UndoManager in NSResponder subclass (macOS)

I am trying to implement undo/redo in my model. So I made my model class a subclass of NSResponder, and then implemented the following:

note: this code is edited based on more research after comments

func setAnnotations(_ newAnnotations: [Annotation]) {
    let currentAnnotations = self.annotations

    self.undoManager.registerUndo(withTarget: self, handler: { (selfTarget) in
        selfTarget.setAnnotations(currentAnnotations)
    })

    self.annotations = newAnnotations
}

Annotation is a struct.

The code inside the closure never gets executed. Initially I noticed that undoManager is nil, but then I found this snippet:

private let _undoManager = UndoManager()
override var undoManager: UndoManager {
   return _undoManager
}

Now undoManager is no longer nil, but the code inside the closure still doesn't get executed.

What am I missing here?

I can't reproduce any issue now that you've made your undo register code make sense. Here is the entire code of a test app (I don't know what an Annotation is so I just used String):

import Cocoa
class MyResponder : NSResponder {
    private let _undoManager = UndoManager()
    override var undoManager: UndoManager {
        return _undoManager
    }
    typealias Annotation = String
    var annotations = ["hello"]
    func setAnnotations(_ newAnnotations: [Annotation]) {
        let currentAnnotations = self.annotations
        self.undoManager.registerUndo(withTarget: self, handler: { (selfTarget) in
            selfTarget.setAnnotations(currentAnnotations)
        })
        self.annotations = newAnnotations
    }
}

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {
    let myResponder = MyResponder()
    @IBOutlet weak var window: NSWindow!
    func applicationDidFinishLaunching(_ aNotification: Notification) {
        print(self.myResponder.annotations)
        self.myResponder.setAnnotations(["howdy"])
        print(self.myResponder.annotations)
        self.myResponder.undoManager.undo()
        print(self.myResponder.annotations)
    }
}

The output is:

["hello"]
["howdy"]
["hello"]

So Undo is working perfectly. If that's not happening for you, perhaps you are mismanaging your "model class" in some way.

---

By the way, a more correct to write your registration closure is this:

    self.undoManager.registerUndo(withTarget: self, handler: {
        [currentAnnotations = self.annotations] (selfTarget) in
        selfTarget.setAnnotations(currentAnnotations)
    })

This ensures that self.annotations is not captured prematurely.