To open an iMessage window, you only need to create MFMessageComposeViewController instance. It is very easy to use and present it like other UIViewController.
Your viewcontroller should conform to protocol MFMEssageComposeViewControllerDelegate to handle actions within the MFMMessageComposeViewController.
I thought this was a simple task, but the transparent background took me awhile to figure it out, so I am going to write it down here.
First must set the current view controller definesPresntationContext to YES, and popup view controller’s modalPresentationStyle to UIModalPresentationOverCurrentContext
This setting allows the current context to be displayed as background when popup the new view controller.