在点击按钮时更改视图的 ViewModel 会导致：“AttributeGraph：通过属性警告检测到循环”答案

【问题标题】：Changing View's ViewModel on button tap causes: "AttributeGraph: cycle detected through attribute warning"在点击按钮时更改视图的 ViewModel 会导致：“AttributeGraph：通过属性警告检测到循环”
【发布时间】：2021-08-03 11:39:00
【问题描述】：

我做了一个简单的 Playground 示例，试图弄清楚如何使用 SwiftUI 和 Combine 将 View Model 连接到 View。我相信我已经按书完成了（检查下面的代码示例），但是当我点击“增量”按钮时，此打印出现在控制台中：

 === AttributeGraph: cycle detected through attribute 22736 ===
 === AttributeGraph: cycle detected through attribute 18320 ===
 === AttributeGraph: cycle detected through attribute 17200 ===
 === AttributeGraph: cycle detected through attribute 18320 ===
 === AttributeGraph: cycle detected through attribute 21592 ===
 === AttributeGraph: cycle detected through attribute 21592 ===
 === AttributeGraph: cycle detected through attribute 21768 ===
 === AttributeGraph: cycle detected through attribute 21768 ===
 === AttributeGraph: cycle detected through attribute 18152 ===
 === AttributeGraph: cycle detected through attribute 21768 ===
 === AttributeGraph: cycle detected through attribute 20224 ===
 === AttributeGraph: cycle detected through attribute 18152 ===
 === AttributeGraph: cycle detected through attribute 18152 ===
 === AttributeGraph: cycle detected through attribute 18280 ===
 === AttributeGraph: cycle detected through attribute 22568 ===
 === AttributeGraph: cycle detected through attribute 22568 ===
 === AttributeGraph: cycle detected through attribute 22736 ===
 === AttributeGraph: cycle detected through attribute 22568 ===
 === AttributeGraph: cycle detected through attribute 22736 ===
 === AttributeGraph: cycle detected through attribute 22568 ===

在那之后，我开始试验代码并发现延迟调用 View Model 的增量方法可以解决这个问题。（取消注释示例中的注释行以对此进行测试。）现在困扰我的是，我不知道为什么会出现这个警告，也不知道为什么延迟方法调用会修复它。

如果有人能解释幕后发生的事情以及为什么会发生这种情况，我将不胜感激。

环境：

macOS Big Sur Version 11.4
Xcode Version 12.5.1 (12E507)

示例代码：

import Combine
import SwiftUI
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

class ViewModel: ObservableObject {
    @Published private (set) var counter: Int = 0
    
    func increment() {
        counter += 1
    }
}

struct SomeView: View {
    @ObservedObject var viewModel: ViewModel
    
    var body: some View {
        VStack(alignment: .center, spacing: 8) {
            Text("Counter: \(viewModel.counter)")
                .padding()
            Button("Increment") {
                // uncomment to fix AttributeGraph cycle warning
//                DispatchQueue.main.asyncAfter(deadline: .now() + 0.001) {
                    self.viewModel.increment()
//                }
            }
            .padding()
        }
        .padding()
    }
}

let viewModel = ViewModel()
let view = SomeView(viewModel: viewModel)
PlaygroundPage.current.setLiveView(view)

【问题讨论】：

这不会发生在 xcode 或 iPhone 中。

标签： swiftui combine observableobject observedobject

【解决方案1】：

我不熟悉 Playground，但在应用程序中，我会使用：

 @StateObject viewModel = ViewModel()

不是

 let viewModel = ViewModel()

如果需要在 SomeView 中：

    DispatchQueue.main.async {
        self.viewModel.increment()
    }

【讨论】：