deserialize method

@override String? deserialize(DartType targetType, String expression, TypeHelperContextWithConfig context, bool defaultProvided,)

Returns Dart code that deserializes an expression representing a JSON literal to into targetType.

If targetType is not supported, returns null.

Let's say you want to deserialize a class Foo by taking an int stored in a JSON literal and calling the Foo.fromInt constructor.

Treating expression as a opaque Dart expression representing a JSON literal, the deserialize implementation could be a simple as:

String deserialize(DartType targetType, String expression) =>
  "new Foo.fromInt($expression)";
```.

Note that [targetType] is not used here. If you wanted to support many
types of [targetType] you could write:

```dart
String deserialize(DartType targetType, String expression) =>
  "new ${targetType.name}.fromInt($expression)";
```.

Implementation

@override
String? deserialize(
  DartType targetType,
  String expression,
  TypeHelperContextWithConfig context,
  bool defaultProvided,
) {
  final memberContent = enumValueMapFromType(targetType);

  if (memberContent == null) {
    return null;
  }

  final jsonKey = jsonKeyForField(context.fieldElement, context.config);

  if (!targetType.isNullableType &&
      jsonKey.unknownEnumValue == jsonKeyNullForUndefinedEnumValueFieldName) {
    // If the target is not nullable,
    throw InvalidGenerationSourceError(
      '`$jsonKeyNullForUndefinedEnumValueFieldName` cannot be used with '
      '`JsonKey.unknownEnumValue` unless the field is nullable.',
      element: context.fieldElement,
    );
  }

  String functionName;
  if (targetType.isNullableType || defaultProvided) {
    functionName = r'$enumDecodeNullable';
  } else {
    functionName = r'$enumDecode';
  }

  context.addMember(memberContent);

  final args = [
    constMapName(targetType),
    expression,
    if (jsonKey.unknownEnumValue != null)
      'unknownValue: ${jsonKey.unknownEnumValue}',
  ];

  return '$functionName(${args.join(', ')})';
}