@StateObject 是否适用于 static 属性，例如 SwiftUI 预览？

Question

我不确定这是否是错误，但据我所知，@ static @StateObject一起使用时似乎表现得像@ObservedObject 。

预期行为：制作了一个实例，它在预览的渲染之间共享，因此每次按下按钮都会增加相同的 integer。

实际行为：似乎在每次渲染预览时都会创建一个新实例，导致计数器永久清零。NSLog 语句证实了这种情况。

这是一个说明问题的示例：

import SwiftUI

struct ContentView: View {
    let title: String
    @ObservedObject var state: DummyState

    var body: some View {
        Text("\(title): \(state.counter)").padding()
    }
}

struct ContentView_Previews: PreviewProvider {
    // Doesn't work as expected. Seems to act just like `@ObservedObject`,
    // i.e. a new instance is made on every render.
    @StateObject static var buggyState = DummyState()

    // Works as expected, only 1 instance is created.
    static let tmp = DummyState()
    @ObservedObject static var workingState = tmp // @StateObject also works

    static var previews: some View {
        VStack {
            ContentView(title: "Broken counter", state: buggyState) // Counter is always 0
            ContentView(title: "Working counter", state: workingState)

            Button {
                buggyState.counter += 1;
                workingState.counter += 1;

                let _ = NSLog("""
                    DummyState
                        initCount: \(DummyState.initCount),
                        latestID: \(ObjectIdentifier(buggyState))
                    """)
            }
            label: { Text("++") }
                .padding()
        }.frame(width: 400, height: 200)
    }
}

// This is some state I want to define in my Preview, and pass into my View to render.
class DummyState: ObservableObject {
    /// Tracks how many instances of `DummyState` were ever made
    static var initCount = 0

    @Published var counter = 0

    init() { Self.initCount += 1 }
}

作为一种解决方法，我发现我可以使用一个常规的static变量来保存我的一个DummyState()引用，然后创建第二个@ObservedObject static 变量来为其添加别名，然后它会按预期工作。

那么，这是一个错误，还是我遗漏了什么？

Answer 1

这些属性包装器仅适用于视图。