To be useful as a result builder, a result builder type must provide a sufficient subset of function-building methods that enable the transformation of various statement kinds (if
, switch
, for
..in
, etc.). The following example result builder illustrates the various function-building methods one can define:
@resultBuilder struct ExampleResultBuilder { /// The type of individual statement expressions in the transformed function, /// which defaults to Component if buildExpression() is not provided. typealias Expression = ... /// The type of a partial result, which will be carried through all of the /// build functions. typealias Component = ... /// The type of the final returned result, which defaults to Component if /// buildFinalResult() is not provided. typealias Result = ... /// Required by every result builder to build combined results from /// statement blocks. static func buildBlock(_ components: Component...) -> Component { ... } /// If declared, provides contextual type information for statement /// expressions to translate them into partial results. static func buildExpression(_ expression: Expression) -> Component { ... } /// Enables support for `if` statements that do not have an `else`. static func buildOptional(_ component: Component?) -> Component { ... } /// With buildEither(second:), enables support for 'if-else' and 'switch' /// statements by folding conditional results into a single result. static func buildEither(first component: Component) -> Component { ... } /// With buildEither(first:), enables support for 'if-else' and 'switch' /// statements by folding conditional results into a single result. static func buildEither(second component: Component) -> Component { ... } /// Enables support for..in loops in a result builder by combining the /// results of all iterations into a single result. static func buildArray(_ components: [Component]) -> Component { ... } /// If declared, this will be called on the partial result of an 'if /// #available' block to allow the result builder to erase type /// information. static func buildLimitedAvailability(_ component: Component) -> Component { ... } /// If declared, this will be called on the partial result from the outermost /// block statement to produce the final returned result. static func buildFinalResult(_ component: Component) -> Result { ... } }