Brian Vaughn bvaughn

I've gotten a couple of questions about how the RN sync process works, so I thought I'd write up a high level overview of the current process and some of its challenges.

Sources of truth

As far as JavaScript goes, there are a couple of things that live outside of the React Native GitHub repository that get auto-synced in:

React (core)
ReactNative renderer
Other things (e.g. component types like View, Text, the Animated library, StyleSheet, etc.)

The source of truth for React core is GitHub (https://github.com/facebook/react). It gets published to NPM and React Native depends on it via an NPM dependency. e.g. the most recently release of RN (55) depends on React core 16.3.1

The Rules of React

All libraries have subtle rules that you have to follow for them to work well. Often these are implied and undocumented rules that you have to learn as you go. This is an attempt to document the rules of React renders. Ideally a type system could enforce it.

What Functions Are "Pure"?

A number of methods in React are assumed to be "pure".

On classes that's the constructor, getDerivedStateFromProps, shouldComponentUpdate and render.

Note:

When this guide is more complete, the plan is to move it into Prepack documentation.
For now I put it out as a gist to gather initial feedback.

A Gentle Introduction to Prepack (Part 1)

If you're building JavaScript apps, you might already be familiar with some tools that compile JavaScript code to equivalent JavaScript code:

Babel lets you use newer JavaScript language features, and outputs equivalent code that targets older JavaScript engines.

This API was removed in React 17

Interaction tracing with React

React recently introduced an experimental profiler API. After discussing this API with several teams at Facebook, one common piece of feedback was that the performance information would be more useful if it could be associated with the events that caused the application to render (e.g. button click, XHR response). Tracing these events (or "interactions") would enable more powerful tooling to be built around the timing information, capable of answering questions like "What caused this really slow commit?" or "How long does it typically take for this interaction to update the DOM?".

With version 16.4.3, React added experimental support for this tracing by way of a new NPM package, scheduler. However the public API for this package is not yet finalized and will likely change with upcoming minor releases, so it should be used with caution.

React recently introduced an experimental profiler API. This page gives instructions on how to use this API in a production release of your app.

Table of Contents

Profiling in production
- Create React App
- Webpack 4

Profiling in production

React DOM automatically supports profiling in development mode for v16.5+, but since profiling adds some small additional overhead it is opt-in for production mode. This gist explains how to opt-in.

Chrome's profiler ("Performance tab) is very useful for measuring JavaScript performance, but what if you want to measure the performance of a custom extension?

For example, what if I would like to profile the following interaction:

Speaker Rider

by Tatiana Mac

Last updated 14 April 2021

What is a speaker rider?

As speaking comes with immense privilege, I have crafted a speaker rider to set expectations and boundaries around my engagement. I am grateful to all the conference organisers who have brilliantly hosted me. I would love to continue to exercise this privilege to speak at conferences, and use this privilege to make the landscape more accessible and beneficial to tech's most historically excluded and marginalised communities.

Considerations

😫 I provide a lot of explanations for those of you who never had to consider these things. Most thoughtful conferences I've attended check most of these boxes intrinsically, particularly when conference runners are experienced speakers. They get it.

useSubscription and useMutableSource¹ tearing and deopt behavior.

Mounting a new tree

The tree below represents a React application mounting. During mount, two components read from an external, mutable source. The first one (List) reads version 1 of that data and the second one (Item) reads version 2.

Deopt

`useSubscription` (legacy mode)

N/A.

Unsupported DevTools backend version

This page contains instructions for updating a local React DevTools application to match a version embedded in a renderer such as React Native. Instructions below cover NPM, Flipper, and React Native Debugger. If you use React DevTools in a different way, please let us know.

If you are viewing this page, you have likely seen one of the dialogs below:

	const State = React.createState();

	function MyComponent(props) {
	return (
	<State
	initialState={() => ({ counter: 0, lastProps: props, divRef: React.createRef() }) }
	deriveState={state => ({ lastProps: props }) }
	shouldUpdate={(state, nextState) => ... }
	didMount={(state, setState) => ... }
	getSnapshotBeforeUpdate={state => ... }