Add support for nullable schemas

Sources/_OpenAPIGeneratorCore/FeatureFlags.swift

@@ -27,9 +27,11 @@
 /// 1.0 is released.)
 public enum FeatureFlag: String, Hashable, Codable, CaseIterable {
 
-    /// Has to be here until we add more feature flags, otherwise the enum
-    /// doesn't compile.
-    case empty
+    /// Support for `nullable` schemas.
+    ///
+    /// A dedicated field in OpenAPI 3.0, a `null` value present in
+    /// the `types` array in OpenAPI 3.1.
+    case nullableSchemas
 }
 
 /// A set of enabled feature flags.

Sources/_OpenAPIGeneratorCore/Translator/FileTranslator+FeatureFlags.swift

@@ -15,4 +15,10 @@ import OpenAPIKit
 
 extension FileTranslator {
     // Add helpers for reading feature flags below.
+
+    /// A Boolean value indicating whether the `nullable` field on schemas
+    /// should be taken into account.
+    var supportNullableSchemas: Bool {
+        config.featureFlags.contains(.nullableSchemas)
+    }
 }

Sources/_OpenAPIGeneratorCore/Translator/TypeAssignment/TypeAssigner.swift

@@ -45,6 +45,10 @@ struct TypeAssigner {
     /// safe to be used as a Swift identifier.
     var asSwiftSafeName: (String) -> String
 
+    /// A Boolean value indicating whether the `nullable` field on schemas
+    /// should be taken into account.
+    var supportNullableSchemas: Bool
+
     /// Returns a type name for an OpenAPI-named component type.
     ///
     /// A component type is any type in `#/components` in the OpenAPI document.
@@ -256,7 +260,10 @@ struct TypeAssigner {
         // Check if this type can be simply referenced without
         // creating a new inline type.
         if let referenceableType =
-            try TypeMatcher(asSwiftSafeName: asSwiftSafeName)
+            try TypeMatcher(
+                asSwiftSafeName: asSwiftSafeName,
+                supportNullableSchemas: supportNullableSchemas
+            )
             .tryMatchReferenceableType(for: schema)
         {
             return referenceableType
@@ -452,12 +459,18 @@ extension FileTranslator {
 
     /// A configured type assigner.
     var typeAssigner: TypeAssigner {
-        TypeAssigner(asSwiftSafeName: swiftSafeName)
+        TypeAssigner(
+            asSwiftSafeName: swiftSafeName,
+            supportNullableSchemas: supportNullableSchemas
+        )
     }
 
     /// A configured type matcher.
     var typeMatcher: TypeMatcher {
-        TypeMatcher(asSwiftSafeName: swiftSafeName)
+        TypeMatcher(
+            asSwiftSafeName: swiftSafeName,
+            supportNullableSchemas: supportNullableSchemas
+        )
     }
 }
 

Sources/_OpenAPIGeneratorCore/Translator/TypeAssignment/TypeMatcher.swift

@@ -20,6 +20,10 @@ struct TypeMatcher {
     /// safe to be used as a Swift identifier.
     var asSwiftSafeName: (String) -> String
 
+    /// A Boolean value indicating whether the `nullable` field on schemas
+    /// should be taken into account.
+    var supportNullableSchemas: Bool
+
     /// Returns the type name of a built-in type that matches the specified
     /// schema.
     ///
@@ -78,8 +82,11 @@ struct TypeMatcher {
                 guard case let .reference(ref, _) = schema else {
                     return nil
                 }
-                return try TypeAssigner(asSwiftSafeName: asSwiftSafeName)
-                    .typeName(for: ref).asUsage
+                return try TypeAssigner(
+                    asSwiftSafeName: asSwiftSafeName,
+                    supportNullableSchemas: supportNullableSchemas
+                )
+                .typeName(for: ref).asUsage
             },
             matchedArrayHandler: { elementType in
                 elementType.asArray
@@ -88,7 +95,7 @@ struct TypeMatcher {
                 TypeName.arrayContainer.asUsage
             }
         )?
-        .withOptional(!schema.required)
+        .withOptional(!schema.required || (supportNullableSchemas && schema.nullable))
     }
 
