swiftui - UndoManager 的 canUndo 属性未在 SwiftUI 中更新

Question

为什么@Environment UndoManager不更新其 canUndo当它的堆栈中有 Action 时的属性？我有一个 View ，它有一个可以使用取消/重做功能的 child ，但由于某种原因，我无法根据管理器禁用撤消按钮。

struct MyView: View {
    @Environment(\.undoManager) var undoManager: UndoManager?

    var body: some View {
        Button("Undo") { ... }
            .disabled(!self.undoManager!.canUndo)
    }
}

Answer 1

UndoManager.canUndo不符合 KVO，所以使用一些通知发布者来跟踪状态，如下所示

struct MyView: View {
    @Environment(\.undoManager) var undoManager
    @State private var canUndo = false

    // consider also other similar notifications
    private let undoObserver = NotificationCenter.default.publisher(for: .NSUndoManagerDidCloseUndoGroup)

    var body: some View {
        Button("Undo") { }
            .disabled(!canUndo)
            .onReceive(undoObserver) { _ in
                self.canUndo = self.undoManager!.canUndo
            }
    }
}