如何创建一个带有可选辅助 View 参数的 SwiftUI 视图？

Question

我正在尝试创建一个自定义 SwiftUI 视图，该视图的行为类似于默认视图，我可以在其中使用方法或可选的初始化程序参数向视图添加额外内容。

SomeCustomView(title: "string argument") {
    // some view
}

SomeCustomView(title: "hello") {
    // some view
}.sideContent {
    // another view
}

// This style is acceptable too
SomeCustomView(title: "hello", sideContent: { /* another view */ }) {
    // some view
}

我怎样才能修改这个视图结构使其表现得像上面的例子？

struct SomeCustomView<Content>: View where Content: View {
    let title: String
    let content: Content

    init(title: String, @ViewBuilder content: () -> Content) {
        self.title = title
        self.content = content()
    }

    var body: some View {
        VStack {
            Text(title)
            content
        }
    }
}

理想情况下，我有两个不同的主体“模板”，我可以根据是调用 sideContent 方法还是设置了 sideContent 参数来切换它们。例如，

var body: some View {
    VStack {
        Text(title)
        content
    }
}

// or

var otherBody: some View {
    HStack {
        VStack {
            Text(title)
            content
        }
        sideContent
    }
}

Answer 1

2021 年 11 月更新（适用于 Xcode 11.x、12.x 和 13.x）

经过一番思考和反复试验，我想通了。事后看来有点明显。

struct SomeCustomView<Content>: View where Content: View {
    let title: String
    let content: Content

    init(title: String, @ViewBuilder content: @escaping () -> Content) {
        self.title = title
        self.content = content()
    }

    // returns a new View that includes the View defined in 'body'
    func sideContent<SideContent: View>(@ViewBuilder side: @escaping () -> SideContent) -> some View {
        HStack {
            self     // self is SomeCustomView
            side() 
        }
    }

    var body: some View {
        VStack {
            Text(title)
            content
        }
    }
}

不管有没有方法调用都可以。

SomeCustomView(title: "string argument") {
    // some view
}

SomeCustomView(title: "hello") {
    // some view
}.sideContent {
    // another view
}

以前的代码有细微的错误：body 应该是self

    func sideContent<SideContent: View>(@ViewBuilder side: @escaping () -> SideContent) -> some View {
        HStack {
            body // <--- subtle bug, updates to the main View are not propagated 
            side() 
        }
    }

感谢 Jordan Smith 很久以前就指出了这一点。

Answer 2

我为容器视图遵循的一种模式是使用条件扩展一致性来支持不同变体的初始化程序。

这是一个带有可选页脚的简单面板视图示例。

struct Panel<Content: View, Footer: View>: View {
    
    let content: Content
    let footer: Footer?
    
    init(@ViewBuilder content: () -> Content, footer: (() -> Footer)? = nil) {
        self.content = content()
        self.footer = footer?()
    }
    
    var body: some View {
        VStack(spacing: 0) {
            content

            // Conditionally check if footer has a value, if desirable.
            footer
        }
    }
}

// Support optional footer
extension Panel where Footer == EmptyView {
    init(@ViewBuilder content: () -> Content) {
        self.content = content()
        self.footer = nil
    }
}

---

我相信这类似于 Apple 为支持所有内置类型的变体所做的工作。例如，这是 Button 的标头的 sn-p。

@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
extension Button where Label == PrimitiveButtonStyleConfiguration.Label {

    /// Creates an instance representing the configuration of a
    /// `PrimitiveButtonStyle`.
    @available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
    public init(_ configuration: PrimitiveButtonStyleConfiguration)
}

@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)
extension Button where Label == Text {

    /// Creates an instance with a `Text` label generated from a localized title
    /// string.
    ///
    /// - Parameters:
    ///     - titleKey: The key for the localized title of `self`, describing
    ///       its purpose.
    ///     - action: The action to perform when `self` is triggered.
    public init(_ titleKey: LocalizedStringKey, action: @escaping () -> Void)

    /// Creates an instance with a `Text` label generated from a title string.
    ///
    /// - Parameters:
    ///     - title: The title of `self`, describing its purpose.
    ///     - action: The action to perform when `self` is triggered.
    public init<S>(_ title: S, action: @escaping () -> Void) where S : StringProtocol
}

Answer 3

我建议使用ViewModifyer 而不是自定义视图。这些工作如下：

struct SideContent<SideContent: View>: ViewModifier {

    var title: String
    var sideContent: (() -> SideContent)?

    init(title: String) {
         self.title = title
    }

    init(title: String, @ViewBuilder sideContent: @escaping () -> SideContent) {
         self.title = title
         self.sideContent = sideContent
    }

    func body(content: Content) -> some View {
        HStack {
          VStack {
             Text(title)
             content
           }
           sideContent?()
        }
    }
}

这个可以作为SomeView().modifier(SideContent(title: "asdasd") { Text("asdasd")})使用，不过如果省略边，还是需要指定它的类型SomeView().modifier(SideContent<EmptyView>(title: "asdasd"))

更新

删除它简化的标题，正如你提到的。

struct SideContent<SideContent: View>: ViewModifier {

    var sideContent: (() -> SideContent)

    init(@ViewBuilder sideContent: @escaping () -> SideContent) {
        self.sideContent = sideContent
    }

    func body(content: Content) -> some View {
        HStack {
            content
            sideContent()
        }
    }
}

另外，您可以为Title 进行修饰符。

struct Titled: ViewModifier {

    var title: String

    func body(content: Content) -> some View {
        VStack {
            Text(title)
            content
        }
    }
}

SomeView()
   .modifier(Titled(title: "Title"))
   .modifier(SideContent { Text("Side") })