Rubén NorteandFacebook GitHub Bot
387bd70e49
Summary: Pull Request resolved: https://github.com/facebook/react-native/pull/37853 This adds a basic implementation of `IntersectionObserver`. This will not be available yet and is only compatible with the new React Native architecture. This shouldn't show up in the changelog until we're ready to enable this in some form. Changelog: [Internal] ## Context This implements a basic version of the `IntersectionObserver` API (as defined on the Web) for React Native. The motivation for this is supporting several use cases that are not possible in React Native at the moment, most importantly: * Tracking paint times for elements in the screen. * Tracking precise visibility of elements in the screen outside the context of a `VirtualizedList` (with an even better precision and control). ## Implementation details This API is implemented as a native module that registers a mount hook in Fabric. Whenever there's a mount (an update to the UI of the host platform) we check for intersections in the shadow tree. The shadow tree contains information about the representation of the UI in a given time (including scroll position), which we use as source of truth for this in a cross-platform fashion. We rely on the fact that scroll position is updated regularly in the shadow tree to provide an up-to-date view into the UI. **This implementation is completely cross-platform.** The only platform-specific part is the report of mounts in mount hooks from the host platform to Fabric. This API uses a centralized entity in JS and native to handle registration of observers and dispatch of notifications. The dispatch the notifications for all observers in the same callback so we can easily change the sequencing of events easily (for example, we can change this to use microtasks when they're available in RN). ## Known limitations * Timestamps are generally accurate for paint (as we report mounts right after they happen in the host platform), but **state updates (like scroll) are reported with a slight delay**. * In regular rendering, we first update the shadow tree and then mount it (paint), which is generally precise. In state updates, the UI is updated first and then the shadow tree is updated. In this case, we're not correctly reporting the timestamp of the scroll event (which we should be using) but the timestamp of when the update is processed. We'll fix this in a following diff. * The IntersectionObserver API has a concept of initial notification. This is a mechanism to report the initial state of an observed target. If we start observing a target when it's added to the tree but before it's painted, this initial notification is supposed to provide initial paint time (which is important for performance measurements). This implements some logic to handle that correctly (we check if there is a pending transaction) but it's currently unreliable: * React Native does not currently block paint on microtasks or layout effects, so setting up an observer in these stages could have race conditions with actual mount. If mount happens before the observation is started, the initial notification doesn't report initial time but observation time. If mount happens after, the initial notification should be fine (except in some cases on Android, see the next point). * On Android, we have a push model to send mutations to the host platform, we means we consume transactions after commit, not immediately before mount. This breaks this logic and we need to figure out a solution in a following diff. ---- Reviewed By: sammy-SC, rshest Differential Revision: D45278720 fbshipit-source-id: de350388c6325128f1cf73328779a9d3577a258a
React Native
Learn once, write anywhere:
Build mobile apps with React.
Getting Started · Learn the Basics · Showcase · Contribute · Community · Support
React Native brings React's declarative UI framework to iOS and Android. With React Native, you use native UI controls and have full access to the native platform.
- Declarative. React makes it painless to create interactive UIs. Declarative views make your code more predictable and easier to debug.
- Component-Based. Build encapsulated components that manage their state, then compose them to make complex UIs.
- Developer Velocity. See local changes in seconds. Changes to JavaScript code can be live reloaded without rebuilding the native app.
- Portability. Reuse code across iOS, Android, and other platforms.
React Native is developed and supported by many companies and individual core contributors. Find out more in our ecosystem overview.
Contents
- Requirements
- Building your first React Native app
- Documentation
- Upgrading
- How to Contribute
- Code of Conduct
- License
📋 Requirements
React Native apps may target iOS 12.4 and Android 5.0 (API 21) or newer. You may use Windows, macOS, or Linux as your development operating system, though building and running iOS apps is limited to macOS. Tools like Expo can be used to work around this.
🎉 Building your first React Native app
Follow the Getting Started guide. The recommended way to install React Native depends on your project. Here you can find short guides for the most common scenarios:
📖 Documentation
The full documentation for React Native can be found on our website.
The React Native documentation discusses components, APIs, and topics that are specific to React Native. For further documentation on the React API that is shared between React Native and React DOM, refer to the React documentation.
The source for the React Native documentation and website is hosted on a separate repo, @facebook/react-native-website.
🚀 Upgrading
Upgrading to new versions of React Native may give you access to more APIs, views, developer tools, and other goodies. See the Upgrading Guide for instructions.
React Native releases are discussed in this discussion repo.
👏 How to Contribute
The main purpose of this repository is to continue evolving React Native core. We want to make contributing to this project as easy and transparent as possible, and we are grateful to the community for contributing bug fixes and improvements. Read below to learn how you can take part in improving React Native.
Code of Conduct
Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please read the full text so that you can understand what actions will and will not be tolerated.
Contributing Guide
Read our Contributing Guide to learn about our development process, how to propose bugfixes and improvements, and how to build and test your changes to React Native.
Open Source Roadmap
You can learn more about our vision for React Native in the Roadmap.
Good First Issues
We have a list of good first issues that contain bugs which have a relatively limited scope. This is a great place to get started, gain experience, and get familiar with our contribution process.
Discussions
Larger discussions and proposals are discussed in @react-native-community/discussions-and-proposals.
📄 License
React Native is MIT licensed, as found in the LICENSE file.
React Native documentation is Creative Commons licensed, as found in the LICENSE-docs file.