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Flutter – How to convert/wrap Stream exceptions to data

Lets say there is a Stream<Foo> that can throw exceptions but we need to convert it to Stream<Result<Foo>> and instead of throwing, returning a Result<Foo> with the exception.
How can it be done?

For clarity that’s what the Result class looks like:

sealed class Result<T> {
  Result();

  factory Result.success(T data) = ResultSuccess<T>;

  factory Result.error(String message) = ResultError<T>;
}

final class ResultSuccess<T> extends Result<T> {
  final T data;

  ResultSuccess(this.data);
}

final class ResultError<T> extends Result<T> {
  final String message;

  ResultError(this.message);
}
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2

Answers


  1. Chosen as BEST ANSWER
    0

    We can use StreamTransformer.fromHandlers.

    StreamTransformer<Foo>, Result<Foo>.fromHandlers(
  handleError: (error, stackTrace, sink) {
    sink.add(Result.error('Error: $error'));
  },
  handleData: (data, sink) {
    sink.add(Result.success(data));
  },
)

    and then use it with our initial stream:

    final transformer = StreamTransformer<Foo>, Result<Foo>.fromHandlers(
  handleError: (error, stackTrace, sink) {
    sink.add(Result.error('Error: $error'));
  },
  handleData: (data, sink) {
    sink.add(Result.success(data));
  },
)
final resultStream = initialStream.transform(transformer);

  2. 0

    You might also consider writing an "asResult" operator as an extension method which would enable you to then write something like…

    myStream.asResult().listen(...

    Pseudocodish implementation follows …

    extension ResultOperator<T> on Stream<T> {
  Stream<Result<T?>> asResult() {
    final controller = StreamController<Result<T?>>();
    final subscription = listen( ...

    // ... handle error/value/done etc, and add your Results via
    // the sink, before returning the stream.
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