这篇文章是针对environmentvalues/dismiss 进行的解读和相关发散思考。

dismiss

An action that dismisses the current presentation.

var dismiss: DismissAction { get }

https://docs.swift.org/swift-book/documentation/the-swift-programming-language/properties/

详细读一下 properties 这章节，了解 { get } 这种写法。

Discussion

Use this environment value to get the DismissAction instance for the current Environment. Then call the instance to perform the dismissal. You call the instance directly because it defines a callAsFunction() method that Swift calls when you call the instance.

For information about how Swift uses the callAsFunction() method to simplify call site syntax, see Methods with Special Names in The Swift Programming Language.

You can use this action to :

• Dismiss a model presentation, like a sheet or a popover.
• Pop the current view from a NavigationStack.

On apps targeting iOS 18 and aligned release, you can use the dismiss action to pop the implicit stack of a collapsed NavigationSplitView, or clear the equivalent state in an expanded split view.

这段文字比较难理解，我们翻译成中文： 在针对iOS18和相应版本的应用程序中，您还可以使用 dismiss 操作来弹出折叠状态下 NavigationSplitView 的隐式堆栈，或在展开状态的分栏视图中清除相应的状态。

另外一个有趣的地方是，我们只能在需要 dismiss 的页面中去定义

比如下面的代码：

private struct DetailView: View {
    @State private var isSheetPresented = false
    @Environment(\.dismiss) private var dismiss // Applies to DetailView.


    var body: some View {
        Button("Show Sheet") {
            isSheetPresented = true
        }
        .sheet(isPresented: $isSheetPresented) {
            Button("Done") {
                dismiss() // Fails to dismiss the sheet.
            }
        }
    }
}

我们想通过弹出的"Done" 按钮去 dismiss 这个弹出框，这样做会失败。因为 dismissAction 只对定义它的环境生效，这里我们是定义在 DetailView 里面,那么就应该是 dismiss DetailView 而不是 .sheet 中的视图。

如果我们想要达到 dismiss .sheet弹出视图，那么我们可以这样做：

private struct DetailView: View {
    @State private var isSheetPresented = false


    var body: some View {
        Button("Show Sheet") {
            isSheetPresented = true
        }
        .sheet(isPresented: $isSheetPresented) {
            SheetContents()
        }
    }
}

private struct SheetContents: View {
    @Environment(\.dismiss) private var dismiss


    var body: some View {
        Button("Done") {
            dismiss()
        }
    }
}

通过新定义一个 SheetContents 视图，来在其中定义 @Environment(\.dismiss) private var dismiss 来达到我们想要的效果。

如何查看一个 View 是否被 presented

The dismiss action has no effect on a view that itn't currently presented. If you need to query whether SwiftUI is currently presenting a view, read the isPresented environment value.

意思就是我们可以在视图中查询当前的 isPresented 环境变量，来找到当前的视图是否被 presented。 通过下面的代码： environmentvalues/isPresented

@Environment(\.isPresented) private var isPresented
.onChange(of: isPresented) { _, isPresented in
    if isPresented {
        // Do something when first presented.
    }
}

但是这段代码我在 macOS15.2下验证，onChange 没有观察到 isPresented 值变化。

还需要在 iOS 下验证一下结果。