Microsoft revealed two upcoming dual-screen devices last year: the Android-based Surface Duo, and the larger Surface Neo running Windows 10X. Today the company released a preview SDK for the Surface Duo, so developers can start working on apps that take advantage of both displays.
Since the Surface Duo will run Android with all of Google's usual apps and services, developing apps for it won't be too different than creating a typical Android app. However, Microsoft is offering native Java APIs for accessing the hinge angle sensor, the DisplayMask API, and other device capabilities. The company will also have a Surface Duo image designed for the emulator in Android Studio, so developers don't have to use special tools just for testing Duo apps.
"Your app by default will occupy a single screen," Microsoft wrote in a blog post, "but users can span the app to cover both screens when the device is in a double-portrait or double-landscape layout."
Microsoft is also working to establish new web APIs for dual-screen devices, so Progressive Web Applications can also use second screen effectively. "We expect to provide an experimental implementation of these features in preview builds of the browser soon," the blog post said.