To initiate a file transfer from an iOS device, tap the Share icon in any app that supports it, such as Photos, and a recipient will display within seconds in the AirDrop field. Tap the recipient and the file is sent (upon confirmation and approval on the other side, if needed).
If you're having trouble finding an AirDrop user, swipe up from the bottom of the screen on an iOS device and toggle AirDrop off and then on. On the Mac, open Finder and click the AirDrop sidebar. Toggle "Allow me to be discovered by:" off and on again.
On a recent hiking trip, my friends and I used AirDrop to exchange videos and photos on the fly, and I was then able to send that media to my Mac without plugging in my phone.
SMS relay: This lets Messages on the Mac (and iPad) send and receive the SMS messages you get on the iPhone (which it couldn't before).
The Messages app picks up a few other tricks, too. Like Finder, the Message app title bar and sidebar now feature slick translucency effects, but the real improvements are in the Details field. From here you can access the message recipient's contact information, if any, and you have the ability to make a voice or video call, share displays and mute the conversation. If you're following the location of the recipients, a Maps field will show you where they are, and pictures that have been sent will be consolidated in this view as well.
A chat with more than one recipient lets you name the chat, add more contacts via the Add Contact... option -- and, if the conversation notifications become too unruly, you can mute the conversation via Do not disturb. There's also an option that lets you leave the conversation entirely. And next to the messages input field is a microphone that allows you to send audio snippets, similar to that of Messages in iOS 8.
Notifications now supports a new Today view that includes customizable and interactive widgets, letting you see useful info at a quick glimpse. You can add more widgets (or remove them) by clicking the Edit button at the bottom of the Notification screen.
Personally, I don't like the way the Today and Notifications area are divided; because it toggles between the two, you're either looking at your Today view or your Notifications. Why not both? Why can't the app notifications be a customizable field in the Today view? As implemented, I have to scroll to view all of the widgets in my Today view; since I'm scrolling anyway, why not allow notifications to be mixed in?
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