SwiftUI 仅在预览中存在主要属性的问题

问题描述

我正在使用 App 方法运行 SwiftUI 应用程序 (Xcode 12.3)：

@main
struct ThingsApp: App {
...
}

这在模拟器和我的设备上都按预期工作。但它在预览中不起作用：在那里，我收到 'main' attribute cannot be used in a module that contains top-level code 的错误。

一定是我的应用程序引起了它，因为如果我在 Xcode 中从头开始设置一个项目，这种方法是有效的。但我不太知道如何弄清楚究竟是什么导致了这种情况。更完整的跟踪如下：

'main' attribute cannot be used in a module that contains top-level code

----------------------------------------

CompileDylibError: Failed to build ThingsApp.swift

Compiling Failed: 'main' attribute cannot be used in
a module that contains top-level code

/Users/cg/Library/Developer/Xcode/DerivedData/
Things-bkpepcogttixysdvumdszlfwxfix/Build/
Intermediates.noindex/
Previews/Things/Intermediates.noindex/Things.build/
Debug-iphonesimulator/Things.build/Objects-normal/x86_64/
ThingsApp.2.preview-thunk.swift:8:1: error: 'main' 
attribute cannot be used in a module that contains top-level code
@main extension ThingsApp {
^
/Users/cg/Library/Developer/Xcode/DerivedData/
Things-bkpepcogttixysdvumdszlfwxfix/Build/
Intermediates.noindex/
Previews/Things/Intermediates.noindex/
Things.build/
Debug-iphonesimulator/Things.build/
Objects-normal/x86_64/
ThingsApp.2.preview-thunk.swift:1:1: note: top-level code defined in this source file
@_private(sourceFile: "ThingsApp.swift") import Things

解决方法

如果你正在使用环境你的代码应该是这样的

    @main
struct YourApp: App {
    @StateObject private var someData = SomeData()
    
    var body: some Scene {
        WindowGroup {
            ContentView()
                .environmentObject(someData)
        }
    }
}

对我来说，这是我在 App 扩展中创建的顶级类类型：

@main
struct MyApp: App {
   var body: some Scene {}

   class AppDelegate: NSObject,UIApplicationDelegate {...}
}

一旦我将它移到外面并进行了干净的构建，预览又可以工作了：

@main
struct MyApp: App {
   var body: some Scene {...}
}

private class AppDelegate: NSObject,UIApplicationDelegate {...}

swiftui swiftui-previews