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This adds a new capability for renderers (React DOM, React Native): prevent a tree from being displayed until it is ready, showing a fallback if necessary, but without blocking the React components from being evaluated in the meantime. A concrete example is CSS loading: React DOM can block a commit from being applied until the stylesheet has loaded. This allows us to load the CSS asynchronously, while also preventing a flash of unstyled content. Images and fonts are some of the other use cases. You can think of this as "Suspense for the commit phase". Traditional Suspense, i.e. with `use`, blocking during the render phase: React cannot proceed with rendering until the data is available. But in the case of things like stylesheets, you don't need the CSS in order to evaluate the component. It just needs to be loaded before the tree is committed. Because React buffers its side effects and mutations, it can do work in parallel while the stylesheets load in the background. Like regular Suspense, a "suspensey" stylesheet or image will trigger the nearest Suspense fallback if it hasn't loaded yet. For now, though, we only do this for non-urgent updates, like with startTransition. If you render a suspensey resource during an urgent update, it will revert to today's behavior. (We may or may not add a way to suspend the commit during an urgent update in the future.) In this PR, I have implemented this capability in the reconciler via new methods added to the host config. I've used our internal React "no-op" renderer to write tests that demonstrate the feature. I have not yet implemented Suspensey CSS, images, etc in React DOM. @gnoff and I will work on that in subsequent PRs.
react-test-renderer
This package provides an experimental React renderer that can be used to render React components to pure JavaScript objects, without depending on the DOM or a native mobile environment.
Essentially, this package makes it easy to grab a snapshot of the "DOM tree" rendered by a React DOM or React Native component without using a browser or jsdom.
Documentation:
https://reactjs.org/docs/test-renderer.html
Usage:
const ReactTestRenderer = require('react-test-renderer');
const renderer = ReactTestRenderer.create(
<Link page="https://www.facebook.com/">Facebook</Link>
);
console.log(renderer.toJSON());
// { type: 'a',
// props: { href: 'https://www.facebook.com/' },
// children: [ 'Facebook' ] }
You can also use Jest's snapshot testing feature to automatically save a copy of the JSON tree to a file and check in your tests that it hasn't changed: https://jestjs.io/blog/2016/07/27/jest-14.html.