iMessage supplants iChat

In 2011 Apple introduced the iMessage communication system, a replacement for text messaging that let iOS devices communicate directly with one another. Unlike SMS text messages, the iMessage system transfers data (not just text, but images and files) via the Internet, so there are no text charges.

With Mountain Lion, support for iMessage comes to the Mac as well. And it happens via the Messages app, which is a renamed version of iChat with all its old features intact, plus support for iMessage.

Like Messages on the iPhone, Messages for Mac lets you hold multi-person chats and can optionally let people know when youve received and read their messages and when youre typing a reply. An integrated video-chat button allows you to kick off a video chat with capable devices, either over AIM (as iChat has always done) or by launching the FaceTime app.

Theres a lot to like about having access to iMessage on the Mac. When Id receive a message on my iPhone while working on my Mac, Id be frustrated that I had to type out a response on my iPhone keyboard rather than the big Mac keyboard right in front of me. Its now really easy to send a quick text message to my wife when Im at workall I have to do is type her name in a new Messages window and then type a message.

This isnt to say that there aren't frustrating things about Messages, or iMessage in general. Its great that using iMessage means you have a record of your conversations on all of your devices, and it makes it easy to keep on having a conversation even if you have to shut down your Mac and head for the bus stop. But every time I receive a message via iMessage on my Mac, my iPad and iPhone also chime or vibrate. Not just at the beginning, but every single time I receive a message. There should be a way for Apple to detect which device Im actively using to have the iMessage conversation and stop ringing the rest of them. I shouldnt have to mute my iPad and pull my iPhone out of my pocket in order to have a conversation on my Mac.

This feature has another odd side effect, too: When I opened my Mac up after I had been having an iMessage conversation on my iPhone, Messages opened and proceeded to open a new chat window and display the old messages from that conversation. Its great that I have a transcript of that chat on my Mac now, but it seems like an inelegant way to do it. (And often times I only see part of a conversation, which is less than helpful.)

I also found a strange, recurring bug when I was logged out of all my services and couldnt log back in until I opened the preferences window, deactivated the account, and then re-activated it.

The bottom line is that I love the idea of iMessage, relish not worrying about the cost of text messages, and am happy that I can send things via iMessage from my Mac. But Apple needs to focus on making the cross-device experience a little less obtrusive.

Alert! Notification Center appears

Sometimes your Mac needs to get your attention. For years, many Mac app developers have built their ownthink of meeting reminder pop-ups in iCal or Microsoft Office, for example. For years, the open-source project Growl has attempted to create a more general notification system, and its supported by lots of apps.

With Mountain Lion, OS X gains a system-level notification system accessible to every developer, with features much like those already found in iOS. Alerts appear in the top right corner of the screen in a small bubble. Notifications remain there for five seconds, and then slide off screen to the right (unless you swipe them away first, or click on them to open the relevant app). Alerts, on the other hand, remain on-screen until you click on the Show or Close (or in the case of some alerts, Snooze) buttons.

In iOS 5, you see all your recent notifications by pulling down from the top of the screen to reveal Notification Center. In Mountain Lion, the Notification Center list is a narrow band that lives just to off the right side of your screen. You can reveal it either by clicking on the new Notification Center icon at the far right of the menu bar, or by swiping with two fingers starting at the far right edge of the trackpad. Either way, your entire Mac interface will slide to the left, revealing a list of whats been trying to get your attention recently.

Not all notifications come from apps, either: Because Mountain Lion includes integrated support for Twitter and (coming this fall) Facebook, Notification Center can display notifications from either servicedirect messages and/or mentions on Twitter, and a whole host of optional items (events, application requests, nearby friends, friend requests, comments, wall posts, messages, photo tags, friend confirmations, and place tags) on Facebook. Apple has even built quick sharing links into the very top of the Notification Center list, so you can click to quickly write a tweet or Facebook status post.

Theres also a new Notifications pane in the System Preferences app, analogous to the Notifications submenu in iOSs Settings app. From here, you can choose which apps appear within Notification Center and how their alert bubbles behave.

Notifications are good when you want to see them, but they can also get in your way, depending on context. Apple has made some smart decisions in order to let you squelch notifications when theyre not appropriate. If you scroll up in the Notification Center list, a new option is revealed: Show Alerts and Banners. If you flip the switch to Off, notifications are mutedbut only until tomorrow. Apple assumes that you just dont want to be bugged right now, but doesnt want you to miss out on important notifications in the future.

Another clever feature is Notification Centers auto-sensing when a Mac is connected to an external display. I use a second display at my desk, and Notification Center has no problem displaying alerts there. But if I hook up my MacBook to an HDTV or a projector, the alerts will be suppressed. Mountain Lion actually looks for clues that the external display youve hooked up to is a TV or projector, and if it is, it wont show any notifications. Because the last thing you want is for a message from one of your friends on Twitter to float over a slide in an important presentation youre giving.

Ive found Notification Center to be a useful addition to my Mac. This is the sort of feature that needed to be a part of the operating system for the sake of consistency and ubiquity, and Apples done a good job of implementing it. I appreciate being alerted when someones sent me a Direct Message on Twitter or when Ive received an important email, and the settings in the Notification Center control panel are granular enough to allow me to suppress any notifications that get in my way. (In a way, they may be too granularI wish there was a way to more broadly set notification settings, rather than going app by app.)

Gatekeeper eyes your apps

Ever since Apple introduced the Mac App Store, many people have speculated that it was only a matter of time until the Mac, like iOS, could only run software sold directly via the store.

I never really thought that was a serious possibility, and Mountain Lion seems to clinch it. The new Gatekeeper feature, found in Mountain Lions Security & Privacy preference pane, adds an intermediate level of protection between fully-approved App Store apps and random files downloaded from unknown sources over the Internet. Its Apples attempt to bring more iOS-style security to Mac users even if the apps they use are not from the App Store, and its a great move.

By default, Mountain Lion will launch newly-downloaded apps from the Mac App Store as well as any apps written by identified developers without complaint. Identified developers are members of Apples Mac developer program who have obtained a certificate linked to their identity, which they use to cryptographically sign their apps. (Apple doesnt do any sort of background check on the developer, and it doesnt approve any of this software. All it means is that Apple knows who the developer who signed the app wasand that gives Apple the ability to revoke the developers license if theyre discovered to be a distributor of malware.) The act of cryptographically signing apps also prevents legitimate apps from being tampered with after the fact, since any modified apps will fail the check Mountain Lion performs.

Most people will only run into Gatekeeper when downloading an app that hasnt been updated with a developer signature. You can turn off Gatekeeper altogether, of course, but you can also choose to open unidentified apps manually: Just control-click on the app in the Finder and choose Open. Gatekeeper wont stop you.

Its also important to note that, as the name implies, Gatekeeper is not a system that continually scans your Mac looking for malware. It works only the very first time you try to open an app, using the same system that warns you before you open just about any file that you downloaded from the Internet. Once you give that app entry through the gate and into your Mac, theres no more security.

Developers have known since February that Gatekeeper was coming; Id wager that most Mac developers have acquired their certificates and signed their apps. And the ones that havent been signed will still run, once youve used your own judgment to decide whether you let them through the gate. Its a sensible strategy that doesnt leave developers whose apps cant be in the Mac App Store out in the cold, and most users wont notice a thing.

Gatekeepers not the only security addition to Mountain Lion. The Security & Privacy preference panes Privacy tab is now more granular. In addition to control over location-based data (introduced in Lion) and the sending of diagnostic information to Apple, you can also control access to Contacts, Twitter, and Facebook.