可选的@ViewBuilder 闭包 [英] Optional @ViewBuilder closure

问题描述

在 SwiftUI 中是否可以有一个可选的 @ViewBuilder 闭包?例如，假设我想开发一个自定义视图，它需要两个视图构建器闭包，如下所示:

Is it possible in SwiftUI to have an optional @ViewBuilder closure? For example, let's say I want to develop a custom view that takes two view builder closures like this:

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")
        })
    }
}

但我希望底部视图是可选的.我试过:

But I'd like the bottom view to be optional. I tried with:

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:

函数构建器属性ViewBuilder"只能应用于函数类型参数.

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

谢谢.

推荐答案

考虑到 ViewBuilder 的 buildIf 特性，以下方法是可能的，允许保留 init 中的 ViewBuilder(最好)

Taking into account buildIf feature of ViewBuilder the following approach is possible that allows to keep ViewBuilder in init (that is preferable)

经过测试适用于 Xcode 11.2/iOS 13.2

Tested & works with Xcode 11.2 / iOS 13.2

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()
        }
    }
}

就像这个一样

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

还有这个

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

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