viewModifier相当前段中的css，定义一种样式，其他地方都可以使用
只需要实现viewModifier协议就可以嘞

struct myModifier: ViewModifier {
    // opaque return types
    func body(content: Content) -> some View {
    }
}

some --- 用在当返回值为不确定类型的情况。

struct myModifier: ViewModifier {
    // opaque return types
    func body(content: Content) -> some View {
        content.foregroundColor(.red).font(Font.system(size: 20,weight: .bold)).border(.green, width: 2)
    }
}
struct ViewModifierDemo: View {
    var body: some View {
        
        VStack{
            Text("Hello, World!").modifier(myModifier())
            Text("hhh").modifier(myModifier())
        }
    }
}

仿写redacted

public enum RedactionReason {
    case placeh
    case black
    case blurred
}

struct Placeholder: ViewModifier{
    func body(content: Content) -> some View {
        content
            .opacity(0)
            .overlay {
                RoundedRectangle(cornerRadius: 2).fill(.black.opacity(0.16))
                    .padding(20)
            }
    }
}

struct Confidential: ViewModifier{
    func body(content: Content) -> some View {
        content
            .opacity(0)
            .overlay {
                Color.black
            }
    }
}

struct Blurred: ViewModifier{
    func body(content: Content) -> some View {
        content
            .blur(radius: 4)
    }
}

struct Redactable: ViewModifier {
    let reason : RedactionReason?
    @ViewBuilder
    func body(content: Content) -> some View {
        switch reason {
        case .placeh:
            content
                .modifier(Placeholder())
        case .black:
            content
                .modifier(Confidential())
        case .blurred:
            content
                .modifier(Blurred())
        case nil:
            content
        }
    }
}


extension View {
    func redacted(reason: RedactionReason?) -> some View {
        self.modifier(Redactable(reason: reason))
    }
}
struct RedactedDemo: View {
    var body: some View {
        Text("Hello, World!")
            .padding()
//            .modifier(Placeholder())
            .redacted(reason: .placeh)
    }
}