Introduce resolveValue method to Interface and Union types

[ruudk]

Note

This is an alternative (and I think better) approach to:

• [RFC] Pass $objectValue by reference when calling resolveType #1774

This allows transforming the object value after type resolution.

This is useful when you have a single entity that needs different representations. For example, when your data layer returns a generic PetEntity with a type discriminator, but your GraphQL schema has separate Dog and Cat types.

Example:

$PetType = new InterfaceType([
    'name' => 'Pet',
    'resolveType' => static function (PetEntity $objectValue): string {
        if ($objectValue->type === 'dog') {
            return 'Dog';
        }
        return 'Cat';
    },
    'resolveValue' => static function (PetEntity $objectValue) {
        if ($objectValue->type === 'dog') {
            return new Dog($objectValue->name, $objectValue->woofs);
        }
        return new Cat($objectValue->name, $objectValue->meows);
    },
    'fields' => ['name' => ['type' => Type::string()]],
]);

Now field resolvers receive the properly typed Dog or Cat object instead of the generic PetEntity,
allowing for type-safe resolution without needing to transform objects before they reach the type
resolver.

Common use cases:

• Database polymorphism (single table with type column)
• External APIs returning generic objects with type discriminators

[ruudk]

Instead of calling it resolveValue we could also go with transformValue indicating that this is about transforming the value from A to B.

[spawnia]

The name resolveValue makes sense to me, especially considering the symmetry with resolveType.

[spawnia]

Released with https://github.com/webonyx/graphql-php/releases/tag/v15.25.0.