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This is really the essence mechanism of the `useSwipeTransition` feature. We don't want to immediately switch to the destination state when starting a gesture. The effects remain mounted on the current state. We want the current state to be "live". This is important to for example allow a video to keeping playing while starting a swipe (think TikTok/Reels) and not stop until you've committed the action. The only thing that can be live is the "new" state. Therefore we treat the destination as the "old" state and perform a reverse animation from there. Ideally we could apply the old state to the DOM tree, take a snapshot and then revert it back in the mutation of `startViewTransition`. Unfortunately, the way `startViewTransition` was designed it always paints one frame of the "old" state which would lead this to cause a flicker. To work around this, we need to create a clone of any View Transition boundary that might be mutated and then render that offscreen. That way we can render the "current" state on screen and the "destination" state offscreen for the screenshots. Being mutated can be either due to React doing a DOM mutation or if a child boundary resizes that causes the parent to relayout. We don't have to do this for insertions or deletions since they only appear on one side. The worst case scenario is that we have to clone the whole root. That's what this first PR implements. We clone the container and if it's not absolutely positioned, we position it on top of the current one. If the container is `document` or `<html>` we instead clone the `<body>` tag since it's the only one we can insert a duplicate of. If the container is deep in the tree we clone just that even though technically we should probably clone the whole document in that case. We just keep the impact smaller. Ideally though we'd never hit this case. In fact, if we clone the document we issue a warning (always for now) since you probably should optimize this. In the future I intend to add optimizations when affected View Transition boundaries are absolutely positioned since they cannot possibly relayout the parent. This would be the ideal way to use this feature most efficiently but it still works without it. Since we render the "old" state outside the viewport, we need to then adjust the animation to put it back into the viewport. This is the trickiest part to get right while still preserving any customization of the View Transitions done using CSS. This current approach reapplies all the animations with adjusted keyframes. In the case of an "exit" the pseudo-element itself is positioned outside the viewport but since we can't programmatically update the style of the pseudo-element itself we instead adjust all the keyframes to put it back into the viewport. If there is no animation on the group we add one. In the case of an "update" the pseudo-element is positioned on the new state which is already inside the viewport. However, the auto-generated animation of the group has a starting keyframe that starts outside the viewport. In this case we need to adjust that keyframe. In the future I might explore a technique that inserts stylesheets instead of mutating the animations. It might be simpler. But whatever hacks work to maximize the compatibility is best. |
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React · 
React is a JavaScript library for building user interfaces.
- Declarative: React makes it painless to create interactive UIs. Design simple views for each state in your application, and React will efficiently update and render just the right components when your data changes. Declarative views make your code more predictable, simpler to understand, and easier to debug.
- Component-Based: Build encapsulated components that manage their own state, then compose them to make complex UIs. Since component logic is written in JavaScript instead of templates, you can easily pass rich data through your app and keep the state out of the DOM.
- Learn Once, Write Anywhere: We don't make assumptions about the rest of your technology stack, so you can develop new features in React without rewriting existing code. React can also render on the server using Node and power mobile apps using React Native.
Learn how to use React in your project.
Installation
React has been designed for gradual adoption from the start, and you can use as little or as much React as you need:
- Use Quick Start to get a taste of React.
- Add React to an Existing Project to use as little or as much React as you need.
- Create a New React App if you're looking for a powerful JavaScript toolchain.
Documentation
You can find the React documentation on the website.
Check out the Getting Started page for a quick overview.
The documentation is divided into several sections:
- Quick Start
- Tutorial
- Thinking in React
- Installation
- Describing the UI
- Adding Interactivity
- Managing State
- Advanced Guides
- API Reference
- Where to Get Support
- Contributing Guide
You can improve it by sending pull requests to this repository.
Examples
We have several examples on the website. Here is the first one to get you started:
import { createRoot } from 'react-dom/client';
function HelloMessage({ name }) {
return <div>Hello {name}</div>;
}
const root = createRoot(document.getElementById('container'));
root.render(<HelloMessage name="Taylor" />);
This example will render "Hello Taylor" into a container on the page.
You'll notice that we used an HTML-like syntax; we call it JSX. JSX is not required to use React, but it makes code more readable, and writing it feels like writing HTML.
Contributing
The main purpose of this repository is to continue evolving React core, making it faster and easier to use. Development of React happens in the open on GitHub, and we are grateful to the community for contributing bugfixes and improvements. Read below to learn how you can take part in improving React.
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.
Good First Issues
To help you get your feet wet and get you familiar with our contribution process, we have a list of good first issues that contain bugs that have a relatively limited scope. This is a great place to get started.
License
React is MIT licensed.