Samuel Susla
ef438464fa
Summary: Pull Request resolved: https://github.com/facebook/react-native/pull/39357 changelog: [internal] # Problem ## When React clones the wrong node revision Whenever React wants to commit a new change, it first needs to clone shadow nodes. React sometimes clones from the wrong revision. This has mostly been fine, Fabric does state reconciliation to pass newest state forward. State reconciliation is needed, as we need to keep native state in the shadow tree. However, when React clones a node that has never been through layout step, it will clone a node without any layout information and its yoga node is dirtied. Even though there might be a subsequent revision of the node with layout information already calculated. As a result, Yoga needs to traverse bigger parts of the tree, even though layout has been calculated before. It is just cached on a different revision that was used as a source. There are two main sources (there is more but they don't help to paint the picture) when this can happen. Background Executor and State Progression. Let's start with the simpler one but less severe: Background Executor. Background Executor moves layout from JavaScript thread. React can start cloning nodes right away, even though they might not have layout information calculated yet. This is a race condition and depending on when the node is cloned, we can see different results. In this case, React eventually clones node from the correct revision with the layout cache. It will be in a correct state in the end. This case is not as bad as far as I can tell but I included it here because it better illustrates what is going on. State Progression is where things get worse. In this scenario, React will never clone from the correct revision and will never recover from this. Anytime React clones node with a state that needs to be progressed, it will get cloned one more time during commit but React will hold the wrong revision. Depending on where this node is located in the view hierarchy, it may lead to expensive layout calculations. Example: Let's use notation A/r1 as node of family A revision 1. - React calls create node. Node A/r1 is created and React holds reference to this. It will later use it to clone it. Node A has native state that was updated. New revision A/r2 is created. Now React and RN do not observe the same node anymore (this is sometimes necessary). - React now clones node A to create A/r3. This revision may have the wrong yoga cache. Now this might sound like one off but let's explore what happens next. - During commit, Fabric must do state progression to give node A/r3 state from A/r2. This requires cloning and new revision A/r4 is created. React has again a wrong node that does not have Yoga cache and can't recover from this state. The blast radius of this varies depending on where in the tree the node is. # Solution Clone-less state progression. In this algorithm, state progression never clones. It checks if a ShadowNode has ever been mounted via `ShadowNode::hasBeenMounted_` to check if a node can be safely mutated without the need to clone and checks if current state the node is holding is obsolete (like the previous algorithm). The clone-less state progression depends on the fact that any shadow node cloned from React is still unsealed and is not exposed to a multithreaded environment. This makes it safe to mutate it in place, without the need to clone. WARNING: there is important semantic difference with the approach. With the old algorithm, you couldn't go back to a shadow node with old state. New state was always enforced when state reconciliation was enabled. The clone-less algorithm does not support that, because it can't mutate such a node in place. Reviewed By: javache Differential Revision: D49012353 fbshipit-source-id: 2d322f0587042c13761c83f7e916d1623159773d
React Native
Learn once, write anywhere:
Build mobile apps with React.
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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 13.4 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.