How to Apply a Custom Protocol in Swift Macro?

Question

I have a question while studying Swift Macros.

When a specific type wants to conform to a custom protocol in Swift Macros, should that custom protocol be declared within the Macro Package? Or can it be declared in the main app project?

For example, if the @ServiceProvidable macro makes a specific class conform to the ServiceProvidable protocol and expands the code with a computed property, how should it be implemented?

@ServiceProvidable
class ViewControllerWrapper {

}

// Expanding a macro
class ViewControllerWrapper { } 
extension ViewControllerWrapper: ServiceProvidable {
    var service: ServiceProviderProtocol {
        guard 
            let serviceProvider = (UIApplication.shared.delegate as? AppDelegate)?.serviceProvider
        else { return ServiceProvider() }
        return serviceProvider
    }
}

Answer 1

You need to specify the conformances that a macro provides, when declaring the macro. e.g.

@attached(extension, conformances: FooProtocol) // You need to be able to say "FooProtocol" here
public macro MyMacro() = #externalMacro(module: "SomeImplementationModule", type: "MyMacro")

Therefore, the protocol should be declared in a module that the code above can access, so either in the same module as the above code, or one of its dependencies. I usually just put it right next to the macro declaration.

@attached(extension, conformances: FooProtocol)
public macro MyMacro() = #externalMacro(module: "SomeImplementationModule", type: "MyMacro")

public protocol FooProtocol {
    func foo()
}

Note that this is not the module where you implement the macro.

You should implement the macro in another module (in the above example, in a module called "SomeImplementationModule"), created using .macro(...) instead of .target(...) in Package.swift.

Here is an example implementation:

enum MyMacro: ExtensionMacro {
    static func expansion(
        of node: AttributeSyntax,
        attachedTo declaration: some DeclGroupSyntax,
        providingExtensionsOf type: some TypeSyntaxProtocol,
        conformingTo protocols: [TypeSyntax],
        in context: some MacroExpansionContext
    ) throws -> [ExtensionDeclSyntax] {
        let name = declaration.as(ClassDeclSyntax.self)!.name
        return [
            try ExtensionDeclSyntax("extension \(name): FooProtocol") {
                """
                public func foo() { print("example implementation") }
                """
            }
        ]
    }
}


@main
struct MyMacroPlugin: CompilerPlugin {
    let providingMacros: [Macro.Type] = [
        MyMacro.self
    ]
}