Attributes #

Attributes is a library package for Dart and Flutter mobile developers providing generic data structures for values, properties, identifiers and entities with type and null safe accessors. It also contains utility functions to convert dynamic values to typed null-safe primitive values.

Key features:

• PropertyMap: type and null safe access for Map<String, dynamic> data
• PropertyList: type and null safe access for List<dynamic> data
• Identifier: an identifier, represented as String, int or BigInt
• Entity: a dynamic data entity with optional id and required properties

This package is at BETA stage, interfaces not fully final yet.

Usage #

Property maps #

Dynamic property maps or JSON Objects are often represented as Map<String, dynamic> objects.

Accessing dynamic data from Map<String, dynamic> objects normally means also a need for many checks or type conversions if you cannot be 100% sure that dynamic data is exactly what you are expecting.

Some solutions:

• Serializing JSON inline
  • access Map<String, dynamic> data directly using the [] operator
• Serializing JSON inside model classes
  • to read JSON implement a fromJson(Map<String, dynamic> json) constructor
  • or to output JSON implement a Map<String, dynamic> toJson() method
• Serializing JSON using code generation libraries
  • many library choices like json_serializable or built_value, and others

These solutions are great and working, and they are suggested by the official Dart and Flutter documentation.

However, for use cases when you just need to access dynamic data from some decoded JSON content without code generated classes or even specific model classes, then PropertyMap helps you on type and null safe access to property values.

Imports for examples below:

import 'package:attributes/attributes.dart';

At first, to create a property map, you can simply decode JSON data:

  // sample JSON data
  const sample = '''
    {
      "name": "Dash",
      "type": "mascot",
      "introduced": 2018,
      "fainted": "2021-03-03",
      "language": {
        "name": "Dart",
        "isNullSafe": true,
        "nullProperty": null,
        "nonNullProperty": "nonNull"
      },
      "toolkit": {
        "name": "Flutter",
        "fps": 60.0,
        "platforms": [ "iOS", "Android", "Windows", "macOS", "Linux", "Web"]
      }
    }
  ''';

  // Decode JSON data as a property map.
  final props = PropertyMap.decodeJson(sample); 

PropertyMap has two main type of accessors for primitive values like:

/// Returns a non-null `String` value or throws `FormatException` if data is 
/// unavailable or cannot be converted to a `String`. 
String getString(String key);

/// Returns a nullable `String` value (null when data is unavailable or cannot
/// be converted to a `String`).
String? tryString(String key);

Similar accessors are available also for int, BigInt, num, double, bool and DateTime values.

Some examples to access primitive values from a property map:

  // Access required null-safe properties using type-safe getXXX accessors.
  // These calls throw if a property is missing or does not convert to a type.
  final name = props.getString('name');
  final introduced = props.getInt('introduced');
  final fainted = props.getTimeUTC('fainted');

  // Access optional nullable properties using type-safe tryXXX accessors.
  // These calls never throw but return null if a property is missing or does 
  // not convert to a type. An optional default value cab be given after 
  // null-aware operator `??`.
  final web = props.tryString('web') ?? 'https://flutter.dev/dash';
  
  /// It's easy to check nullable values from accessors of optional properties.
  final users = props.tryBigInt('knownUsers'); 
  if(users != null) {
    print('The number of users ($users) is now known and it is huge!');
  }

  // Hierarchical data is represented by sub property maps (JSON Objects) or 
  // sub property lists (JSON Arrays). For example property maps can be 
  // accessed by "getMap" (required data) or "tryMap" (optional data) accessors.
  final toolkit = props.getMap('toolkit');

  // Numeric values can be clamped to a range if value validation is needed.
  // Min and max limits are optional parameters when accessing num, int, double,
  // or BigInt.
  final fps = toolkit.getDouble('fps', min: 60.0, max: 120.0);

  // As already described dynamic data like JSON may also contain nulls or an 
  // element for a certain key might not exist at all. Sometimes it's reasonable
  // just to check whether an value exists without trying to access it.
  final lang = props.getMap('language');
  if(lang.exists('nullProperty')) {
    // executes when exists and a value is either null or non-null
  }
  if(lang.existsNull('nullProperty')) {
    // executes when exists and a value is null
  }
  if(lang.existsNonNull('nonNullProperty')) {
    // executes when exists and a value is NOT null
  }

