雪だるま型パラから型を順番に取り出すやつ

TypeExtract.swift

import Foundation

protocol _View {
    static var specialType: Any.Type { get }
}

protocol View: _View {
    associatedtype Body: View
    associatedtype Special
    var body: Body { get }
    init()
}

extension View {
    static var specialType: Any.Type {
        return Special.self
    }
}

struct Hoge<T: View>: View {
    var body: some View {
        T()
    }
    
    typealias Special = Int
}

struct Fuga<T: View>: View {
    var body: some View {
        Hoge<T>()
    }
    
    typealias Special = Float
}

struct Box<T>: View {
    var body: Never {
        fatalError()
    }
    
    typealias Special = T
}

struct Piyo<T: View, U: View>: View {
    var body: Never {
        fatalError()
    }
    
    typealias Special = (T, U)
}

extension Never: View {
    var body: Never {
        fatalError()
    }
    
    init() {
        fatalError()
    }
    
    typealias Special = Void
}

class ExtractTypeTests: XCTestCase {

    func testHoge() {
        let hogefuga = Hoge<Fuga<Box<String>>>()
        
        func extract<T: View>(_ v: T) -> [_View.Type] {
            return _extract(type(of: v))
        }
        
        func _extract<T: View>(_ t: T.Type = T.self) -> [_View.Type] {
            if T.self == Never.self {
                return []
            }
            
            return [T.self] + _extract(T.Body.self)
        }

        extract(hogefuga).forEach {
            print($0)
        }
        
        extract(hogefuga).forEach {
            print($0.specialType)
        }
    }

}