Another interesting idea added to Mail this time around is a feature called Markup, which allows you to add simple annotations to images and PDFs from directly within a Mail window. Here's how it works: You click on a file and an icon appears in the top-left corner of the file's preview. Click it and select Markup, and the item zooms out, with a toolbar appearing directly above the item.
Markup is actually an example of the new Extensions technology that's debuting in both Yosemite and iOS 8, in which code from an entirely separate application can appear inside another app's window. In this case, Apple's written a Markup extension that lets you draw lines, shapes, text, and more on PDFs and images. You can stick your signature on PDFs without ever leaving Mail. It's a pretty clever idea.
Unfortunately, I'm not thrilled about the implementation. Marking up a file really does feel like you're using a different program entirely--but one without keyboard shortcuts or a menu bar. I kept pressing Command-Z in order to undo mistakes I made in Markup, and it did nothing. The Markup controls are pretty simple--they feel more like an iOS app than a part of Mail, to be honest--but they definitely did the job once I got the hang of them.
Extensions have the potential to dramatically reshape how we use Mac apps, but they will take some getting used to. Also, it's early days—it seems that Apple does not currently give third-party developers the level of access that the Markup extension has. Right now, developers are limited in the locations where they can deploy Extensions, and attachments in a Mail message isn't one of them. That's probably a good thing, because it feels like Apple is experimenting with features like Markup. It might be wise to let Apple's own developers realize the strengths and weaknesses of Extensions before opening it up to everyone else.
Messages, Apple's go-to app for iMessage and miscellaneous other chat services (that's clearly the order of priority), gets a bunch of new upgrades in Yosemite that serve to enhance your conversations, so long as everyone is using iMessage. (Those miscellaneous other chat services are still supported, it's just that support for them appears to have been frozen in amber for several updates. If you truly love IM like many of my friends do, you'd probably be better off using an app such as Adium for those services, and keep iChat focused on iMessage.)
The marquee feature in Messages on Yosemite is probably Soundbites, which adds a microphone button next to your chat window. Click it, and you'll be able to record a brief audio message and send it via iMessage. (This feature is probably more useful on iOS devices, where it will arrive as part of iOS 8.)
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