It is important for two reasons. First, it adds full fat Windows and Xbox to increase the market for the app ‘pie’ by about 1 billion units and second, developers can create tailored experiences for each device type but reuse the bulk of the code for all the apps.

Over time, many of the 240,000 apps in Windows Phone store will make the update to universal apps (UA). Developers can allow users to buy an app once, and have it run on all Windows devices without needing to buy the app again. Similarly, in-app purchases, scores, achievements, rewards, and notifications can be shared across devices due to Microsoft’s OneDrive cloud.

There have been some concerns from developers that apps for mobile devices are priced lower than for desktops. The counter argument is that the market will expand exponentially. There is also the concern that apps written for the small touch screen will not translate to larger non-touch screens. The counter argument is that no one forces developers to write universal apps although the incidence of touch is now at about 40% of notebook, and a growing number of desktop, sales.

The first two true commercial universal apps are Halo: Spartan Assault and Skulls of Shogun. Microsoft has also turn several of its free games into UAs including Wordament, Hexic, Mahjong, Minesweeper, and Solitaire.

Apple and Google do offer some so called universal app ‘purchases’ but these are more in name than function. Developers still need to write separate programs for each device – Mac OSX and iOS and Android in its various formats – they just charge once for the app.

Technical stuff

The majority of Windows Phone apps were written using Microsoft Silverlight (xap) as the base. A new Windows Runtime platform (appx, and appxbundle) is now used to maximise code reuse. The only minor issue is that apps must run in its Tiled UI environment – not the desktop.

App developers also get the benefit of an enhanced speech recognition and search system called Cortana and many of Bing’s location services. Ultimately there will be only one Windows app store and developers can reserve ‘names’ for apps that they intend to make universal.