DispatchQueue+isRunningOnMainQueue.swift

/// Inspired by
/// http://blog.benjamin-encz.de/post/main-queue-vs-main-thread/
/// and https://stackoverflow.com/a/60348601
/// Associates a predefined key/value pair with the main queue and later uses the presence of it
/// to determine whether the current queue is the main queue.
extension DispatchQueue {
    private static let mainQueueCheckKey: DispatchSpecificKey<String> = {
        let mainQueueCheckKey = DispatchSpecificKey<String>()
        DispatchQueue.main.setSpecific(key: mainQueueCheckKey, value: mainQueueCheckValue)
        return mainQueueCheckKey
    }()
    private static let mainQueueCheckValue = "MainQueueCheck"

    public static var isRunningOnMainQueue: Bool {
        getSpecific(key: mainQueueCheckKey) == mainQueueCheckValue
    }
}