I'm looking for a WebDriver implementation for Microsoft UI Automation (UIA) to automate desktop applications and store apps.
Just like what Appium and Appium for Mac have done, in addition to web applications and mobile apps, I think WebDriver API should be applicable to Windows desktop applications and store apps (formerly known as Metro apps) as well.
I'm going to implement a server that exposes WebDriver API and interacts with UI elements via UI Automation API internally. Do you know if there exist some open source projects doing this?
I don't know of an existing project that wraps UIA with the WebDriver API.
But RAutomation might be a useful example to take a look at - it sounds a bit like what you describe, but with the Watir API as the design inspiration instead of WebDriver's API:
https://github.com/jarmo/RAutomation
Note that there's an experimental UIA driver included with RAutomation.
Also, in case it's useful, I have a github repo at the link below that contains a hello world program (ported across several languages) implemented using both the COM and .NET flavors of UIA:
https://github.com/billagee/test-polyglot/tree/master/printDesktopUiaElementName