transaction_daily_balance.dart

class Transaction {
  final int amount;
  final DateTime date;
  final String title;
  final String accountType;
  final String notes;
  final String id;
  final String repeat;
  
  Transaction({
    this.amount,
    this.date,
    this.title,
    this.accountType,
    this.notes,
    this.id,
    this.repeat,
  });
}

Map<String, Transaction> _userTransactions;

int getDailyBalance(DateTime targetDate) {
  // (Start with a list of all the values in the map)
  return _userTransactions.values
  // ...identify all Transaction that have a certain value for the argument date
    .where((t) => t.date == targetDate)
  // Then I need access to the values from the argument amount
    .map((t) => t.amount)
  // ...add up all values for the daily balance
    .reduce((a1, a2) => a1 + a2) 
  // (Default to zero if no matching values are found)
    ?? 0;
}

void main() {
  final now = DateTime.now();
  
  _userTransactions = {
    'one': Transaction(amount: -45, date: now, title: 'socks', accountType: 'timr', notes: 'joa' , id: 'kololdcd', repeat: 'always'),
    'two': Transaction(amount: -60, date: now, title: 'present', accountType: 'timr', notes: 'brother' , id: 'kolofkfrcd', repeat: 'never'),
  } ;
  
  print(getDailyBalance(now));
}