AnyMusicStaffViewElement.swift

struct AnyMusicStaffViewElement: MusicStaffViewElement {
    private var wrapped: MusicStaffViewElement
    
    init(_ wrapped: MusicStaffViewElement) {
        self.wrapped = wrapped
    }
  
    var unboxed: MusicStaffViewElement {
        return self.wrapped
    }
  
    var asAnyMusicStaffViewElement: AnyMusicStaffViewElement {
        AnyMusicStaffViewElement(self)
    }
    
    //MusicStaffViewElement protocol adoption
  
    func path(in frame: CGRect) -> CGPath {
        return wrapped.path(in: frame)
    }
    
    var aspectRatio: CGFloat {
        return wrapped.aspectRatio
    }
    
    var heightInStaffSpace: CGFloat {
        return wrapped.heightInStaffSpace
    }
    
    var anchorPoint: CGPoint {
        return wrapped.anchorPoint
    }
    
    func offset(in clef: MusicClef) -> Int {
        return wrapped.offset(in: clef)
    }
    
    func direction(in clef: MusicClef) -> MusicStaffViewElementDirection {
        return wrapped.direction(in: clef)
    }
}

//Tests
XCTAssertNotNil(MusicAccidental.natural.asAnyMusicStaffViewElement) //pass
XCTAssertNotNil(MusicAccidental.natural.asAnyMusicStaffViewElement.unboxed as? MusicAccidental) //pass

XCTAssertNotNil(AnyMusicStaffViewElement(MusicClef.bass) as? MusicClef) //fails
//Cast from 'AnyMusicStaffViewElement' to unrelated type 'MusicClef' always fails