Result Builder — 它是什么、语法和应用

@resultBuilder
struct StringBuilder {
    static func buildBlock(_ parts: String...) -> String {
        parts.joined(separator: " ")
    }
}

@StringBuilder
func greeting() -> String {
    "Hello"
    "World"
    "from"
    "Swift"
}
// 编译器将其转换为:
// StringBuilder.buildBlock("Hello","World","from","Swift")
// 结果: "Hello World from Swift"

@resultBuilder
enum HTMLBuilder {
    static func buildBlock(_ components: String...) -> String {
        components.joined()
    }

    static func buildOptional(_ component: String?) -> String {
        component ?? ""
    }

    static func buildEither(first component: String) -> String {
        component
    }

    static func buildEither(second component: String) -> String {
        component
    }

    static func buildArray(_ components: [String]) -> String {
        components.joined()
    }
}

func div(@HTMLBuilder _ content: () -> String) -> String {
    "<div>\(content())</div>"
}

func p(_ text: String) -> String {
    "<p>\(text)</p>"
}

let page = div {
    p("Hello")
    p("World")

    if showFooter {
        p("页脚")
    }
}
// 结果: <div><p>Hello</p><p>World</p><p>Footer</p></div>

struct GreetingView: View {
    let isLoggedIn: Bool

    var body: some View {
        VStack {
            Image(systemName: "person.circle")
            Text("个人资料")
                .font(.title)

            if isLoggedIn {
                Text("欢迎回来!")
                    .foregroundColor(.green)
            } else {
                Button("登录") { }
            }
        }
    }
}