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可选@ViewBuilder闭包

swiftui swift ios

superpuccio Kasra · 技术社区 · 6 年前

在SwiftUI中是否可以有一个可选的 @ViewBuilder 关闭?例如,假设我要开发一个自定义视图,它采用两个视图生成器闭包,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")
        })
    }
}

但我希望底部视图是可选的。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!()
            }
        }
    }
}

但我有个错误:

函数生成器属性“ViewBuilder”只能应用于函数类型的参数。

谢谢。

0 回复 | 直到 6 年前

Asperi 6 年前

考虑到 buildIf 特征 ViewBuilder 以下方法可能允许 视图生成器 在里面 init (这更可取)

使用Xcode 11.2/iOS 13.2测试和工作

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()
        }
    }
}

所以像这个一样

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")
        })
    }
}

Mamaessen 6 年前

似乎您不需要在初始值设定项中使用@ViewBuilder,这样就可以:

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!()
            }
        }
    }
}