How to detect whether custom keyboard is activated from the keyboard's container app?

Question

I was wondering if there is a method that would allow me to detect from the keyboard container app whether the associated keyboard has been activated in the

Answer 1

Just in case here is Swift version of Kurt's brilliant and awesome answer:

   func isKeyboardExtensionEnabled() -> Bool {
    guard let appBundleIdentifier = Bundle.main.bundleIdentifier else {
        fatalError("isKeyboardExtensionEnabled(): Cannot retrieve bundle identifier.")
    }

    guard let keyboards = UserDefaults.standard.dictionaryRepresentation()["AppleKeyboards"] as? [String] else {
        // There is no key `AppleKeyboards` in NSUserDefaults. That happens sometimes.
        return false
    }

    let keyboardExtensionBundleIdentifierPrefix = appBundleIdentifier + "."
    for keyboard in keyboards {
        if keyboard.hasPrefix(keyboardExtensionBundleIdentifierPrefix) {
            return true
        }
    }

    return false
}