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 fromRecord behavior.

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

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

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

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

var record = new PersonRecord()
  ..first_name.value = "John"
  ..last_name.value = "Doe"
  ..dob.value = new 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 = new PersonRecord.fromRecord(
  "Benjamin            Franklin            1706-01-171600003.00");

Field Types:


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

StringField name = new 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 = new DateTimeField(10, format: new 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 = new 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 = new 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 = new 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 = new SignedImpliedDecimalField(8, decimals: 2);

String Representation: : 0030075-

Dart Value: num - -300.75

BooleanField isActive = new 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 = new NullBooleanField();

String Representation: Y, N, or

Dart Value: bool


RecordField can be thought of as a "sub-record", or a "record within a record". i.e. Sometimes 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 = new StringField(60);
    StringField city = new StringField(30);
    StringField state = new StringField(2);
    StringField postalCode = new StringField(12);

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

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

String Representation: the entire padded string of the declared record

Dart Value: an instance of the declared Record


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

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

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

class Transaction extends Record {
    ListField orderItems = new ListField(record: ItemRecord, occurs: 25);
    Transaction.fromRecord(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 = new 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 = new StringField(10);

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


'Pigeons are super cool'
'Pigeons ar'

See the examples for additional usage samples.

Features and bugs

Please file feature requests and bugs at the issue tracker.