可选的@ViewBuilder闭包

Question

在SwiftUI中是否可以有一个可选的@ViewBuilder闭包？例如，假设我要开发一个自定义视图，该视图需要两个这样的视图构建器闭包：

import SwiftUI

struct TopAndBottomView<Content>: View where Content: View {
    let topContent: () -> Content
    let bottomContent: () -> Content

    init(@ViewBuilder topContent: @escaping () -> Content, @ViewBuilder bottomContent: @escaping () -> Content) {
        self.topContent = topContent
        self.bottomContent = bottomContent
    }

    var body: some View {
        VStack {
            topContent()
            Spacer()
            bottomContent()
        }
    }
}

struct TopAndBottomView_Previews: PreviewProvider {
    static var previews: some View {
        TopAndBottomView(topContent: {
            Text("TOP")
        }, bottomContent: {
            Text("BOTTOM")
        })
    }
}

但是我希望底视图是可选的。我尝试过：

struct TopAndBottomView<Content>: View where Content: View {
    let topContent: () -> Content
    let bottomContent: (() -> Content)?

    init(@ViewBuilder topContent: @escaping () -> Content, @ViewBuilder bottomContent: (() -> Content)? = nil) {
        self.topContent = topContent
        self.bottomContent = bottomContent
    }

    var body: some View {
        VStack {
            topContent()
            Spacer()
            if bottomContent != nil {
                bottomContent!()
            }
        }
    }
}

但我收到此错误：

函数生成器属性'ViewBuilder'只能应用于函数类型的参数。

谢谢。

Answer 1

似乎您在初始化程序中不需要@ViewBuilder，因此可以正常工作：

struct TopAndBottomView<Content>: View where Content: View {
    let topContent: () -> Content
    let bottomContent: (() -> Content)?

    init(@ViewBuilder topContent: @escaping () -> Content, bottomContent: (() -> Content)?) {
        self.topContent = topContent
        self.bottomContent = bottomContent
    }

    var body: some View {
        VStack {
            topContent()
            Spacer()

            if bottomContent != nil {
                bottomContent!()
            }
        }
    }
}

Answer 2

考虑到buildIf的ViewBuilder特性，可以采用以下方法将ViewBuilder保留在[​​C0]中（最好）

经过测试并与Xcode 11.2 / iOS 13.2配合使用

init

如此工作）>

struct TopAndBottomView<Content>: View where Content: View {
    let topContent: () -> Content
    let bottomContent: () -> Content?

    init(@ViewBuilder topContent: @escaping () -> Content, 
         @ViewBuilder bottomContent: @escaping () -> Content? = { nil }) {
        self.topContent = topContent
        self.bottomContent = bottomContent
    }

    var body: some View {
        VStack {
            topContent()
            Spacer()
            bottomContent()
        }
    }
}
和这一个

struct TopAndBottomView_Previews: PreviewProvider {
    static var previews: some View {
        TopAndBottomView(topContent: {
            Text("TOP")
        }, bottomContent: {
            Text("BOTTOM")
        })
    }
}