Swift UndoManager usage

good-way.swift

import UIKit

class Counter {
    let manager = UndoManager()
    var count = 0

    @objc func increment() {
        count += 1
        manager.registerUndo(withTarget: self, selector: #selector(Counter.decrement), object: nil)
    }

    @objc func decrement() {
        count -= 1
        manager.registerUndo(withTarget: self, selector: #selector(Counter.increment), object: nil)
    }
}

let counter = Counter()

print("Let's increment!")
counter.increment()
print("count: \(counter.count)")
print("Let's undo!")
counter.manager.undo()
print("count: \(counter.count)")
print("Let's redo!")
counter.manager.redo()
print("count: \(counter.count)") // It returns 1 :)

/** Outputs
Let's increment!
count: 1
Let's undo!
count: 0
Let's redo!
count: 1
*/

ng.swift

import UIKit

class Counter {
    var count = 0
}

let counter = Counter()
let manager = UndoManager()

print("Let's increment!")
counter.count += 1
manager.registerUndo(withTarget: counter, handler: { counter in
    counter.count -= 1
    print("Decremented by undo")
})

print("count: \(counter.count)")
print("Let's undo!")
manager.undo()
print("count: \(counter.count)")
print("Let's redo!")
manager.redo()
print("count: \(counter.count)") // It should be 1, but returns 0. Why?

/** Outputs
Let's increment!
count: 1
Let's undo!
Decremented by undo
count: 0
Let's redo!
count: 0
*/

ok.swift

import UIKit

class Counter {
    var count = 0
}

let counter = Counter()
let manager = UndoManager()

print("Let's increment!")
counter.count += 1
manager.registerUndo(withTarget: counter, handler: { counter in
    counter.count -= 1
    print("Decremented by undo")
    // It needs registering for redo
    manager.registerUndo(withTarget: counter, handler: { counter in
        counter.count += 1
    })
})

print("count: \(counter.count)")
print("Let's undo!")
manager.undo()
print("count: \(counter.count)")
print("Let's redo!")
manager.redo()
print("count: \(counter.count)") // It returns 1 :)

/** Outputs
Let's increment!
count: 1
Let's undo!
Decremented by undo
count: 0
Let's redo!
count: 1
*/