How do I change Stream<List<T1>> to Stream<List<T2>> in Flutter-CodePudding

New to Flutter so be gentle...

I want to isolate a specific package from the rest of my app to minimise the spread of changes should the package need to be replaced.

The package in question provides a Stream<List<TYPE1>> and I want to provide a Stream<List<TYPE2>> where each TYPE2 item in my list is created from some properties of each corresponding TYPE1 item in the original list.

The resultant Stream<List<TYPE2>> will then be made available to the rest of the app by means of a StreamProvider.

Hope someone can point me in the right direction?

CodePudding user response：

Hopefully that will be of help:

void main() {
  final intsStream = Stream.fromIterable([
    [1, 2, 3],
    [4, 5, 6],
  ]);

  final mappedIntStream = intsStream.map(
    (list) => list
        .map(
          (number) => (number * 2).toString(),
        )
        .toList(),
  );

  mappedIntStream.forEach(
    (list) => print(
      "list contents: ${list}, first element type: ${list.first.runtimeType}",
    ),
  );
}

Basically, just use map method on stream, which will allow you to modify each list, and then use map on list so you can change list contents

CodePudding user response：

This is a very common use case that you can want to transfer an existing stream to another stream. For example, You have a stream of Int that you want to transform to String. The most general approach is to create a new stream that waits for events on the original stream and then outputs new events. Example:

void main() {
  Stream<List<int>> listGenerator(Duration interval, [int? maxCount]) async* {
    int i = 0;
    List<int> list = [];
    while (true) {
      await Future.delayed(interval);
      i  ;

      list.add(i);
      yield list;

      if (i == maxCount) break;
    }
  }

  // stream with Stream<List<TYPE1>>
  final stream = listGenerator(Duration(seconds: 3), 10);

  // define transformer
  Stream<List<String>> streamTransformer(Stream<List<int>> source) async* {
    await for (final chunk in source) {
      final list = <String>[];
      for (final item in chunk) {
        list.add('${item.toString()}_string'); // the conversion can happen here
      }
      yield list;
    }
  }

  // transform
  // stream with Stream<List<TYPE2>>
  final newStream = streamTransformer(stream);

  // print out put
  newStream.forEach(print);
}

For many common transformations, you can use Stream-supplied transforming methods such as map(), where(), expand(), and take().

If you take the example above, the easier way to transform would be

void main() {
  Stream<List<int>> listGenerator(Duration interval, [int? maxCount]) async* {
    int i = 0;
    List<int> list = [];
    while (true) {
      await Future.delayed(interval);
      i  ;

      list.add(i);
      yield list;

      if (i == maxCount) break;
    }
  }

  // stream with Stream<List<TYPE1>>
  final stream = listGenerator(Duration(seconds: 3), 10);

  final newStream = stream.map(
    (list) {
      return list.map(
        ((item) {
          return '${item}_string';
        }),
      ).toList();
    },
  );

  // print out put
  newStream.forEach(print);
}

So, your output would become in both cases

[1_string]
[1_string, 2_string]
[1_string, 2_string, 3_string]
[1_string, 2_string, 3_string, 4_string]
[1_string, 2_string, 3_string, 4_string, 5_string]
[1_string, 2_string, 3_string, 4_string, 5_string, 6_string]
[1_string, 2_string, 3_string, 4_string, 5_string, 6_string, 7_string]
[1_string, 2_string, 3_string, 4_string, 5_string, 6_string, 7_string, 8_string]
[1_string, 2_string, 3_string, 4_string, 5_string, 6_string, 7_string, 8_string, 9_string]
[1_string, 2_string, 3_string, 4_string, 5_string, 6_string, 7_string, 8_string, 9_string, 10_string]