According to the latest buzz, Apple is planning something big for WWDC 2019: It may release a formal Marzipan SDK for porting iOS apps directly to the Mac App Store. Bloomberg reports that the SDK set for possible release at WWDC will be unique to iPad apps. Once the SDK is implemented, developers will be able to ship an iPad app to both the iOS App Store and Mac App Store without having to create two unique codebases. We’ve seen previews of this already, with Apple’s own ‘Marzipan’-based apps: Home, News, Stocks, and Voice Memos. All four were written in Swift; let's call them the "four horsemen of the app apocalypse." The report states this iPad-to-Mac SDK will arrive “later this year,” and could make its debut at WWDC in June. This release schedule would follow the normal rhythms of Apple’s developer ecosystem. Typically, WWDC serves as the launch for APIs and SDKs developers can’t formally utilize until later in the year, when the ‘gold master’ versions of iOS and macOS (and tvOS and watchOS) hit user’s devices. The report says 2020 will see this SDK hit the iPhone: “Apple engineers have found this challenging because iPhone screens are so much smaller than Mac computer displays.” This suggests iPhone apps may not be ‘windowed’ apps for the Mac (the same way Android apps are displayed on Chrome OS). In theory, 2021 is the year all three platforms (iPad, iPhone, and Mac) will reportedly be able to access one single app binary. If this timeline is accurate, we should expect the SDK Apple releases for the iPad-to-Mac app merger will be a watershed moment for its platforms as a whole, and a turning point for iOS and macOS engineers. But the report leaves a lot of questions unanswered. A major consideration is where this leaves the UIKit/AppKit framework partnership. Currently, Apple says macOS apps written as ‘Marzipan’ apps are mostly AppKit, which is the legacy framework for macOS apps. At WWDC 2018, the company suggested the SDK was how it made its four Marzipan horsemen of the app apocalypse available for the Mac from the iPad. At the time, we noted these apps were almost direct ports of iPad apps, and that each adopted the iPad's user interface – but it’s unclear how these apps use AppKit and UIKit. Apple’s on-screen graphics during that keynote (seen at the top of this article) would lead you to believe the ‘Marzipan’ apps are mostly AppKit, but Senior Vice President Craig Federighi said AppKit was simply handling things like trackpad input and window resizing. He also said the apps were able to go cross-platform with “very few code changes” from the iPad version to the Mac version, which suggests Apple simply added an SDK to its code. And while Federighi didn’t explicitly say there was an internal SDK at play last year, he did say the ability to ‘port’ iPad apps to the Mac was going to be made available to developers in 2019. Again, the most likely scenario is a WWDC release for developers, with cross-platform apps going live as new versions of macOS and iOS drop later this year. We’re also unsure how tightly Apple will control this reported SDK. Specifically, it could block certain types of apps or features from running on the Mac. For instance, we’re not sure how you’d be able to have augmented reality features for the Mac, while there are plenty of iPad apps that have them. This also casts a shadow on the Mac App Store, which was redesigned at WWDC 2018 to look more like the iOS App Store. It’s entirely possible the redesign was made to allow the two to merge more seamlessly later on. If apps are made available across platforms easily, or even by default, there’s simply no need for multiple app stores. The reported timeline for this SDK rollout is also not really news. Apple admitted this whole iOS-apps-on-the-Mac thing would take years, so a potential 2021 date is no surprise. With regard to timelines, let's blend in the separate rumor of ‘rOS,’ allegedly Apple’s augmented reality operating system, reportedly set to launch in 2020. If Apple launches rOS at WWDC 2020, making the ‘Marzipan’ SDK available for iPhone in 2020 divests augmented reality in a purer way, and allows Apple the opportunity to spin up a totally new app store or category for augmented reality apps.