ios - 可选的@ViewBuilder 闭包

Question

在 SwiftUI 中是否有可能有一个可选的 @ViewBuilder关闭？例如，假设我想开发一个自定义 View ，它需要两个 View 构建器闭包，如下所示:

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")
        })
    }
}

但我希望底部 View 是可选的。我试过:

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:

Function builder attribute 'ViewBuilder' can only be applied to a parameter of function type.

谢谢。

Answer 1

考虑到buildIf ViewBuilder的特点以下方法允许保留 ViewBuilder在 init (这是最好的)

经过测试并与 Xcode 11.2/iOS 13.2 一起使用

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()
        }
    }
}

所以就像这个一样

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

和这个

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