dart:I How to get Total Event Price?

Here I have GroupBy data but I want Total price of event.
How to total this eventPrice?
Please help me, Thank you?

import 'dart:convert';
void main() {
  final data = [
   {
      "FullName":"Harry Potter",
      "DateOfBirth": "2020/02/16",
      "Department":"Branch Operation",
      "BirthDay":"Friday",
      "PriceOfEvent":35000,
   },
   {
      "FullName":"John Wick",
      "DateOfBirth": "2020/02/16",
      "Department":"Finance",
      "BirthDay":"Friday",
      "PriceOfEvent":20000,
   },
   {
      "FullName":"Solomon Kane",
      "DateOfBirth":"2020/02/19",
      "Department":"Loan",
      "BirthDay":"Monday",
      "PriceOfEvent":22000,
   }
];

double total = 0;
 final value = data.fold(<String, List<dynamic>>{}, (Map<String, List<dynamic>> a, b) {
   a.putIfAbsent(b['DateOfBirth'].toString(), () => []).add(b);
   return a;
 }).values
   .where((l) => l.isNotEmpty)
   .map((l) => {
     'DateOfBirth': l.first['DateOfBirth'],
     'BirthDay': l.first['BirthDay'],
     "TotalEventPrice":total+=l.first["PriceOfEvent"],
     'Data': l.map((e) => {
       'Department': e['Department'],
       'FullName': e['FullName'],
     }).toList()
 }).toList();

  print(json.encode(value));
}

I want like this.

[
  {
    "DateOfBirth": "2020/02/16",
    "BirthDay": "Friday",
    "TotalEventPrice": 55000,
    "Data": [
      {
        "Department": "Branch Operation",
        "FullName": "Harry Potter"
      },
      {
        "Department": "Finance",
        "FullName": "John Wick"
      }
    ]
  },
  {
    "DateOfBirth": "2020/02/19",
    "BirthDay": "Monday",
    "TotalEventPrice": 22000,
    "Data": [
      {
        "Department": "Loan",
        "FullName": "Solomon Kane"
      }
    ]
  }
]

Here is your solution

  final groupMap = <String, dynamic>{};
  
  // group by date
  for(var item in data) {
    final birthday = item['DateOfBirth'] as String;
    if(groupMap.containsKey(birthday)) {
      groupMap[birthday].add(item);
    } else {
      groupMap[birthday] = [item];
    }
  }
  
  // groupMap
  // {2020/02/16: [{FullName: Harry Potter, DateOfBirth: 2020/02/16, Department: Branch Operation, BirthDay: Friday, PriceOfEvent: 35000}, {FullName: John Wick, DateOfBirth: 2020/02/16, Department: Finance, BirthDay: Friday, PriceOfEvent: 20000}], 2020/02/19: [{FullName: Solomon Kane, DateOfBirth: 2020/02/19, Department: Loan, BirthDay: Monday, PriceOfEvent: 22000}]}
  
  final groupedList = [];
  
  // iterate each date
  for(var key in groupMap.keys) {
    num total = 0;
    final dataItem = [];
    
    // calculate total event price
    for(var item in groupMap[key]) {
      total += item['PriceOfEvent'] as num;
      dataItem.add({
        "Department": item["Department"],
        "FullName": item["FullName"]
      });
    }
    
    // re-group
    groupedList.add({
      "DateOfBirth": key,
      "BirthDay": groupMap[key].first['BirthDay'],
      "TotalEventPrice": total,
      "Data": dataItem
    });
  }
  
  print(jsonEncode(groupedList));