Using NSUndoManager from Swift

Until Apple releases an undo manager that can accept closures, we will need to use the present NSUndoManager to implement the undo/redo functionality that all users expect. When I tried to use NSUndoManager, I ran into a few problems. Not unsurmountable, but using NSUndoManager is not as simple as some other Cocoa/Foundation classes.

Here are my experiences:

1) The registerUndoWithTarget function pretty much works as advertised. But the selector that is called must be marked with "@objc". That would be fine, but sometimes I want to pass an parameter type that is not known to Objective-C, which won't work because all functions marked with "@objc" must use types that are known to Objective-C. Besides, the parameter must be an object, this means no struct's or enum's.
These two problems caused me to implement a generic Wrapper class and define the @objc func as accepting AnyObject. Example:

class Wrapper<T> {

    var payload: T

    init(_ data: T) {

        self.payload = data

    }

}

enum Action {

    case SET_STRING(String),

}

extension MyClass {

       

    func dispatch(action: Action) -> ActionResult {

        

        let oldString = string

        

        let result = doAction(action)

        

        if result == .CHANGED {

            if let undoManager = g_undoManager {

                undoManager.registerUndoWithTarget(self, selector: "dispatchFromWrapper:", object: Wrapper(Action.SET_STRING(oldString)))

            }

        }

        

        return result

    }

    

    @objc func dispatchFromWrapper(wrapper: AnyObject) {

        dispatch((wrapper as! Wrapper<Action>).payload)

    }

Note 1: When instantiating a Wrapper object, we can use the abbreviated form: Wrapper(Action.SET_STRING(oldString)) instead of the full form: Wrapper<Action>(Action.SET_STRING(oldString)) thanks to type inference.

Note 2: The MyClass definition contains a var called "string" of the type "String".

Don't let the g_undoManager throw you for a loop: each application (normally) uses only one NSUndoManager. The NSUndoManager is part of every NSWindow and NSView. To gain global access to the undoManager I used the following code in the AppDelegate:

// Make the undo manager global

var g_undoManager: NSUndoManager?

class AppDelegate: NSObject, NSApplicationDelegate {

    @IBOutlet weak var window: NSWindow!

    func applicationDidFinishLaunching(aNotification: NSNotification) {

        

        g_undoManager = window.undoManager

...

Perhaps not the best design possible, but it works!

PS: for more on the dispatch pattern that I used here, see: Swift "Dispatch" Design Pattern.

2) The prepareWithInvocationTarget does not seem usable. I tried several variations, including introducing a category on NSUndoManager, but even though some variations were compilable the app always fails at runtime with an "unrecognized selector sent to" error.

Hopefully Apple will bring out a closure oriented approach soon.

Happy coding...

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