batcher.dart

import 'dart:async';

///
/// Simulating DB
///

Map<String, String> data = {
  'id1': 'data for id1',
  'id2': 'data for id2',
  'id3': 'data for id3',
  'id4': 'data for id4',
  'id5': 'data for id5',
};

///
/// Simulating API Call
///
Future<Map<String, String>> getDataById(List<String> ids) async {
  Map<String, String> result = {};

  for (final entry in data.entries) {
    if (ids.contains(entry.key)) result.addEntries([entry]);
  }
  // simulating network delay
  await Future.delayed(const Duration(seconds: 1));

  return result;
}

///
/// api Batcher: Functional
///

typedef Future<String> MakeApiCall(String id);

final List<String> requestedIds = [];
Completer? apiBatcherCompleter;

MakeApiCall apiBatcher(Duration batchDelay) {
  return (id) async {
    if (apiBatcherCompleter == null || apiBatcherCompleter!.isCompleted) {
      requestedIds.clear();
      apiBatcherCompleter = Completer();
      apiBatcherCompleter!.complete(Future.delayed(batchDelay)
          .then((value) => getDataById(requestedIds)));
    }
    requestedIds.add(id);
    final resp = await apiBatcherCompleter!.future;
    return Future.value(resp[id]);
  };
}

MakeApiCall makeApiCall = apiBatcher(const Duration(seconds: 1));

/// Run using `dart ./batcher.dart`
main(List<String> args) {
  // first batch
  Future.delayed(const Duration(milliseconds: 200)).then((value) {
    makeApiCall('id1').then((value) {
      print("\nfirst batch");
      print(value);
    });
  });

  Future.delayed(const Duration(milliseconds: 600)).then((value) {
    makeApiCall('id1').then((value) {
      print(value);
    });
  });

  Future.delayed(const Duration(milliseconds: 1100)).then((value) {
    makeApiCall('id1').then((value) {
      print(value);
    });
  });

  // second batch
  Future.delayed(const Duration(milliseconds: 1300)).then((value) {
    makeApiCall('id2').then((value) {
      print("\n\nsecond batch");
      print(value);
    });
  });
  Future.delayed(const Duration(milliseconds: 1600)).then((value) {
    makeApiCall('id2').then((value) {
      print(value);
    });
  });
}

Run using dart ./batcher.dart

Expected output:

first batch
data for id1
data for id1
data for id1


second batch
data for id2
data for id2