MyDocument.swift

//
// Logic necessary to implement a "closing handler" for a NSDocument
//
// This is inspired by: 
// - https://stackoverflow.com/questions/49343307/best-practice-to-implement-canclosewithdelegateshouldclosecontextinfo
// - https://github.com/keith/radars/blob/master/NSDocumentSwiftAnnotations/NSDocumentSwiftAnnotations/Document.swift
//   Keith's supporting documentation for his radar submission: http://www.openradar.me/33422662  
// Documentation:
// - https://developer.apple.com/library/content/releasenotes/AppKit/RN-AppKitOlderNotes/
//   Paragraph called: "Advice for Overriders of Methods that Follow the delegate:didSomethingSelector:contextInfo: Pattern"
//
// I'm not even sure I understand all that's happening here, use it at you own risk!
//

class MyDocument: NSDocument {

    private /* weak */ var delegate: NSObjectProtocol?
    private var shouldCloseSelector: Selector?

    override func canClose(withDelegate delegate: Any, shouldClose shouldCloseSelector: Selector?, contextInfo: UnsafeMutableRawPointer?) {
        self.delegate = delegate as? NSObjectProtocol
        self.shouldCloseSelector = shouldCloseSelector
        super.canClose(withDelegate: delegate, shouldClose: shouldCloseSelector, contextInfo: contextInfo)
    }

    @objc func document(_ document: NSDocument, shouldClose: Bool, contextInfo: UnsafeMutableRawPointer?) {
        typealias FunctionSignature = (@convention(c) (AnyObject?, Selector?, NSDocument, Bool, UnsafeMutableRawPointer?) -> Void)?
        let implementation = shouldCloseSelector.flatMap { unsafeBitCast(class_getMethodImplementation(type(of: delegate) as? AnyClass, $0), to: FunctionSignature.self) }
        implementation?(delegate, shouldCloseSelector, self, shouldClose, contextInfo)
    }
}