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A library for working with fixed width files in dart.

This library helps you easily create a Record layout that clearly describes the desired layout size and output formats. You can go both ways - taking a fixed width record and turning it into a dart object, or loading a dart object and outputting a fixed width string.


To get started - first define your Record layout. Note that class constructors are not inherited and you'll need to include them in your own Record definition to get the fromString behavior.

import 'package:intl/intl.dart';
import 'package:fixedwidth/fixedwidth.dart';

class PersonRecord extends Record {
  StringField first_name = StringField(20);
  StringField last_name = StringField(20);
  DateTimeField dob =
      DateTimeField(10, format: DateFormat("yyyy-MM-dd"));
  IntegerField num_siblings = IntegerField(2, defaultValue: 0);
  DecimalField amount_due = DecimalField(8, decimals: 2);

  PersonRecord.fromString(String record) : super.fromString(record);

You can take a Record, populate it, and turn it to a fixed width string.

var record = PersonRecord()
  ..first_name.value = "John"
  ..last_name.value = "Doe"
  ..dob.value = DateTime(1980, 4, 16)
  ..num_siblings.value = 2
  ..amount_due.value = 24.75;
print("Total Record Length: ${record.length()}");

You can also take a fixed width string and turn it into the appropriate dart object. Calling a field's value gives you the dart typed object.

var record2 = PersonRecord.fromString(
  "Benjamin            Franklin            1706-01-171600003.00");

Field Types:


This is the most common field. Just a string, padded to the right.

StringField name = StringField(15);

String Representation: right padded to the given length i.e.: "Peter          "

Dart Value: the raw string assigned ie. "Peter"


DateTimeField is for dates or timestamps. You can set desired output format on the field to control the string output.

Uses the intl library for date formatting.

DateTimeField completedDate = DateTimeField(10, format: DateFormat("yyyy-MM-dd"));

String Representation: Formatted date as defined by format. i.e. 2018-03-16

Dart Value: DateTime - 2018-03-16 14:53:02.030

IntegerField amount = IntegerField(5);

String Representation: zero padded integer: 00300

Dart Value: int - 300


A num type that outputs the desired length w/ decimals. Value will be rounded if necessary to shorten decimal precision.

DecimalField amount = DecimalField(8, decimals: 2);

String Representation: zero padded decimal: 00300.50

Dart Value: num - 300.50


With COBOL programs it used to be common to have implied decimals which means a decimal is implied at a certain spot, but not present.

ImpliedDecimalField amount = ImpliedDecimalField(7, decimals: 2);

String Representation: zero padded decimal: 0030075

Dart Value: num - 300.75


Same thing as Implied Decimal Field, but has a trailing sign that indicates whether the value is positive or negative.

If you'd like a signed integer field, just set decimals to 0

SignedImpliedDecimalField amount = SignedImpliedDecimalField(8, decimals: 2);

String Representation: : 0030075-

Dart Value: num - -300.75

BooleanField isActive = BooleanField();

String Representation: 'Y' or 'N'

Dart Value: bool


Like a BooleanField, but can also be null. This is sometimes used when a value is unset or unknown.

NullBooleanField likesPizza = NullBooleanField();

String Representation: 'Y', 'N', or ' '

Dart Value: bool


ListField is like a nested record, but it is a list of records. Very similar to COBOL occurs functionality.

class ItemRecord extends Record {
    StringField sku = StringField(10);
    IntegerField qty = IntegerField(2);
    DecimalField amount = StringField(9, decimals: 2);

    ItemRecord.fromString(String record) : super.fromString(record);

class Transaction extends Record {
    ListField orderItems = ListField(record: ItemRecord, occurs: 25);
    Transaction.fromString(String record) : super.fromString(record);

String Representation: the entire padded string of the list of declared records

Dart Value: List of Record instances

Other Features


each field type accepts a defaultValue parameter. A field's value is initially set to the default value when it is instantiated.

StringField name = StringField(10, defaultValue: "Peter");


'Peter     '

set autoTruncate = true If you want to automatically truncate the string value to fit the necessary fixed width length (instead of throwing an exception),

This is useful when you store data in fields longer than what the fixed width file layout requirement is.

class MyRecord extends Record {
    bool autoTruncate = true;
    StringField description = StringField(10);

var record = MyRecord()
  ..description.value = "Pigeons are super cool";


'Pigeons are super cool'
'Pigeons ar'
Nested Records

You can nest an entire record among other record fields.

Useful when parts of a record (like an address) are repeated in multiple places and it is less work to define the "sub-record" only once.

class AddressRecord extends Record {
    StringField address = StringField(60);
    StringField city = StringField(30);
    StringField state = StringField(2);
    StringField postalCode = StringField(12);

    AddressRecord.fromString(String record) : super.fromString(record);

class Transaction extends Record {
    AddressRecord billingAddress = AddressRecord();
    AddressRecord shippingAddress = AddressRecord();
    Transaction.fromString(String record) : super.fromString(record);

var txn = Transaction()
  ..billingAddress.address.value = "PO Box 255" = "Des Moines"
  ..billingAddress.state.value = "IA"
  ..billingAddress.state.value = "50306"
  ..shippingAddress.address.value = "123 1st St" = "West Des Moines"
  ..shippingAddress.state.value = "IA"
  ..shippingAddress.state.value = "50266";

See the examples for additional usage samples.

Features and bugs

Please file feature requests and bugs at the issue tracker.