a typesafe wrapper for accessing json

json.dart

class Json extends Iterable<Json> {
  Json(this.value);

  Json.from(String s) : this(jsonDecode(s));

  final dynamic value;

  @override
  String toString() => jsonEncode(value);

  bool get isNull => value == null;

  bool get isBool => value is bool;
  bool get isNum => value is num;
  bool get isInt => value is int;
  bool get isDouble => value is double;
  bool get isString => value is String;
  bool get isArray => value is List;
  bool get isObject => value is Map;

  bool get asBool => value as bool;
  num get asNum => value as num;
  int get asInt => asNum.toInt();
  double get asDouble => asNum.toDouble();
  String get asString => value as String;
  List<Json> get asArray => (value as List).map((e) => Json(e)).toList();
  Map<String, Json> get asObject => (value as Map).map((k, v) => MapEntry(k, Json(v)));

  bool? get optBool => value as bool?;
  num? get optNum => value as num?;
  int? get optInt => optNum?.toInt();
  double? get optDouble => optNum?.toDouble();
  String? get optString => value as String?;
  List<Json>? get optArray => (value as List?)?.map((e) => Json(e)).toList();
  Map<String, Json>? get optObject => (value as Map?)?.map((k, v) => MapEntry(k, Json(v)));

  /// Returns the JSON value at [key] which must be either an `int` or a `String`, depending
  /// on whether the receiver is an array or an object. If the receiver is `null`, `null` is
  /// returned and the [key] is ignored. If the [key] doesn't exist, `Json(null)` is returned.
  Json operator [](dynamic key) {
    if (value == null) return this;
    if (value is List) {
      final index = (key as num).toInt();
      return Json(index >= 0 && index < value.length ? value[index] : null);
    }
    if (value is Map<String, dynamic>) return Json(value[key.toString()]);
    throw TypeError();
  }

  @override
  Iterator<Json> get iterator => asArray.iterator;

  /// Creates a instance of [T] by applying [decode] to the receiver.
  /// If the receiver is `null`, `null` is returned.
  /// The [decode] function should throw a [TypeError] is something doesn't match.
  T? of<T>(T Function(Json) decode) => isNull ? null : decode(value);

  /// Creates a list of instances of [T] by applying [decode] to all array elements if the receiver is an array.
  /// If the receiver is `null`, `null` is returned.
  /// If the receiver isn't an array and isn't `null`, a [TypeError] is thrown.
  /// The [decode] function should throw a [TypeError] is something doesn't match.
  List<T>? mapOf<T>(T Function(Json) decode) => (value as List?)?.map((e) => decode(Json(e))).toList();
}