SwiftUI 视图的默认成员初始化器 VS 自定义初始化器

简而言之--使用ViewModel("MyView"/"MyViewModel")构建自定义SwiftUI视图时，如下所示:

struct ContentView: View {

    var body: some View {
        MyView(viewModel: MyViewModel())
    }
}

为什么会这样:

struct MyView: View {

    @StateObject var viewModel: MyViewModel

    var body: some View {
        Text("Hello world!")
    }
}

与此不同:

struct MyView: View {

    @StateObject var viewModel: MyViewModel

    init(viewModel: MyViewModel) {
        self._viewModel = StateObject(wrappedValue: viewModel)
    }

    var body: some View {
        Text("Hello world!")
    }
}

在调试时，第二个选项似乎会在SwiftUI每次重新构建MyView时创建MyViewModel的新实例.但在第一个选项中，StateObject包装器似乎完成了它的工作，并确保只为MyView的所有重建创建一个MyViewModel的实例.

是否有一些额外的SwiftUI魔力应用于使用视图的默认成员式初始化器与自定义初始化器？也许是通过ViewBuilder？

下面是一个简单的示例应用程序MyApp.swft，它可以查看这种行为的实际情况.

//
//  MyApp.swift
//

import SwiftUI

@main
struct MyApp: App {

    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}

struct ContentView: View {

    @AppStorage("redBackground") private var redBackground: Bool = false

    var body: some View {
        ZStack {
            // Flipping "redBackground" will cause a reconstruction of the view hierarchy
            if redBackground {
                Color.red
            } else {
                Color.green
            }
            MyView(viewModel: MyViewModel())
        }
    }
}

final class MyViewModel: ObservableObject {

    init() {
        print("MyViewModel.init")
    }
}

struct MyView: View {

    @StateObject var viewModel: MyViewModel

    @AppStorage("redBackground") private var redBackground: Bool = false

    // WARNING: Uncommenting this causes the view model to be recreated every reconstruction of the view!
    //    init(viewModel: MyViewModel) {
    //        self._viewModel = StateObject(wrappedValue: viewModel)
    //    }

    var body: some View {
        VStack {
            Button("Toggle background") {
                redBackground = !redBackground
            }
        }
    }
}