Microsoft is open-sourcing an early version of its toolkit for iOS developers to enable them to move their apps more easily to Windows 10.

Microsoft demonstrated this toolkit, codenamed "Project Islandwood," at its Build developers conference at the end of April. Islandwood is one of four "bridges" that Microsoft announced at Build. These bridges are meant to increase the number of applications available in the new unified Windows Store that is part of Windows 10.

An early preview of the Windows Bridge for iOS is available today, August 6, on GibHub under the MIT open-source license. Microsoft's goal is to release the final version some time later this fall, timed to coincide with an upcoming Visual Studio 2015 update.

The iOS bridge enables developers to build apps that work with Windows 8.1 and Windows 10 apps that run on both x86 and x64 systems. Support for ARM-based systems, specifically for mobile devices, will be coming this fall. Prior to today, the iOS bridge was available currently as an invite-only preview.

The target audience are iOS developers who are willing to import their Xcode projects into Visual Studio using a newly developed Microsoft Objective-C compiler and extend their code to "take advantage of Universal Windows Platform capabilities," such as Cortana.

The other three bridges are all on different timelines. Microsoft already made its Windows Bridge for web apps, codenamed "Project Westminster" available and usable with Visual Studio 2015.

Visual Studio 2015 was released to manufacturing on July 20, and the new Windows Store opened for app submissions on July 29, the same day Windows 10 began rolling out.

The "Project Astoria" bridge for Android developers also is available as an invitation-only tech preview. Microsoft is planning to deliver the bridge for Android in public beta form this fall.

Astoria is meant to allow developers to use their existing Android code to build apps that will work on Windows 10 phones. Astoria includes a Windows Phone emulator.

Microsoft is adding a layer to its Windows 10 Mobile operating system that will allow Android Open Source Project (AOSP) to run as a subsystem -- similar to the way that it traditionally ran POSIX as a subsystem in Windows. Android developers will be able to submit versions of their apps, written in Java or C++, to the Windows Store in he form of APKs and have those apps work on Windows Phone 10 devices, Microsoft officials have said.

The "Project Centennial" bridge, which is designed to bring existing .NET and Win32-based apps to the Windows Store is the furthest behind. It will be available for testing some time in 2016, Microsoft officials said.