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我正在试验 Dart,我无法解释两个观察结果。
onDone
”处理程序没有启动。 onPause
”和“onResume
”处理程序只启动一次。 import 'dart:async';
import 'dart:io';
/// This class encapsulates all the necessary data used by the "onValue" event
/// handler (the construct avoids using global variables).
class OnValueHandlerContainer {
static StreamSubscription<int> _streamSubscriber;
static setStreamSubscriber(StreamSubscription<int> stream) {
_streamSubscriber = stream;
}
// This static method is the handler executed when a event is received through
// the stream.
//
// WARNING: you have absolutely no idea when this handler will be executed.
// Do not assume that it will be executed right after the execution of the code
// that emits an event. It may be executed several lines (of codes) below the
// line that emits the event. It may well be executed after the end of the
// script.
static void onValue(int value) {
// At this point: the state of the subscription is (inevitably) "active".
print("onValue: An event has been raised. The associated value is ${value}!");
print(" Pause the subscription. Wait for 1 second. Resume the subscription");
// Note 1: once a Dart function starts executing, it continues executing until
// it exits. When managing interrupts in C, it is necessary to protect
// interrupt handlers from being interrupted. This is not the case in
// Dart : a function (and, thus, an event handler) cannot be interrupted
// by the occurrence of another event.
// => The code below has no sense, other than experimentation.
// Note 2: while paused, the subscription will not fire any events. If it receives
// events from its source, they will be buffered until the subscription
// is resumed.
_streamSubscriber.pause();
sleep(Duration(seconds: 1));
_streamSubscriber.resume();
// At this point: the state of the subscription is "active".
}
}
main() async {
// Create a controller.
// A StreamController gives you a new stream and a way to add events to the stream
// at any point, and from anywhere. The stream has all the logic necessary to handle
// listeners and pausing. You return the stream and keep the controller to yourself.
StreamController<int> sc = StreamController<int>(
onListen: () => print("Controller: the stream has been assigned a listener!"),
onCancel: () => print("Controller: the stream has been canceled!"),
// As you may notice, the event handlers are not executed every time the
// subscription gets paused or resumed.
//
// This behaviour comes from these facts:
// - Dart is single-threaded.
// - An event handler cannot be interrupted: once a Dart function starts
// executing, it continues executing until it exits. In other words, Dart
// functions can’t be interrupted by other Dart code.
// See https://webdev.dartlang.org/articles/performance/event-loop
// - A stream is a FIFO.
onPause: () => print("Controller: the stream has been paused!"),
onResume: () => print("Controller: the stream has been resumed!")
);
// Get the stream created by the stream controller.
// Right now, this stream has no assigned listener.
Stream<int> stream = sc.stream;
print("Does the stream provided by the controller have a listener ? ${sc.hasListener ? 'yes' : 'no'} - the answer should be no.");
// Push values into the stream controlled by the stream controller.
// Because no listener subscribed to the stream, these values are just stored
// into the stream.
for(int i=0; i<3; i++) {
print("Send the value ${i} into the stream.");
sc.add(i);
}
// Add a listener to the stream.
// Now the stream has an assigned listener.
StreamSubscription<int> subscriber = stream.listen(OnValueHandlerContainer.onValue);
OnValueHandlerContainer.setStreamSubscriber(subscriber);
subscriber.onDone(() => print("The subscription is done!"));
print("Does the stream provided by the controller have a listener ? ${sc.hasListener ? 'yes' : 'no'} - the answer should be yes.");
// Wait for 10 seconds.
print("Start waiting for 10 seconds");
Future.delayed(Duration(seconds: 10)).then((var v) => print("10 seconds ellapsed!"));
print("End of script");
}
Does the stream provided by the controller have a listener ? no - the answer should be no.
Send the value 0 into the stream.
Send the value 1 into the stream.
Send the value 2 into the stream.
Controller: the stream has been assigned a listener!
Does the stream provided by the controller have a listener ? yes - the answer should be yes.
Start waiting for 10 seconds
End of script
onValue: An event has been raised. The associated value is 0!
Pause the subscription. Wait for 1 second. Resume the subscription
Controller: the stream has been paused!
onValue: An event has been raised. The associated value is 1!
Pause the subscription. Wait for 1 second. Resume the subscription
onValue: An event has been raised. The associated value is 2!
Pause the subscription. Wait for 1 second. Resume the subscription
Controller: the stream has been resumed!
10 seconds ellapsed!
onDone
”处理程序。 OnValueHandlerContainer::onValue
) 中,我们暂停和恢复订阅。 onDone
”处理程序永远不会执行。我希望它在脚本执行结束时执行,而 Controller 正在被销毁(因此,订阅被关闭)。 onPause
”和“onResume
”处理程序只启动一次。我希望他们被执行 3 次。 最佳答案
你没有得到“完成”事件的原因是你从来没有close
流订阅。
您没有收到更多“暂停”事件的原因是流订阅很聪明。
您要做的第一件事是在任何人收听流之前添加很多事件。你不应该在实际代码中这样做,而是只在 onListen
时才开始添加事件。被调用,并在 onPause
时再次停止被调用,直到订阅恢复。
在这里,流订阅被许多事件填满,然后它传递一个事件,然后订阅被暂停。
订阅会尽职尽责地将其报告给 Controller 。
然后订阅会得到一份简历。这就是它变得聪明的地方。由于它已经有要传递的事件,它不会将简历报告回 Controller 。它现在实际上并不想要更多的事件,有很多要交付的。因此,它以一秒的间隔传递缓冲的事件,直到缓冲区为空。此时,它将恢复报告给 Controller 。
Controller 报告工作已恢复,但由于没有人添加更多事件,也没有人调用 close
, 不会有任何进一步的事情发生。
关于events - CLI Dart : onPause, onResume,onDone 未按预期启动,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/53336297/
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