This week, Eli White gave a talk at F8 2019 about React Native in Facebook's Android and iOS applications. We are excited to share what we've been up to for the past two years and what we're doing next.

Check out the video on Facebook's developer website:

Highlights from the talk:​

• We spent 2017 and 2018 focused on React Native's largest product, Facebook's Marketplace. We collaborated with the Marketplace team to improve quality and add delight to the product. At this point, Marketplace is one of the highest quality products in the Facebook app both on Android and iOS.
• Marketplace's performance was a big challenge as well, especially on mid-end Android devices. We cut startup time by more than 50% over the last year with more improvements on the way! The biggest improvements are being built into React Native and will be coming to the community later this year.
• We have the confidence that we can build the high quality and performant apps that Facebook needs with React Native. This confidence has let us invest in bigger bets, like rethinking the core of React Native.
• Microsoft supports and uses React Native for Windows, enabling people to use their expertise and codebase to render to Microsofts's Universal Windows Platform. Check out Microsoft Build next week to hear them talk about that more.

React Radio Podcast about Open Source​

Eli's talk concludes by talking about our recent open source work. We gave an update on our progress in March and recently Nader Dabit and Gant Laborde invited Christoph for a chat on their podcast, React Native Radio, to chat about React Native in open source.

Highlights from the podcast:​

• We talked about how the React Native team at Facebook thinks about open source and how we are building a sustainable community that scales for a project of React Native's size.
• We are on track to remove multiple modules as part of the Lean Core effort. Many modules like WebView and the React Native CLI have received more than 100 Pull Requests since they were extracted.
• Next, we'll be focusing on overhauling the React Native website and documentation. Stay tuned!

You'll find the episode in your favorite podcasting app soon or you can listen to the recording right here:

Welcome to the 0.59 release of React Native! This is another big release with 644 commits by 88 contributors. Contributions also come in other forms, so thank you for maintaining issues, fostering communities, and teaching people about React Native. This month brings a number of highly anticipated changes, and we hope you enjoy them.

🎣 Hooks are here​

React Hooks are part of this release, which let you reuse stateful logic across components. There is a lot of buzz about hooks, but if you haven't heard, take a look at some of the wonderful resources below:

• Introducing Hooks explains why we’re adding Hooks to React.
• Hooks at a Glance is a fast-paced overview of the built-in Hooks.
• Building Your Own Hooks demonstrates code reuse with custom Hooks.
• Making Sense of React Hooks explores the new possibilities unlocked by Hooks.
• useHooks.com showcases community-maintained Hooks recipes and demos.

Be sure to give this a try in your apps. We hope that you find the reuse as exciting as we do.

📱 Updated JSC means performance gains and 64-bit support on Android​

React Native uses JSC (JavaScriptCore) to power your application. JSC on Android was a few years old, which meant that a lot of modern JavaScript features weren't supported. Even worse, it performed poorly compared iOS's modern JSC. With this release, that all changes.

Thanks to some awesome work by @DanielZlotin, @dulmandakh, @gengjiawen, @kmagiera, and @kudo JSC has caught up with the past few years. This brings with it 64-bit support, modern JavaScript support, and big performance improvements. Kudos for also making this a maintainable process now so that we can take advantage of future WebKit improvements without so much legwork, and thank you Software Mansion and Expo for making this work possible.

💨 Faster app launches with inline requires​

We want to help people have performant React Native apps by default and are working to bring Facebook's optimizations to the community. Applications load resources as needed rather than slowing down launch. This feature is called "inline requires", as it lets Metro identify components to be lazy loaded. Apps with a deep and varied component architecture will see the most improvement.

We need the community to let us know how it works before we turn it on by default. When you upgrade to 0.59, there will be a new metro.config.js file; flip the options to true and give us your feedback! Read more about inline requires in the performance docs to benchmark your app.

🚅 Lean core is underway​

React Native is a large and complex project with a complicated repository. This makes the codebase less approachable to contributors, difficult to test, and bloated as a dev dependency. Lean Core is our effort to address these issues by migrating code to separate libraries for better management. The past few releases have seen the first steps of this, but let's get serious.

