ComposeMessage.swift

private func composeMessage() {
    let session = MSSession()
    
    let layout = MSMessageTemplateLayout()
    layout.image = UIImage(named: "message-background.png")
    layout.imageTitle = "iMessage Extension"
    layout.caption = "Hello world!"

    let message = MSMessage(session: session)
    message.layout = layout
    message.summaryText = "Sent Hello World message"
}