# StreamTransformer<S, T> constructor

const StreamTransformer<S, T>(StreamSubscription<T> onListen(Stream<S> stream, bool cancelOnError))

Creates a StreamTransformer based on the given `onListen` callback.

The returned stream transformer uses the provided `onListen` callback when a transformed stream is listened to. At that time, the callback receives the input stream (the one passed to bind) and a boolean flag `cancelOnError` to create a StreamSubscription.

The `onListen` callback does not receive the handlers that were passed to Stream.listen. These are automatically set after the call to the `onListen` callback (using StreamSubscription.onData, StreamSubscription.onError and StreamSubscription.onDone).

Most commonly, an `onListen` callback will first call Stream.listen on the provided stream (with the corresponding `cancelOnError` flag), and then return a new StreamSubscription.

There are two common ways to create a StreamSubscription:

1. by allocating a StreamController and to return the result of listening to its stream. It's important to forward pause, resume and cancel events (unless the transformer intentionally wants to change this behavior).
2. by creating a new class that implements StreamSubscription. Note that the subscription should run callbacks in the Zone the stream was listened to (see Zone and Zone.bindCallback).

Example:

    /// Starts listening to [input] and duplicates all non-error events.
    StreamSubscription<int> _onListen(Stream<int> input, bool cancelOnError) {
      StreamSubscription<String> subscription;
      // Create controller that forwards pause, resume and cancel events.
      var controller = new StreamController<String>(
          onPause: () {
            subscription.pause();
          },
          onResume: () {
            subscription.resume();
          },
          onCancel: () => subscription.cancel(),
          sync: true); // "sync" is correct here, since events are forwarded.

      // Listen to the provided stream using `cancelOnError`.
      subscription = input.listen((data) {
        // Duplicate the data.
        controller.add(data);
        controller.add(data);
      },
          onError: controller.addError,
          onDone: controller.close,
          cancelOnError: cancelOnError);

      // Return a new [StreamSubscription] by listening to the controller's
      // stream.
      return controller.stream.listen(null);
    }

    // Instantiate a transformer:
    var duplicator = const StreamTransformer<int, int>(_onListen);

    // Use as follows:
    intStream.transform(duplicator);

## Implementation

    const factory StreamTransformer(
            StreamSubscription<T> onListen(
                Stream<S> stream, bool cancelOnError)) =
        _StreamSubscriptionTransformer<S, T>;