MacOS Marzipan
Some people have been clamouring for a way to run iOS apps on an Mac for a long time, and now here it is.When Marzipan was first announced, Apple was very careful to temper people's expectations.  Marzipan is not about some grand new cross-platform development framework.  It's not about merging Mac and iOS.  It's about allowing iOS developers to port certain iOS apps to the Mac with as little friction as possible.And in that sense it's a complete success.  Many 3rd-party apps can already run inside of Marzipan with little or no code changes.  That's a huge win.There are some very obvious downsides.  Most iOS apps are a poor fit on the desktop and won't conform to the expected Mac desktop expectations.  I suppose Apple could have gone the Windows 8 route and put these iOS apps in a completely separate environment, but we all know how that turned out on Windows.  At least these apps attempt to mesh with the regular desktop environment.Another downside is that it's a hack.  These are pure iOS apps running inside of an iOS simulator designed to look like a Mac app.  These apps have no access to the world outside of iOS.If you're looking to make a great modern Mac app, Marzipan isn't for you.  If you're looking for an easier way to share UI code between your AppKit and UIKit apps, Marzipan has nothing for you.  If you have a relatively simple iOS app that you think might be useful on a Mac, Marzipan is exactly what you've been waiting for.If you want a peek into what a grand new cross-platform development framework might be, take a look at the JetEngine.framework embedded in the Mac App Store app.  It appears to be a separate UI framework that is neither AppKit nor UIKit, but can run on top of either.  However, it's full of App Store-specific UI components and I would expect that it was developed only for internal use at Apple.