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UIActivity is an awesome API for sharing content with built-in iOS services. It's really easy to share via email and text message with the built-in activities, but for one really fatal flaw: Apple's API doesn't allow for customizing content specific to the built-in activities.
Take MFMailComposeViewController: incredibly useful, except that there's no API for setting the subject and adding attachments. Another example: The text you put on there for a Tweet might be different from the text you put into a text message. But there's no API to be able to specify special text for the text message.
WHTextActivity and WHMailActivity are UIActivity subclasses that allow you to use these powerful controllers, but be able to customize their content.