Question

您好！

我试图了解Dart中溪流的运作方式这是一个简单的例子：

我们有Publisher

class Publisher {

    StreamController<String> _publishCtrl = new StreamController<String>();
    Stream<String> onPublish;

    Publisher() {
        onPublish = _publishCtrl.stream.asBroadcastStream();
    }

    void publish(String s) {
        _publishCtrl.add(s);
    }
}

Reader

class Reader {
    String name;
    Reader(this.name);
    read(String s) {
        print("My name is $name. I read string '$s'");
    }
}

简单函数main()：

main() {
    Publisher publisher = new Publisher();

    Reader john = new Reader('John');
    Reader smith = new Reader('Smith');

    publisher.onPublish.listen(john.read);
    publisher.onPublish.listen(smith.read);

    for (var i = 0; i < 5; i++) {
        publisher.publish("Test message $i");
    }
}

作为代码的结果，我从阅读器John获得5条控制台消息，从阅读器Smith获得5条消息。

My name is John. I read string 'Test message 0'
My name is Smith. I read string 'Test message 0'
My name is John. I read string 'Test message 1'
My name is Smith. I read string 'Test message 1'
My name is John. I read string 'Test message 2'
My name is Smith. I read string 'Test message 2'
My name is John. I read string 'Test message 3'
My name is Smith. I read string 'Test message 3'
My name is John. I read string 'Test message 4'
My name is Smith. I read string 'Test message 4'

一切正常。但是，如果我尝试更改周期for，那么在2步读取Smith阻止接收消息之后，该消息将只接收读者John。以下是更改函数main ()的示例：

    main() {
        Publisher publisher = new Publisher();

        Reader john = new Reader('John');
        Reader smith = new Reader('Smith');

        publisher.onPublish.listen(john.read);
        var smithSub = publisher.onPublish.listen(smith.read);

        for (var i = 0; i < 5; i++) {
            publisher.publish("Test message $i");

            if (i > 2) {
                smithSub.cancel();
            }
        }
    }

如果您运行此代码，则控制台只会通过 John ：

My name is John. I read string 'Test message 0'
My name is John. I read string 'Test message 1'
My name is John. I read string 'Test message 2'
My name is John. I read string 'Test message 3'
My name is John. I read string 'Test message 4'

但我认为应该有来自读者史密斯的3条消息。

如果我知道的话，请告诉我吗？如果没有，请帮助我，理解为什么会发生这种情况。

非常感谢。

Answer 1

创建同步StreamController

StreamController<String> _publishCtrl = new StreamController<String>(sync: true);

或允许控制器在发送新项目之前处理这些项目

  int i = 0;
  Future.doWhile(() {
    i++;

    publisher.publish("Test message $i");

    if (i > 2) {
      subscriptions
        ..forEach((s) => s.cancel())
        ..clear();
    }
    return i < 5;
  }

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