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 var originalShouldCloseSelectorBlock : ((Bool) -> Void)? = nil

    override func canClose(withDelegate delegate: Any, shouldClose shouldCloseSelector: Selector?, contextInfo: UnsafeMutableRawPointer?) {
        guard let selector = shouldCloseSelector, let context = contextInfo,
            let object = delegate as? NSObject, let objcClass = objc_getClass(object.className) as? AnyClass,
            let method = class_getMethodImplementation(objcClass, selector) else {
                super.canClose(withDelegate: delegate, shouldClose: shouldCloseSelector, contextInfo: contextInfo)
                return
        }

        typealias FunctionSignature = @convention(c) (AnyObject, Selector, AnyObject, Bool, UnsafeMutableRawPointer?) -> Void
        let function = unsafeBitCast(method, to: FunctionSignature.self)

        self.originalShouldCloseSelectorBlock = {[unowned object, selector, context] (flag) -> Void in
            function(object, selector, self, flag, context)
        }

        let delegate: Any = self
        let shouldCloseSelector: Selector = #selector(document(_:shouldClose:contextInfo:))
        super.canClose(withDelegate: delegate, shouldClose: shouldCloseSelector, contextInfo: contextInfo)
    }

    @objc func document(_ document: NSDocument, shouldClose: Bool, contextInfo: UnsafeMutableRawPointer?) {
        //
        // This is where you can do you own document-closing related logic
        //

        self.originalShouldCloseSelectorBlock?(shouldClose)
        self.originalShouldCloseSelectorBlock = nil
    }
}