You may notice that additional components are now officially deprecated. This is great news, as there are now owners for these features actively maintaining them. Heed the warning messages and migrate to the new libraries for these features, because they will be removed in a future release. Below is a table indicating the component, its status, and where you may migrate your use to.

Over the coming months, there will be many more components following this path to a leaner core. We're looking for help with this — head over to the lean core umbrella to pitch in.

👩🏽‍💻 CLI improvements​

React Native's command line tools are developer's entry point to the ecosystem, but they had long-standing issues and lacked official support. The CLI tools have been moved to a new repository, and a dedicated group of maintainers have already made some exciting improvements.

Logs are formatted much better now. Commands now run nearly instantly — you'll immediately notice a difference:

🚀 Upgrading to 0.59​

We heard your feedback regarding the React Native upgrade process and we are taking steps to improve the experience in future releases. To upgrade to 0.59, we recommend using rn-diff-purge to determine what has changed between your current React Native version and 0.59, then applying those changes manually. Once you've upgraded your project to 0.59, you will be able to use the newly improved react-native upgrade command (based on rn-diff-purge!) to upgrade to 0.60 and beyond as newer releases become available.

🔨 Breaking Changes​

Android support in 0.59 has been cleaned up following Google's latest recommendations, which may result in potential breakage of existing apps. This issue might present as a runtime crash and a message, "You need to use a Theme.AppCompat theme (or descendant) with this activity". We recommend updating your project's AndroidManifest.xml file, making sure that the android:theme value is an AppCompat theme (such as @style/Theme.AppCompat.Light.NoActionBar).

The react-native-git-upgrade command has been removed in 0.59, in favor of the newly improved react-native upgrade command.

🤗 Thanks​

Lots of new contributors helped with enabling generation of native code from flow types and resolving Xcode warnings - these are a great way to learn how React Native works and contributing to the greater good. Thank you! Look out for similar issues in the future.

While these are the highlights that we noted, there are many others to be excited about. To see all of the updates, take a look at the changelog. 0.59 is a huge release – we can't wait for you to try it out.

We have even more improvements coming throughout the rest of the year. Stay tuned!

Ryan and the whole React Native core team

We announced our React Native Open Source roadmap in Q4 2018 after deciding to invest more in the React Native open source community.

For our first milestone, we focused on identifying and improving the most visible aspects of our community. Our goals were to reduce outstanding pull requests, reduce the project's surface area, identify leading user problems, and establish guidelines for community management.

In the past two months, we made more progress than we expected. Read on for more details:

Pull Requests​

In order to build a healthy community, we must respond quickly to code contributions. In past years, we de-prioritized reviewing community contributions and accumulated 280 pull requests (December 2018). In the first milestone, we reduced the number of open pull requests to ~65. Simultaneously, the average number of pull requests opened per day increased from 3.5 to 7 which means we have handled about 600 pull requests in the last three months.

We merged almost two-thirds and closed one-third of the pull requests. They were closed without being merged if they are obsolete or low quality, or if they unnecessarily increase the project's surface area. Most of the merged pull requests fixed bugs, improved cross-platform parity, or introduced new features. Notable contributions include improving type safety and the ongoing work to support AndroidX.

At Facebook, we run React Native from master, so we test all changes first before they make it into a React Native Release. Out of all the merged pull requests, only six caused issues: four only affected internal development and two were caught in the release candidate state.

One of the more visible community contributions was the updated “RedBox” screen. It's a good example of how the community is making the developer experience friendlier.

Lean Core​

React Native currently has a very wide surface area with many unmaintained abstractions that we do not use a lot at Facebook. We are working on reducing the surface area in order to make React Native smaller and allow the community to take better care of abstractions that are mostly unused at Facebook.

In the first milestone, we asked the community for help on the Lean Core project. The response was overwhelming and we could barely keep up with all the progress. Check out all the work completed in less than a month!

What we are most excited about is that maintainers have jumped in fixing long standing issues, adding tests, and supporting long requested features. These modules are getting more support than they ever did within React Native, showing that this is a great step for the community. Examples of such projects are WebView that has received many pull requests since their extraction and the CLI that is now maintained by members of the community and received much needed improvements and fixes.

