Standalone button handling and affording undo/redo actions (implemented without overgeneralizing the implementation above my own needs). An answer to https://stackoverflow.com/questions/60647857/undomanagers-canundo-property-not-updating-in-swiftui/67354446#67354446

UndoManagerActionButton.swift

import SwiftUI
import Combine

struct UndoManagerActionButton: View {
    enum Action {
        case
            undo,
            redo
    }
    
    private let action: Action
    private let willChangePublisher: AnyPublisher<Void, Never>
    
    @Environment(\.undoManager) private var undoManger: UndoManager!
    @State private var canPerformAction = false
    
    init<WillChangePublisher>(
        _ action: Action,
        willChangePublisher: WillChangePublisher
    ) where WillChangePublisher: Publisher, WillChangePublisher.Failure == Never {
        self.action = action
        self.willChangePublisher = willChangePublisher
            .map { _ in () }
            .eraseToAnyPublisher()
    }
    
    var body: some View {
        Group {
            if canPerformAction {
                Button(
                    action: action == .undo ? undoManger.undo : undoManger.redo,
                    label: { Text(action == .undo ? "Undo" : "Redo") }
                )
            }
            else {
                EmptyView()
            }
        }
        .onReceive(
            willChangePublisher,
            perform: { _ in
                canPerformAction = action == .undo ? undoManger.canUndo : undoManger.canRedo
            }
        )
    }
}