swift - ResultBuilder

struct ContentView: View {
     var body: some View {
         // This is inside a result builder
         VStack {
             Text("Hello World!") // VStack and Text are 'build blocks'
         }
     }
 }

@ViewBuilder var body: Self.Body { get }

NSAttributedString {
    "Hello "
      .color(.red)
    "World"
      .color(.blue)
      .underline(.blue)
}

@resultBuilder
struct AttributedStringBuilder {
  static func buildBlock(_ segments: NSAttributedString...) -> NSAttributedString {
    let string = NSMutableAttributedString()
    segments.forEach { string.append($0) }
    return string
  }
}

extension NSAttributedString {
  convenience init(@AttributedStringBuilder _ content: () -> NSAttributedString) {
    self.init(attributedString: content())
  }
}

// MARK: - extension
extension NSAttributedString {
    func apply(_ attributes: [NSAttributedString.Key:Any]) -> NSAttributedString {
        let mutable = NSMutableAttributedString(string: self.string, attributes: self.attributes(at: 0, effectiveRange: nil))
        mutable.addAttributes(attributes, range: NSMakeRange(0, (self.string as NSString).length))
        return mutable
    }

    func foregroundColor(_ color: UIColor) -> NSAttributedString {
        self.apply([.foregroundColor : color])
    }

    func font(_ font: UIFont) -> NSAttributedString {
        self.apply([.font: font])
    }
}

extension String {
    func foregroundColor(_ color: UIColor) -> NSAttributedString {
        NSAttributedString(string: self, attributes: [.foregroundColor : color])
    }
    func font(_ font: UIFont) -> NSAttributedString {
        NSAttributedString(string: self, attributes: [.font: font])
    }
}

        let str = NSAttributedString {
            "hello "
                .foregroundColor(.red)
                .font(.systemFont(ofSize: 14))
            "world!"
                .foregroundColor(.label)
                .font(.systemFont(ofSize: 20, weight: .bold))
        }

// 对应 block 中没有值返回的情况
static func buildBlock(_ components: Component...) -> Component

// 通过一个或多个值来构建    必须实现
static func buildBlock(_ components: Component...) -> Component
Combines an array of partial results into a single partial result. A result builder must implement this method.

// 通过可选值来构建，可以支持不包含 else 的 if 语句
static func buildOptional(_ component: Component?) -> Component
Builds a partial result from a partial result that can be nil. Implement this method to support if statements that don’t include an else clause.

// 支持通过条件来变化的值，通过下面这两个方法支持包含 switch 和 if-else 的语句
static func buildEither(first: Component) -> Component
Builds a partial result whose value varies depending on some condition. Implement both this method and buildEither(second:) to support switch statements and if statements that include an else clause.

static func buildEither(second: Component) -> Component
Builds a partial result whose value varies depending on some condition. Implement both this method and buildEither(first:) to support switch statements and if statements that include an else clause.

// 通过数组来构建 可以支持 forin 语句
static func buildArray(_ components: [Component]) -> Component
Builds a partial result from an array of partial results. Implement this method to support for loops.

// 通过 Expression 来生成，Expression 为自定义的表达式 具体可以看官方文档中的例子
static func buildExpression(_ expression: Expression) -> Component
Builds a partial result from an expression. You can implement this method to perform preprocessing—for example, converting expressions to an internal type—or to provide additional information for type inference at use sites.

// 用于对最外层的 `buildBlock` 结果的再包装。例如，让结果构建器隐藏一些它并不想对外的类型（转换成可对外的类型）。
static func buildFinalResult(_ component: Component) -> FinalResult
Builds a final result from a partial result. You can implement this method as part of a result builder that uses a different type for partial and final results, or to perform other postprocessing on a result before returning it.

// 如果提供了 `buildLimitedAvailability` 的实现，构建器提供了对 API 可用性检查（如 `if #available(..)`）的支持。
static func buildLimitedAvailability(_ component: Component) -> Component
Builds a partial result that propagates or erases type information outside a compiler-control statement that performs an availability check. You can use this to erase type information that varies between the conditional branches.