Leading User Problems​

In December, we asked the community what they disliked about React Native. We aggregated the responses and replied to each and every problem. Fortunately, many of the issues that our community faces are also problems at Facebook. In our next milestone, we plan to address some of the main problems.

One of the highest voted problems was the developer experience of upgrading to newer versions of React Native. Unfortunately, this is not something that we experience ourselves because we run React Native from master. Thankfully, members from the community already stepped up to address this problem:

• Michał Pierzchała from Callstack improved react-native upgrade by using rn-diff-purge under the hood. We also updated the website to remove outdated upgrade instructions.
• We plan on recommending CocoaPods by default for iOS projects which will reduce churn in project files when upgrading React Native. This will make it easier for people to install and link third-party modules which is even more important in the context of Lean Core as we expect projects to link more modules by default.

0.59 Release​

Without the help of the React Native community, especially Mike Grabowski and Lorenzo Sciandra, we would not be able to ship releases. We want to improve the release management process and plan to be more involved from now on:

• We will work with community members to create a blog post for each major release.
• We will show breaking changes directly in the CLI when people upgrade to new versions.
• We will reduce the time it takes to make a release. We are exploring ways to increase automated testing and also creating an improved manual test plan.

Many of these plans will be incorporated in the upcoming React Native 0.59 release. 0.59 will ship with React Hooks, a new 64-bit version of JavaScriptCore for Android, and many performance and functionality improvements. It is currently published as a release candidate and is expected to be stable within the next two weeks.

Next Steps​

For the next two months, we will continue managing pull requests to stay on track while also starting to reduce the number of outstanding GitHub issues. We will continue reducing the surface area of React Native through the Lean Core project. We plan to address 5 of the top community problems. As we finalize the community guidelines, we will turn attention to our website and documentation.

We are very excited to host over ten contributors from our community at Facebook London in March to help drive several of these efforts. We are glad that you are using React Native and hope that you'll see and feel the improvements we are working on in 2019. We'll be back with another update in a few months and will be merging your pull requests in the meantime! ⚛️✌️

In 2018 the React Native Community made a number of changes to the way we develop and communicate about React Native. We believe that a few years from now we will look back and see that this shift was a turning point for React Native.

A lot of people are excited about the rewrite of React Native's architecture, widely known as Fabric. Among other things, this will fix fundamental limitations in React Native's architecture and will set up React Native for success in the future together with JSI and TurboModules.

The biggest shift in 2018 was to empower the React Native Community. From the beginning, Facebook encouraged developers from all around the world to participate in React Native's open source project. Since then, a number of core contributors emerged to handle, among other things, the release process.

These members took a few substantial steps towards making the whole community more empowered to shape the future of this project with the following resources:

react-native-releases 📬​

This repository, created in January, serves the dual purpose of allowing everyone to keep up the new releases in a more collaborative manner and opened the conversation of what would be part of a certain release to whomever wanted to suggest a cherry-pick (like for 0.57.8 and all its previous versions).

This has been the driving force behind moving away from a monthly release cycle, and the "long term support" approach currently used for version 0.57.x.

Half of the credit for reaching these decisions goes to the other repository created this year:

discussions-and-proposals 🗣​

This repository, created in July, expanded on the idea of a more open environment for conversations on React Native. Previously, this need was handled by issues labelled For Discussion in the main repository, but we wanted to expand this strategy to an RFC approach that other libraries have (e.g. React).

This experiment immediately found its role in the React Native lifecycle. The Facebook team is now using the community RFC process to discuss what could be improved in React Native, and coordinate the efforts around the Lean Core project - among other interesting discussions.

@ReactNativeComm 🐣​

We are aware that our approach to communicate these efforts has not been as effective as we would have liked, and in an attempt to give you all an easier time keeping up with everything going on in the React Native Community (from releases to active discussions) we created a new twitter account that you can rely on @ReactNativeComm.

If you are not on that social network, remember that you can always watch repositories via GitHub; this feature improved these past few months with the possibility of being notified only for releases, so you should consider using it anyway.

What awaits ahead 🎓​

Over the past 7-8 months, core contributors enhanced the React Native Community GitHub organization to take more ownership over the development of React Native, and enhance collaboration with Facebook. But this always lacked the formal structure that similar projects may have in place.

