因此，我试图使用@EnvironmentObject变量来遵循协议，但编译器会抱怨，而如果使用@ObservedObject，它就可以工作。
使用@EnvironmentObject：

final class ViewModel: ObservableObject {
    @Published var someOutput: String

    init(someOutput: String) {
        self.someOutput = someOutput
    }
}

protocol SomeChildView: View {
    var viewModel: ViewModel { get set }
}

struct FirstChildView: SomeChildView {
    @EnvironmentObject var viewModel: ViewModel // <-- This does not work.

    var body: some View {
        Text("")
    }
}

使用@ObservedObject：

struct FirstChildView: SomeChildView {
    @ObservedObject var viewModel: ViewModel // <-- Works

    var body: some View {
        Text("")
    }
}