NSDocument canClose in Swift

RadarDocument.swift

    /// So, lets talk about AppKit. The way this function works is by passing a delegate (which is required
    /// to be a NSObject, but here is typed as Any) which turns out to be the same type as `self`, a Selector,
    /// which in this case is `_something:didSomething:soContinue:` (not kidding), and some "contextInfo"
    /// (which is actually a block). While all of these arguments appear to be optional, passing nil through
    /// to the function call that `shouldCloseSelector` defines, or passing nil to super, will cause the app
    /// to crash. So then we need to call the function on `delegate` defined by `shouldCloseSelector`.
    ///
    /// According to the documentation the function signature for this selector looks like this:
    /// - (void)document:(NSDocument *)doc shouldClose:(BOOL)shouldClose contextInfo:(void *)contextInfo
    ///
    /// So of course since this is an instance method on NSDocument (or in theory whatever type `delegate` is)
    /// we also have the implicit Objective-C runtime arguments of the object and the selector. Using the
    /// knowledge of the type of this function, we can query the Objective-C runtime in order to get the
    /// function pointer to the implementation, and then cast that into our custom typealias that we create
    /// so that we can call it from Swift. In order to be overly cautious I've treated all these values as
    /// optional, even though they "shouldn't be".
    override func canClose(withDelegate delegate: Any, shouldClose shouldCloseSelector: Selector?,
                           contextInfo: UnsafeMutableRawPointer?)
    {
        if self.fileURL != nil {
            super.canClose(withDelegate: delegate, shouldClose: shouldCloseSelector, contextInfo: contextInfo)
            return
        }

        typealias EverythingIsHorribleAndYouShouldBeSadAboutThat = @convention(c)
            (NSObject, Selector, NSDocument, Bool, UnsafeMutableRawPointer) -> Void

        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
        }

        let function = unsafeBitCast(method, to: EverythingIsHorribleAndYouShouldBeSadAboutThat.self)
        function(object, selector, self, true, context)
    }