Here's a concrete solution for Swift 3.0+ based on the accepted answer. Note that, like the accepted answer, this is known to work only on the iOS Mail app and not necessarily other apps.
Implementation:
class MessageWithSubject: NSObject, UIActivityItemSource {
let subject:String
let message:String
init(subject: String, message: String) {
self.subject = subject
self.message = message
super.init()
}
func activityViewControllerPlaceholderItem(_ activityViewController: UIActivityViewController) -> Any {
return message
}
func activityViewController(_ activityViewController: UIActivityViewController, itemForActivityType activityType: UIActivityType) -> Any? {
return message
}
func activityViewController(_ activityViewController: UIActivityViewController,
subjectForActivityType activityType: UIActivityType?) -> String {
return subject
}
}
Usage:
Here's an example of usage. Note that it works well to use this as the first item in the activityItems array, and include any additional items to follow:
let message = MessageWithSubject(subject: "Here is the subject", message: "An introductory message")
let itemsToShare:[Any] = [ message, image, url, etc ]
let controller = UIActivityViewController(activityItems: itemsToShare, applicationActivities: nil)