  // This is not a check but accesses a boolean value.
  if(lang.getBool('isNullSafe')) {
    print('Dart has stable sound null-safety!');
  }

Other methods available for accessing data from a property map:

/// Returns the number of properties on a property map.
int get length;

/// Get all property keys.  
Iterable<String> get keys;

/// Returns a value at the key, the result can be of any object or null.
Object? operator [](String key);

Property lists #

Dynamic property lists or JSON Arrays are often represented as List<dynamic> objects, at least when handling decoded JSON data.

Just like for property maps, it's possible to decode a property list from JSON using PropertyList.decodeJson factory constructor.

However, below is a example to access an optional property list from a property map (toolkit) of the previous example:

  // Access an optional property list as a nullable variable.
  final platforms = toolkit.tryList('platforms');

  // Trying to get an item by index in a property list (here nullable). Returns
  // null if not available, but in this example should return a String. 
  final android = platforms?.tryString(1);

Property lists also have similar type-safe accessors for nullable and non-null properties as property maps.

Identifiers #

Identifiers could be represented as String or integer values. For dynamic data it's possible that primitive data types for identifiers are not known by code consuming such data.

The Identifier class allows creating an instance from a primitive value that could be either String or integer. Then a client can dynamically check a type, or convert an identifier to String, int or BigInt representation.

This is demonstrated below:

  // Identifiers can be based on String, int or BigInt values, here a String id.
  final dashId = Identifier.fromString('dash-2018');
  if(dashId.isInt) {
    final intId = dashId.asInt();
  } else {
    final stringId = dashId.asString();
  }

Identifier type can be checked using isString, isInteger, isInt and isBigInt properties. When expecting a specific type, asString, asInteger, asInt and asBigInt accessors are used. If not sure about a type, but you want to avoid format exceptions, then tryString, tryInteger, tryInt and tryBigInt returns a value or a null if not available as a requested type.

Entities #

In the context of this package, an Entity represents a structured data entity that has an optional identification by an Identifier object and contains associated property values in a PropertyMap object.

An example how to create an entity (with dashId and props refering to variables from previous examples):

  // An entity contains required properties and an optional id.
  final dash = Entity.of(
    id: dashId,
    properties: props);

The geocore package has a Feature class that extends Entity, and has also geospatial geometry and bounds as fields along with id and properties fields. That is a feature is a geospatial entity object.

Value conversions #

Conversions from JSON elements or other dynamic data structures can be converted to primitive values using utility functions provided by the package. These functions are also used by PropertyMap or PropertyList when accessing primitive property values with methods described earlier like getString, tryString, getInt, etc.

For example there are converter functions:

• String valueToString(Object? value);
  • Converts value to String or throws FormatException if cannot convert.
• int valueToInt(Object? value, {int? min, int? max})
  • Converts value to int or throws FormatException if cannot convert.
  • If provided min and max are used to clamp the returned value.

Similar functions are available for BigInt, double, num, bool and DateTime.

Installing #

The package supports Dart null-safety and using it requires at least Dart 2.12 from the stable channel. Please see the official null-safety migration guide.

In the pubspec.yaml of your project add the dependency:

dependencies:
  attributes: ^0.6.0

All dependencies used by attributes are also ready for null-safety!

Package #

This is a Dart code package named attributes under the dataflow repository.

The package is associated with (but not depending on) the geocore package. The attributes package contains non-geospatial data structures that are extended and utilized by the geocore package to provide geospatial data structures and utilities.

Libraries #

The package contains following mini-libraries:

For example to access a mini library you should use an import like:

import 'package:attributes/entity.dart';

To use all libraries of the package:

import 'package:attributes/attributes.dart';

Authors #

This project is authored by Navibyte.

More information and other links are available at the dataflow repository from GitHub.

License #

This project is licensed under the "BSD-3-Clause"-style license.

Please see the LICENSE.