.environmentObject()视图操作符vs @EnvironmentObject的目的是什么？

Question

我想爬出这俗话说的新手深渊。

直到我注意到文档中的.environmentObject()视图操作符，我才开始掌握@EnvironmentObject的用法。

这是我的密码：

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() {}
}

​

我试图用视图上的.environmentObject()操作符替换@EnvironmentObject的用法。

我有一个编译错误，因为缺少‘设置’def。

但是，没有.environmentObject操作符，代码运行良好。

那么我的问题是，为什么有.environmentObject操作符？

.environmentObject()是实例化environmentObject，还是@environmentObject对象访问实例化对象？

Answer 1

下面是演示代码，用于显示EnvironmentObject & .environmentObject用法的变体(带有内联注释)：

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不声明环境对象的需求，因此不使用.environmentObject修饰符。