问题

我发现了一个令人困惑的 Dart `Stream` 相等性行为。

```dart
final streamController = StreamController<int>();

final stream1 = streamController.stream;
final stream2 = streamController.stream;

print(stream1 == stream2); // 打印 true
print(identical(stream1, stream2)); // 打印 false

如果 stream2 引用了 stream1，则会按预期工作：

final streamController = StreamController<int>();

final stream1 = streamController.stream;
final stream2 = stream1;

print(stream1 == stream2); // 打印 true
print(identical(stream1, stream2)); // 打印 true

对于 StreamController<int>.broadcast()，一切都是相同的。对于这种令人困惑的行为，有什么解释呢？

<details>
<summary>英文:</summary>

I found out a confusing Dart `Stream` equality behavior.

```dart
final streamController = StreamController&lt;int&gt;();

final stream1 = streamController.stream;
final stream2 = streamController.stream;

print(stream1 == stream2); // Prints true
print(identical(stream1, stream2)); // Prints false

If the stream2 refers to the stream1, it works as expected:

final streamController = StreamController<int>();

final stream1 = streamController.stream;
final stream2 = stream1;

print(stream1 == stream2); // Prints true
print(identical(stream1, stream2)); // Prints true

It all works the same with a StreamController<int>.broadcast(). What would be the explanation for this confusing behavior?

答案1

得分: 2

如 stream 是一个 getter，它每次都可以返回相同的对象，但也可以像在这种情况下一样为每次调用创建新的对象。请查看dart/lib/async/stream_controller.dart：

      // 每次都返回一个新的流。这些流是相等的，但不是相同的。
      Stream<T> get stream => _ControllerStream<T>(this);

评论解释了这种令人困惑的行为。

在dart/lib/async/broadcast_stream_controller.dart中也发生了相同的情况：

      Stream<T> get stream => new _BroadcastStream<T>(this);

      // Return a new stream every time. The streams are equal, but not identical.
      Stream<T> get stream => _ControllerStream<T>(this);

      Stream<T> get stream => new _BroadcastStream<T>(this);