How to save the SMS of iPhone 6 Plus as a draft?

Imessage on Apple's ios system doesn't have the function of saving drafts, but as long as you can't quit in the background, when you click into imessage application, the edited SMS still exists.

IMessage is an instant messaging software launched by Apple, which can send short messages, videos and so on. It has high security. Different from the SMS/MMS service of operators, users only need the data support of WiFi or cellular data network to complete communication.