swiftui - .environmentObject() View 运算符与 @EnvironmentObject 的目的是什么？

Question

我正试图从这里众所周知的新手深渊中爬出来。

我开始掌握 @EnvironmentObject 的使用，直到我注意到文档中的 .environmentObject() View 运算符。

这是我的代码:

import SwiftUI

struct SecondarySwiftUI: View {
    @EnvironmentObject var settings: Settings

    var body: some View {
        ZStack {
            Color.red
            Text("Chosen One: \(settings.pickerSelection.name)")
        }.environmentObject(settings) //...doesn't appear to be of use.
    }

    func doSomething() {}
}

我尝试在 View 上用 .environmentObject() 运算符替换 @EnvironmentObject 的使用。
我因缺少“设置”定义而出现编译错误。

但是，如果没有 .environmentObject 运算符，代码也可以正常运行。

所以我的问题是，为什么有 .environmentObject 运算符？

.environmentObject() 是否实例化环境对象，而 @environmentObject 是否访问实例化对象？

Answer 1

这里是演示代码，显示 EnvironmentObject 和 .environmentObject 用法的变体(内嵌注释):

struct RootView_Previews: PreviewProvider {
    static var previews: some View {
        RootView().environmentObject(Settings()) // environment object injection
    }
}

class Settings: ObservableObject {
    @Published var foo = "Foo"
}

struct RootView: View {
    @EnvironmentObject var settings: Settings // declaration for request of environment object

    @State private var showingSheet = false
    var body: some View {
        VStack {
            View1() // environment object injected implicitly as it is subview
            .sheet(isPresented: $showingSheet) {
                View2() // sheet is different view hierarchy, so explicit injection below is a must
                    .environmentObject(self.settings) // !! comment this out and see run-time exception
            }
            Divider()
            Button("Show View2") {
                self.showingSheet.toggle()
            }
        }
    }
}

struct View1: View {
    @EnvironmentObject var settings: Settings // declaration for request of environment object
    var body: some View {
        Text("View1: \(settings.foo)")
    }
}

struct View2: View {
    @EnvironmentObject var settings: Settings // declaration for request of environment object
    var body: some View {
        Text("View2: \(settings.foo)")
    }
}

因此，在您的代码中 ZStack 没有声明环境对象的需求，因此没有使用 .environmentObject 修饰符。