This organization can set the example for everyone in the larger developer community by enforcing a set of standards for all the packages/repos hosted in it, providing a single place for maintainers to help each other and contribute quality code that conforms to community-agreed standards.

In early 2019, we will have this new set of guidelines in place. Let us know what you think in the dedicated discussion.

We are confident that with these changes, the community will become more collaborative so that when we reach 1.0, we will all continue to write (even more) awesome apps by leveraging this joint effort 🤗

I hope you are as excited as we are about the future of this community. We're excited to see all of you involved either in the conversations happening in the repositories listed above or via the awesome code you’ll produce.

Happy coding!

This year, the React Native team has focused on a large scale re-architecture of React Native. As Sophie mentioned in her State of React Native post, we've sketched out a plan to better support the thriving population of React Native users and collaborators outside of Facebook. It's now time to share more details about what we've been working on. Before I do so, I'd like to lay out our long-term vision for React Native in open source.

Our vision for React Native is...

• A healthy GitHub repository. Issues and pull requests get handled within a reasonable period of time.
  • Increased test coverage.
  • Commits that sync out from the Facebook code repository should not break open source tests.
  • A higher scale of meaningful community contributions.
• Stable APIs, making it easier to interface with open source dependencies.
  • Facebook uses the same public API as open source
  • React Native releases that follow semantic versioning.
• A vibrant eco-system. High quality ViewManagers, native modules, and multiple platform support maintained by the community.
• Excellent documentation. Focus on helping users create high quality experiences, and up-to-date API reference docs.

We have identified the following focus areas to help us achieve this vision.

✂️ Lean Core​

Our goal is to reduce the surface area of React Native by removing non-core and unused components. We'll transfer non-core components to the community to allow it to move faster. The reduced surface area will make it easier to manage contributions to React Native.

WebView is an example of a component that we transferred to the community. We are working on a workflow that will allow internal teams to continue using these components after we remove them from the repository. We have identified dozens more components that we'll give ownership of to the community.

🎁 Open Sourcing Internals and 🛠Updated Tooling​

The React Native development experience for product teams at Facebook can be quite different from open source. Tools that may be popular in the open source community are not used at Facebook. There may be an internal tool that achieves the same purpose. In some cases, Facebook teams have become used to tools that do not exist outside of Facebook. These disparities can pose challenges when we open source our upcoming architecture work.

We'll work on releasing some of these internal tools. We'll also improve support for tools popular with the open source community. Here's a non-exhaustive list of projects we'll tackle:

• Open source JSI and enable the community to bring their own JavaScript VMs, replacing the existing JavaScriptCore from RN's initial release. We'll be covering what JSI is in a future post, in the meantime you can learn more about JSI from Parashuram's talk at React Conf.
• Support 64-bit libraries on Android.
• Enable debugging under the new architecture.
• Improve support for CocoaPods, Gradle, Maven, and new Xcode build system.

✅ Testing Infrastructure​

When Facebook engineers publish code, it's considered safe to land if it passes all tests. These tests identify whether a change might break one of our own React Native surfaces. Yet, there are differences in how Facebook uses React Native. This has allowed us to unknowingly break React Native in open source.

We'll shore up our internal tests to ensure they run in an environment that is as close as possible to open source. This will help prevent code that breaks these tests from making it to open source. We will also work on infrastructure to enable better testing of the core repo on GitHub, enabling future pull requests to easily include tests.

Combined with the reduced surface area, this will allow contributors to merge pull requests quicker, with confidence.

📜 Public API​

Facebook will consume React Native via the public API, the same way open source does, to reduce unintentional breaking changes. We have started converting internal call sites to address this. Our goal is to converge on a stable, public API, leading to the adoption of semantic versioning in version 1.0.

📣 Communication​

React Native is one of the top open source projects on GitHub by contributor count. That makes us really happy, and we'd like to keep it going. We'll continue working on initiatives that lead to involved contributors, such as increased transparency and open discussion. The documentation is one of the first things someone new to React Native will encounter, yet it has not been a priority. We'd like to fix that, starting with bringing back auto-generated API reference docs, creating additional content focused on creating quality user experiences, and improving our release notes.

