The best alternatives to Realm are Fieldbook, Native Directory, and Webiny. If these 3 options don't work for you, we've listed a few more alternatives below.
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Native Directory is a destination for finding any libraries that work with React Native. There are four sub categories, libraries that work with iOS, libraries that work with Android, libraries that work with Expo, and libraries that even work on the web. Native Directory takes submissions at https://github.com/react-community/native-directory
Webiny is an open source enterprise CMS platform for users that are looking for great CMS experience, that is underpinned by unparalleled scalability, cost efficiency, infinite customization and data ownership through self-hosting.
Makeroid is a tool that helps Makers build apps without needing to be able to code. You just have to drag and drop components and join blocks to make the code. And then we do the magic transforming into an app! We have powerful tools, like three types of ads or in-app purchases to help you monetize your app or a floating-action-button
We've made a twitch platformer for mobile, And it became a little intense. No Time left is a hard twitch platformer made for mobile and mobile only. Player needs to complete procedurally generated levels before timer runs out. Run fast, dash and complete the levels before the timer runs out. I'd be super happy if you download and give it a spin!