SwiftLint/Source/SwiftLintBuiltInRules/Rules/Lint/ClassDelegateProtocolRule.swift

import SwiftSyntax

@SwiftSyntaxRule
struct ClassDelegateProtocolRule: Rule {
    var configuration = SeverityConfiguration<Self>(.warning)

    static let description = RuleDescription(
        identifier: "class_delegate_protocol",
        name: "Class Delegate Protocol",
        description: "Delegate protocols should be class-only so they can be weakly referenced",
        rationale: """
        Delegate protocols are usually `weak` to avoid retain cycles, or bad references to deallocated delegates.

        The `weak` operator is only supported for classes, and so this rule enforces that protocols ending in \
        "Delegate" are class based.

        For example

        ```
        protocol FooDelegate: class {}
        ```

        versus

        ```
        ↓protocol FooDelegate {}
        ```
        """,
        kind: .lint,
        nonTriggeringExamples: [
            Example("protocol FooDelegate: class {}"),
            Example("protocol FooDelegate: class, BarDelegate {}"),
            Example("protocol Foo {}"),
            Example("class FooDelegate {}"),
            Example("@objc protocol FooDelegate {}"),
            Example("@objc(MyFooDelegate)\n protocol FooDelegate {}"),
            Example("protocol FooDelegate: BarDelegate {}"),
            Example("protocol FooDelegate: AnyObject {}"),
            Example("protocol FooDelegate: AnyObject & Foo {}"),
            Example("protocol FooDelegate: Foo, AnyObject & Foo {}"),
            Example("protocol FooDelegate: Foo & AnyObject & Bar {}"),
            Example("protocol FooDelegate: NSObjectProtocol {}"),
            Example("protocol FooDelegate where Self: BarDelegate {}"),
            Example("protocol FooDelegate where Self: BarDelegate & Bar {}"),
            Example("protocol FooDelegate where Self: Foo & BarDelegate & Bar {}"),
            Example("protocol FooDelegate where Self: AnyObject {}"),
            Example("protocol FooDelegate where Self: NSObjectProtocol {}"),
            Example("protocol FooDelegate: Actor {}"),
        ],
        triggeringExamples: [
            Example("↓protocol FooDelegate {}"),
            Example("↓protocol FooDelegate: Bar {}"),
            Example("↓protocol FooDelegate: Foo & Bar {}"),
            Example("↓protocol FooDelegate where Self: StringProtocol {}"),
            Example("↓protocol FooDelegate where Self: A & B {}"),
        ]
    )
}

private extension ClassDelegateProtocolRule {
    final class Visitor: ViolationsSyntaxVisitor<ConfigurationType> {
        override var skippableDeclarations: [any DeclSyntaxProtocol.Type] {
            .allExcept(ProtocolDeclSyntax.self)
        }

        override func visitPost(_ node: ProtocolDeclSyntax) {
            if node.name.text.hasSuffix("Delegate"),
               !node.hasObjCAttribute(),
               !node.isClassRestricted(),
               !node.inheritsFromObjectOrDelegate() {
                violations.append(node.protocolKeyword.positionAfterSkippingLeadingTrivia)
            }
        }
    }
}

private extension ProtocolDeclSyntax {
    func hasObjCAttribute() -> Bool {
        attributes.contains(attributeNamed: "objc")
    }

    func isClassRestricted() -> Bool {
        inheritanceClause?.inheritedTypes.contains { $0.type.is(ClassRestrictionTypeSyntax.self) } == true
    }

    func inheritsFromObjectOrDelegate() -> Bool {
        if inheritanceClause?.inheritedTypes.contains(where: { $0.type.isObjectOrDelegate() }) == true {
            return true
        }

        guard let requirementList = genericWhereClause?.requirements else {
            return false
        }

        return requirementList.contains { requirement in
            guard let conformanceRequirement = requirement.requirement.as(ConformanceRequirementSyntax.self),
                  let simpleLeftType = conformanceRequirement.leftType.as(IdentifierTypeSyntax.self),
                  simpleLeftType.typeName == "Self"
            else {
                return false
            }

            return conformanceRequirement.rightType.isObjectOrDelegate()
        }
    }
}

private extension TypeSyntax {
    func isObjectOrDelegate() -> Bool {
        if let typeName = `as`(IdentifierTypeSyntax.self)?.typeName {
            let objectTypes = ["AnyObject", "NSObjectProtocol", "Actor"]
            return objectTypes.contains(typeName) || typeName.hasSuffix("Delegate")
        }
        if let combined = `as`(CompositionTypeSyntax.self) {
            return combined.elements.contains { $0.type.isObjectOrDelegate() }
        }
        return false
    }
}