Use os_signpost for performance logging of transformations in RxSwift

RxSignpost.swift

// RxSwift signposts

import os.signpost
import RxSwift

func signpost<T>(log: OSLog, name: StaticString, value: String, _ thing: () throws -> T) rethrows -> T {
    let signpostID = OSSignpostID(log: log)

    os_signpost(
        .begin,
        log: log,
        name: name,
        signpostID: signpostID,
        "%{public}s",
        value
    )

    let result = try thing()

    os_signpost(
        .end,
        log: log,
        name: name,
        signpostID: signpostID,
        "%{public}s",
        value
    )

    return result
}

extension ObservableType {

    /// Projects each element of an observable sequence into a new form.
    /// Also uses os_signpost to log the duration of transformation.
    /// Useful for performance debugging.
    ///
    /// - Parameters:
    ///   - log: an instance of OSLog to use for logging
    ///   - name: The name of the event, e.g. "decode image"
    ///   - logMessageGenerator: A closure that takes the value to be
    ///     transformed as input and returns some meaningful information
    ///     as output. Useful to identify which operation is occurring.
    ///     For example, { $0.imageURL }.
    ///   - transform: A transform function to apply to each source element.
    /// - Returns: An observable sequence whose elements are the result
    ///   of invoking the transform function on each element of source.
    func mapAndSignpost<T>(log: OSLog, name: StaticString, logMessageGenerator: @escaping (Element) -> String, _ transform: @escaping (Self.Element) throws -> T) -> Observable<T> {
        return self.map {
            let signpostID = OSSignpostID(log: log)
            let message = logMessageGenerator($0)
            os_signpost(
                .begin,
                log: log,
                name: name,
                signpostID: signpostID,
                "%{public}s",
                message
            )

            let result = try transform($0)

            os_signpost(
                .end,
                log: log,
                name: name,
                signpostID: signpostID,
                "%{public}s",
                message
            )
            return result
        }
    }

    func startSignpost(on log: OSLog, message: String) -> Observable<Element> {
        let dict = Thread.current.threadDictionary
        let id = OSSignpostID(log: log)
        dict["myCoolSignpostID"] = id
        dict["myCoolLogger"] = log
        return self.do(onNext: { _ in
            os_signpost(
                .begin,
                log: log,
                name: "Name",
                signpostID: id,
                "%{public}s",
                message
            )
        })
    }

    func endSignpost() -> Observable<Element> {
        let dict = Thread.current.threadDictionary
        let id = dict["myCoolSignpostID"] as! OSSignpostID
        let log = dict["myCoolLogger"] as! OSLog

        return self.do(onNext: { _ in
            os_signpost(
                .end,
                log: log,
                name: "Name",
                signpostID: id,
                "%{public}s",
                "message"
            )
        })
    }

}