Google's cross-platform Flutter SDK is hitting yet another release milestone on the way to version 1.0. Flutter is moving out of beta and releasing "Flutter Release Preview 1." Google says the release preview status "signals our confidence in the stability and quality of what we have and our focus on bug fixing and stabilization."

Flutter is Google's second swing at a mobile SDK (the first being a little platform called "Android"). Flutter's claim to fame is that it's cross-platform—Flutter apps run on Android and iOS—and it's really fast. Flutter apps sidestep the app platforms of Android and iOS and instead run on the Flutter rendering engine (written in C++) and Flutter framework (written in Google's Dart language, just like Flutter apps). When it's time to ship a Flutter app off to Google's and Apple's respective app stores, the requisite Flutter engine code gets bundled up with the app code, and the Flutter SDK spits out Android and iOS versions of your single code base. Each version comes complete with built-in app themes for Android or iOS, so they still feel like native apps. Along with Android and iOS, Flutter is also the platform used for apps in Google's experimental Fuchsia OS.

With the release of preview 1, the Flutter team is focusing on "scenario completeness" for a Flutter app. There's an improved video player package, support for older 32-bit iOS devices like the iPhone 5C, and support for Firebase Dynamic Links. The Flutter team is also improving the documentation and tooling for embedding Flutter code into an existing Android or iOS app.

Flutter was a big topic at Google I/O, with six sessions dedicated to getting developers up-and-running on Google's newest platform. Google says that post I/O, the project saw a 50 percent increase in active Flutter users, with the project becoming one of the top 100 software projects on GitHub. Google is announcing the Flutter Release Preview at a conference in China, where big players like Alibaba and Tencent have picked up the platform for their apps. Inside Google itself, it's hard to give a platform a stronger endorsement than to use it to build the Adwords app, the mobile interface for Google's primary money-maker. Flutter is also popular with smaller developers, where learning the iOS and Android app development methods separately can be such a challenge that many don't bother with cross-platform apps at all.

The latest Release Preview 1 SDK should be available from Flutter's site, and users wondering what all the fuss is about should check out something from the Flutter app showcase.