过；也就是子isolate没有向main isolate发送过消息……

而且，代码中的类型判断也有问题。 必须是这样的： MessageWrap<String> 。

修改后的代码如下（可以正常运行）：

import 'dart:isolate';

/// isolate06_my_test2.dart
///
///

/// [MessageWrap] 包装了消息和端口
class MessageWrap<T> {
  final SendPort sendPort;
  final T message;
  final int count;
  MessageWrap(this.sendPort, this.message, this.count);
}

/// main
main() async {
  ReceivePort mainReceivePort = ReceivePort();
  await Isolate.spawn(topNewIsolateFun, mainReceivePort.sendPort);
  mainReceivePort.listen((wrap){
    bool flag = wrap is MessageWrap<String>;
    if (!flag) throw Exception("消息类型传输错误！");
    //wrap = wrap as MessageWrap;
    //final msg = wrap.message.toString();
    //print("Main Isolate 接收到的消息： ${msg}");
    final count = wrap.count;
    if (count == 5) {
      mainReceivePort.close();
    } else {
      print(wrap.message.toString());
      wrap.sendPort.send(MessageWrap(mainReceivePort.sendPort,
          "${count+1} : Main Isolate 向您问好！", count+1));
    }
  });
}

topNewIsolateFun(SendPort sendPort) async {
  ReceivePort localReceivePort = ReceivePort();
  localReceivePort.listen((wrap){
    print(">> wrap's runtimeType is : ${wrap.runtimeType}");
    bool flag = wrap is MessageWrap<String>;
    //print("flag: $flag");
    if (!flag) throw Exception("消息类型传输错误！");
    //wrap = wrap as MessageWrap;
    final msg = wrap.message.toString();
    //print("New Isolate 接收到的消息： ${msg}");
    final count = wrap.count;
    if (msg == 'q') {
      localReceivePort.close();
    } else {
      print(wrap.message.toString());
      wrap.sendPort.send(MessageWrap(localReceivePort.sendPort,
          "${count+1} : New Isolate 向您问好！", count+1));
    }
  });
  sendPort.send(MessageWrap(localReceivePort.sendPort, "${1} : New Isolate 向您问好！", 1));
}