之间存在循环依赖关系
static let preview: QuakesProvider呼叫Quake.makePreviews(count: 10)
以及
static func makePreviews(count: Int) -> [Quake]呼叫QuakesProvider.preview
您可以更改为：

static func makePreviews(context: NSManagedObjectContext, count: Int) -> [Quake] {
    var quakes: [Quake] = []
    for index in 0..<count {
        let quake = Quake(context: context)
        quake.code = UUID().uuidString
        quake.time = Date().addingTimeInterval(Double(index) * -300)
        quake.magnitude = .random(in: -1.1...10.0)
        quake.place = "15km SSW of Cupertino, CA"
        quakes.append(quake)
    }
    return quakes
}

/// A quakes provider for use with canvas previews.
static let preview: QuakesProvider = {
    let provider = QuakesProvider(inMemory: true)
    Quake.makePreviews(context: provider.container.viewContext, count: 10)
    return provider
}()

/// An earthquake for use with canvas previews.
static var preview: Quake {
    let provider = QuakesProvider(inMemory: true)
    let quakes = Quake.makePreviews(context: provider.container.viewContext, count: 1)
    return quakes[0]
}

同样在QuakesProvider.swift中，删除初始化器的private访问级别，以便我们可以在Quake扩展中使用它：

init(inMemory: Bool = false) { ... }

快速用户界面预览：

对于List或在Quake实体上具有FetchRequest的任何视图：

struct ContentView_Previews: PreviewProvider {
    // Use the in-memory and pre-populated QuakesProvider
    static let quakesProvider = QuakesProvider.preview
    static var previews: some View {
        ContentView(quakesProvider: quakesProvider)
            .environment(\.managedObjectContext, quakesProvider.container.viewContext)
    }
}

对于QuakeRow中的单个项目：

struct QuakeRow_Previews: PreviewProvider {
    static var previews: some View {
        // Use the in-memory QuakesProvider to create only 1 item, and returns it
        QuakeRow(quake: .preview)
    }
}