# StreamTransformer<S, T>.fromHandlers constructor

StreamTransformer<S, T>.fromHandlers({void handleData(S data, EventSink<T> sink), void handleError(Object error, StackTrace stackTrace, EventSink<T> sink), void handleDone(EventSink<T> sink) })

Creates a StreamTransformer that delegates events to the given functions.

Example use of a duplicating transformer:

    stringStream.transform(new StreamTransformer<String, String>.fromHandlers(
        handleData: (String value, EventSink<String> sink) {
          sink.add(value);
          sink.add(value);  // Duplicate the incoming events.
        }));

Transformers that are constructed this way cannot use captured state if they are used in streams that can be listened to multiple times.

    StreamController<String> controller;
    controller = new StreamController.broadcast(onListen: () {
      scheduleMicrotask(() {
        controller.addError("Bad");
        controller.addError("Worse");
        controller.addError("Worst");
      });
    });
    var sharedState = 0;
    var transformedStream = controller.stream.transform(
        new StreamTransformer<String>.fromHandlers(
            handleError: (error, stackTrace, sink) {
      sharedState++; // Increment shared error-counter.
      sink.add("Error $sharedState: $error");
    }));

    transformedStream.listen(print);
    transformedStream.listen(print); // Listen twice.
    // Listening twice to the same stream makes the transformer share the same
    // state. Instead of having "Error 1: Bad", "Error 2: Worse",
    // "Error 3: Worst" as output (each twice for the separate subscriptions),
    // this program emits:
    // Error 1: Bad
    // Error 2: Bad
    // Error 3: Worse
    // Error 4: Worse
    // Error 5: Worst
    // Error 6: Worst

## Implementation

    factory StreamTransformer.fromHandlers(
        {void handleData(S data, EventSink<T> sink),
        void handleError(Object error, StackTrace stackTrace, EventSink<T> sink),
        void handleDone(EventSink<T> sink)}) = _StreamHandlerTransformer<S, T>;