Source: Apple Keynote.

Many people expected Apple (NASDAQ:AAPL) to unveil updates to its OS X and iOS operating systems, an updated watchOS development kit, and of course Apple Music at its annual developers conference last week. But it was the unexpected announcement that the tech giant will open source its new programming language, Swift, that received the biggest applause from the developers in attendance.

It's easy to understand why. An open-source Swift will allow developers to extend the reach of their programs beyond Apple products. It means the door is open to port the language to Microsoft (NASDAQ:MSFT) Windows, and Apple is even handing over a Linux compiler. In fact, an open-source Swift could theoretically be used to create apps for Google's (NASDAQ:GOOG) (NASDAQ:GOOGL) Android, making it easy to deploy apps for both iOS and Android at the same time.

What is Apple thinking?
Apple is focused on the bigger picture with Swift. Craig Federighi, Apple's senior vice president for software engineering, told the audience at the conference, "We think Swift is the next big programming language, the one that we'll all be doing application and system programming on for 20 years to come."

Even in its infancy, Swift has struck a chord with developers. A recent survey from StackOverflow found that 78% of developers using Swift prefer to continue using the language. That's higher than any other programming language. Java, one of the programming languages used to develop Android apps, doesn't even make the list. Neither does Swift's predecessor, Objective-C.

Creating an open-source programming language increases the usability and adoption. Many people don't want to learn a programming language just to develop on a single platform, even if it's as popular as the iPhone. As an open-source language, Swift could gain acceptance in the education community, since it will no longer require expensive hardware to compile and run code.

Apple can dramatically increase the adoption of Swift by providing the source code for its compiler and libraries and letting third parties build on top of it. In the long run, that means more developers capable of easily making iOS and OS X apps, which ultimately strengthens its app ecosystem.

Helping the competition
At its annual Build conference a few weeks ago, Microsoft announced its plans to enable Android and iOS developers to port their apps to Windows 10. It will support Android apps developed in C++ and Java and iOS apps written in Objective-C. It's also working to clone Swift, but now that it will be open-source, the task is a bit easier.

Developing a Swift compiler for Windows could help Microsoft attract app developers already writing in the language for OS X or iOS. Microsoft has thus far struggled to attract top app developers to its Windows Store. With the free release of Windows 10, the company is relying on the app store to make up for forgone revenue.

The move may also enable Google to port Swift to Android, so developers can use a single language to make apps for the two most popular mobile platforms. Google (or some third party) will still have to develop libraries for Android and a porting tool, but it could get developers to release apps for both iOS and Android at essentially the same time.

There are other hurdles to overcome with Android development, but using the same language would certainly ease the friction of developing for Android. That could destroy one of Apple's biggest advantages, as the only platform that supports the newest and best apps.

Apple is capable of taking measures to prevent Google and Microsoft from adapting Swift to use on competing platforms, but that would also curb its adoption in the long run. The amount of source code Apple makes available and licenses for free is up to it.

Ultimately, an open-source Swift is great for developers and potential developers who needed more widespread uses before adopting the language. I believe it will be a net positive for Apple since it ultimately makes developing software for its products more accessible. That will extend the use of their devices, leading to more sales.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.