     /// Returns a Boolean value that indicates whether the schema

Sources/swift-openapi-generator/Documentation.docc/Articles/Supported-OpenAPI-features.md

@@ -155,7 +155,7 @@ Supported features are always provided on _both_ client and server.
 - [x] description
 - [x] format
 - [ ] default
-- [ ] nullable (only in 3.0, removed in 3.1)
+- [x] nullable (only in 3.0, removed in 3.1, add `null` in `types` instead)
 - [x] discriminator
 - [ ] readOnly
 - [ ] writeOnly

Tests/OpenAPIGeneratorReferenceTests/SnippetBasedReferenceTests.swift

@@ -101,6 +101,102 @@ final class SnippetBasedReferenceTests: XCTestCase {
         )
     }
 
+    func testComponentsSchemasNullableStringProperty() throws {
+        try self.assertSchemasTranslation(
+            """
+            schemas:
+              MyObj:
+                type: object
+                properties:
+                  fooOptional:
+                    type: string
+                  fooRequired:
+                    type: string
+                  fooOptionalNullable:
+                    type: [string, null]
+                  fooRequiredNullable:
+                    type: [string, null]
+                required:
+                  - fooRequired
+                  - fooRequiredNullable
+            """,
+            """
+            public enum Schemas {
+                public struct MyObj: Codable, Hashable, Sendable {
+                    public var fooOptional: Swift.String?
+                    public var fooRequired: Swift.String
+                    public var fooOptionalNullable: Swift.String?
+                    public var fooRequiredNullable: Swift.String
+                    public init(
+                        fooOptional: Swift.String? = nil,
+                        fooRequired: Swift.String,
+                        fooOptionalNullable: Swift.String? = nil,
+                        fooRequiredNullable: Swift.String
+                    ) {
+                        self.fooOptional = fooOptional
+                        self.fooRequired = fooRequired
+                        self.fooOptionalNullable = fooOptionalNullable
+                        self.fooRequiredNullable = fooRequiredNullable
+                    }
+                    public enum CodingKeys: String, CodingKey {
+                        case fooOptional
+                        case fooRequired
+                        case fooOptionalNullable
+                        case fooRequiredNullable
+                    }
+                }
+            }
+            """
+        )
+        try self.assertSchemasTranslation(
+            featureFlags: [.nullableSchemas],
+            """
+            schemas:
+              MyObj:
+                type: object
+                properties:
+                  fooOptional:
+                    type: string
+                  fooRequired:
+                    type: string
+                  fooOptionalNullable:
+                    type: [string, null]
+                  fooRequiredNullable:
+                    type: [string, null]
+                required:
+                  - fooRequired
+                  - fooRequiredNullable
+            """,
+            """
+            public enum Schemas {
+                public struct MyObj: Codable, Hashable, Sendable {
+                    public var fooOptional: Swift.String?
+                    public var fooRequired: Swift.String
+                    public var fooOptionalNullable: Swift.String?
+                    public var fooRequiredNullable: Swift.String?
+                    public init(
+                        fooOptional: Swift.String? = nil,
+                        fooRequired: Swift.String,
+                        fooOptionalNullable: Swift.String? = nil,
+                        fooRequiredNullable: Swift.String? = nil
+                    ) {
+                        self.fooOptional = fooOptional
+                        self.fooRequired = fooRequired
+                        self.fooOptionalNullable = fooOptionalNullable
+                        self.fooRequiredNullable = fooRequiredNullable
+                    }
+                    public enum CodingKeys: String, CodingKey {
+                        case fooOptional
+                        case fooRequired
+                        case fooOptionalNullable
+                        case fooRequiredNullable
+                    }
+                }
+            }
+            """
+        )
+    }
+
     func testComponentsObjectNoAdditionalProperties() throws {
         try self.assertSchemasTranslation(
             """