问题

我有以下用Swift编写的代码：

```swift
protocol Theme {
    var iconColor: Color { get }
}

struct MyTheme: Theme {
   var iconColor: Color = .white
}

class Appearance {
    static var theme: Theme = MyTheme()
}

我可以在我的视图中使用这个Appearance类来方便地获取当前主题的属性：

Image("my-icon")
    .foregroundColor(Appearance.theme.iconColor)

我在XCode中启用了-warn-concurrency编译标志，作为一个实验，看看我的应用程序会产生多少警告。这是其中一个警告：

Reference to static property 'theme' is not concurrency-safe because it involves shared mutable state

现在我明白为什么会出现这个警告，但我该如何更改代码，以使theme在并发环境中是安全的，同时仍然像现在这样方便？

我可以将theme更改为static let，但那样我将无法在运行时更改应用程序的主题。我还尝试了@MainActor和actor Appearance的组合，但它们要么产生相同的警告，要么产生其他错误。

编辑

@MainActor class Appearance可以消除所有警告，但请考虑以下情况：

struct FooView: View {
    var iconColor: Color = Appearance.theme.iconColor
    
    var body: some View {
        HStack {
            Text("FooView")
            Image("foo-logo")
                .foregroundColor(iconColor)
        }
    }
}

上面的代码会产生错误：Main actor-isolated static property 'theme' cannot be referenced from a non-isolated context

<details>
<summary>英文:</summary>

I have the following code written in Swift:

protocol Theme {
var iconColor: Color { get }
}

struct MyTheme: Theme {
var iconColor: Color = .white
}

class Appearance {
static var theme: Theme = MyTheme()
}

I can use this `Appearance` class in my views to conveniently get the current theme's properties

Image("my-icon")
.foregroundColor(Appearance.theme.iconColor)

I enabled `-warn-concurrency` compiler flag in XCode as an experiment to see how many warnings my app produces. This is one of the warnings: 

`Reference to static property 'theme' is not concurrency-safe because it involves shared mutable state`

Now I understand why it gives this warning, but how could I change the code so that `theme` is concurrency safe and still as convenient as it is?




I could change `theme` to `static let` but then I would not be able to change the theme of the app during runtime. I also tried combinations of `@MainActor` and `actor Appearance` but they give mixed result with either the same warning or some other error.

**Edit**

`@MainActor class Appearance` gets rid of all the warnings, but consider this:

struct FooView: View {
var iconColor: Color = Appearance.theme.iconColor

var body: some View {
    HStack {
        Text("FooView")
        Image("foo-logo")
            .foregroundColor(iconColor)
    }
}

}

The code above will give an error: `Main actor-isolated static property 'theme' can not be referenced from a non-isolated context`

</details>


# 答案1
**得分**: 2

要消除警告，您只需将`@MainActor`添加到`Appearance`类中。如果您在`body`之外使用`FooView`，还需要将其标记为`@MainActor`，因为`body`是`MainActor`隔离的，但`FooView`不是。

话虽如此，这在实践中不会起作用。SwiftUI 不会知道`Appearance.theme`已更改并相应地更新您的视图颜色。在 SwiftUI 中实现这样的“主题”系统的方式是使用`EnvironmentObject`。

```swift
class Appearance: ObservableObject {
    @Published var theme: Theme = MyTheme()
}

struct YourRootView: View {
    // 在每个需要访问主题的视图中编写此代码
    @EnvironmentObject var appearance: Appearance
    
    var body: some View {
        Text("Foo")
            .foregroundColor(appearance.theme.iconColor)
    }
}

当您创建YourRootView时，请为其提供一个新的Appearance对象。

@main
struct FooApp: App {
    var body: some Scene {
        WindowGroup {
            YourRootView()
                .environmentObject(Appearance())
        }
    }
}

现在 SwiftUI 可以观察theme属性的更改。

class Appearance: ObservableObject {
    @Published var theme: Theme = MyTheme()
}

struct YourRootView: View {
    // write this in every view that needs to access the theme
    @EnvironmentObject var appearance: Appearance
    
    var body: some View {
        Text("Foo")
            .foregroundColor(appearance.theme.iconColor)
    }
}

@main
struct FooApp: App {
    var body: some Scene {
        WindowGroup {
            YourRootView()
                .environmentObject(Appearance())
        }
    }
}