解析SwiftUI布局细节（一）

/// A view that arranges its children in a vertical line.
@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, *)
@frozen public struct VStack<Content> : View where Content : View {

    /// Creates an instance with the given spacing and horizontal alignment.
    ///
    /// - Parameters:
    ///   - alignment: The guide for aligning the subviews in this stack. It has
    ///     the same horizontal screen coordinate for all children.
    ///   - spacing: The distance between adjacent subviews, or `nil` if you
    ///     want the stack to choose a default distance for each pair of
    ///     subviews.
    ///   - content: A view builder that creates the content of this stack.
    @inlinable public init(alignment: HorizontalAlignment = .center, spacing: CGFloat? = nil, @ViewBuilder content: () -> Content)

    /// The type of view representing the body of this view.
    ///
    /// When you create a custom view, Swift infers this type from your
    /// implementation of the required `body` property.
    public typealias Body = Never
}

@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, *)
@_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 through unmodified.
    ///
    /// An example of a single view written as a child view is
    /// `{ Text("Hello") }`.
    public static func buildBlock<Content>(_ content: Content) -> Content where Content : View
}

/// 用_functionBuilder修饰TestBuilder
/// 就像用_functionBuilder修饰了ViewBuilder一样
/// 我们就用TestBuilder看看它的实际效果
@_functionBuilder struct TestBuilder {
    
    /// String... 参数 数量可变,你可以传入任意数量的参数
    /// - Parameter items: items description
    /// - Returns: description
    static func buildBlock(_ items: String...) -> [String] {
        
        return items
    }
}

/// 然后我们有这样一个方法
/// @TestBuilder模拟@ViewBuilder
/// - Parameter content: content description
func testBuilder(@TestBuilder _ content:() -> [String]){
        
    print(content())
}

/// 然后我们调用的时候
self.testBuilder {
     "1"
     "2"
     "3"
     "4"
}

@available(iOS 13.0, macOS 10.15, tvOS 13.0, watchOS 6.0, *)
extension ViewBuilder {

    public static func buildBlock<C0, C1, C2, C3, C4, C5, C6, C7, C8, C9>(_ c0: C0, _ c1: C1, _ c2: C2, _ c3: C3, _ c4: C4, _ c5: C5, _ c6: C6, _ c7: C7, _ c8: C8, _ c9: C9) -> TupleView<(C0, C1, C2, C3, C4, C5, C6, C7, C8, C9)> where C0 : View, C1 : View, C2 : View, C3 : View, C4 : View, C5 : View, C6 : View, C7 : View, C8 : View, C9 : View
}

/// 定义一个常见的背景View
struct Background<Content: View>: View {
    
    private var content: Content

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

    var body: some View {
         
        Color.white
        .frame(width: UIScreen.main.bounds.width, height: UIScreen.main.bounds.height)
        .overlay(content)
    }
}

///  UIApplication 的扩展
extension UIApplication {
    
    func endEditing() {
        
        sendAction(#selector(UIResponder.resignFirstResponder), to: nil, from: nil, for: nil)
    }
}

/// 具体的使用就是下面这样，这样就达到了我们的目的，中间的代码我隐藏起来了，代码在BaseLoginView中可以查看到
///
var body: some View {
        
     Background {                
          /// 里面具体的视图内容
     }.onTapGesture {

          self.endEditing()
     }
}