SwiftUI DSL背后的原理

今年的WWDC应该是近几年最让开发者兴奋的一届，新发布的SwiftUI更是让社区一片欢腾，俨然开启了Apple app开发的新纪元。玩了一下SwiftUI，它抛弃了AutoLayout，放弃了Storyboard，带来了手写UI的丝滑体验。爽的同时，我们来看一下SwiftUI DSL到底是如何实现的。

在SwiftUI里我们方法看到了一种新的语法：

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var body: some View {
    VStack(alignment: .leading) {
        Text("Hello, World") // No comma, no separator ?!
        Text("Hello World!")
    }
}

注意上面的两个Text直接是没有逗号或其它什么分隔符的，直接堆砌上去，这到底是怎么做到的？

以下内容来自StackOverflow，一些专用名词实在不好翻译成中文，就直接上英文了：

If you look at the documentation for VStack‘s init(alignment:spacing:content:), you can see that the content: parameter has the attribute @ViewBuilder:

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init(alignment: HorizontalAlignment = .center, spacing: Length? = nil,
     @ViewBuilder content: () -> Content)

This attribute refers to the ViewBuilder type, which if you look at the generated interface, looks like:

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@_functionBuilder public struct ViewBuilder {

    /// Builds an empty view from a block containing no statements, `{ }`.
    public static func buildBlock() -> EmptyView

    /// Passes a single view written as a child view (e..g, `{ Text("Hello") }`)
    /// through unmodified.
    public static func buildBlock(_ content: Content) -> Content 
      where Content : View
}

The @_functionBuilder attribute is a part of an unofficial feature called “function builders“, which has been pitched on Swift evolution here, and implemented specially for the version of Swift that ships with Xcode 11, allowing it to be used in SwiftUI.

Marking a type @_functionBuilder allows it to be used as a custom attribute on various declarations such as functions, computed properties and, in this case, parameters of function type. Such annotated declarations use the function builder to transform blocks of code:

• For annotated functions, the block of code that gets transformed is the implementation.
• For annotated computed properties, the block of code that gets transformed is the getter.
• For annotated parameters of function type, the block of code that gets transformed is any closure expression that is passed to it (if any).

The way in which a function builder transforms code is defined by its implementation of builder methods such as buildBlock, which takes a set of expressions and consolidates them into a single value.

参考：What enables SwiftUI’s DSL?