Timeline​

We're planning to land these projects throughout the next year or so. Some of these efforts are already ongoing, such as JSI which has already landed in open source. Others will take a bit longer to complete, such as reducing the surface area. We'll do our best to keep the community up to date with our progress. Please join us in the Discussions and Proposals repository, a initiative from the React Native community that has led to the creation of several of the initiatives discussed in this roadmap.

For a long time now, Apple has discouraged using UIWebViews in favor of WKWebView. In iOS 12, which will be released in the upcoming months, UIWebViews will be formally deprecated. React Native's iOS WebView implementation relies heavily on the UIWebView class. Therefore, in light of these developments, we've built a new native iOS backend to the WebView React Native component that uses WKWebView.

The tail end of these changes were landed in this commit, and will become available in the 0.57 release.

To opt into this new implementation, please use the useWebKit prop:

<WebView
  useWebKit={true}
  source={{ url: 'https://www.google.com' }}
/>

Improvements​

UIWebView had no legitimate way to facilitate communication between the JavaScript running in the WebView, and React Native. When messages were sent from the WebView, we relied on a hack to deliver them to React Native. Succinctly, we encoded the message data into a url with a special scheme, and navigated the WebView to it. On the native side, we intercepted and cancelled this navigation, parsed the data from the url, and finally called into React Native. This implementation was error prone and insecure. I'm glad to announce that we've leveraged WKWebView features to completely replace it.

Other benefits of WKWebView over UIWebView include faster JavaScript execution, and a multi-process architecture. Please see this 2014 WWDC for more details.

Caveats​

If your components use the following props, then you may experience problems when switching to WKWebView. For the time being, we suggest that you avoid using these props:

Inconsistent behavior:

automaticallyAdjustContentInsets and contentInsets (commit)

When you add contentInsets to a WKWebView, it doesn't change the WKWebView's viewport. The viewport remains the same size as the frame. With UIWebView, the viewport size actually changes (gets smaller, if the content insets are positive).

backgroundColor (commit)

With the new iOS implementation of WebView, there's a chance that your background color will flicker into view if you use this property. Furthermore, WKWebView renders transparent backgrounds differently from UIWebview. Please look at the commit description for more details.

Not supported:

scalesPageToFit (commit)

WKWebView didn't support the scalesPageToFit prop, so we couldn't implement this on the WebView React Native component.

Motivation​

As technology advances and mobile apps become increasingly important to everyday life, the necessity of creating accessible applications has likewise grown in importance.

React Native's limited Accessibility API has always been a huge pain point for developers, so we've made a few updates to the Accessibility API to make it easier to create inclusive mobile applications.

Problems With the Existing API​

Problem One: Two Completely Different Yet Similar Props - accessibilityComponentType (Android) and accessibilityTraits (iOS)​

accessibilityComponentType and accessibilityTraits are two properties that are used to tell TalkBack on Android and VoiceOver on iOS what kind of UI element the user is interacting with. The two biggest problems with these properties are that:

1. They are two different properties with different usage methods, yet have the same purpose. In the previous API, these are two separate properties (one for each platform), which was not only inconvenient, but also confusing to many developers. accessibilityTraits on iOS allows 17 different values while accessibilityComponentType on Android allows only 4 values. Furthermore, the values for the most part had no overlap. Even the input types for these two properties are different. accessibilityTraits allows either an array of traits to be passed in or a single trait, while accessibilityComponentType allows only a single value.
2. There is very limited functionality on Android. With the old property, the only UI elements that Talkback were able to recognize were “button,” “radiobutton_checked,” and “radiobutton_unchecked.”

Problem Two: Non-existent Accessibility Hints:​

Accessibility Hints help users using TalkBack or VoiceOver understand what will happen when they perform an action on an accessibility element that is not apparent by only the accessibility label. These hints can be turned on and off in the settings panel. Previously, React Native's API did not support accessibility hints at all.

Problem Three: Ignoring Inverted Colors:​

Some users with vision loss use inverted colors on their mobile phones to have greater screen contrast. Apple provided an API for iOS which allows developers to ignore certain views. This way, images and videos aren't distorted when a user has the inverted colors setting on. This API is currently unsupported by React Native.

Design of the New API​

Solution One: Combining accessibilityComponentType (Android) and accessibilityTraits (iOS)​

In order to solve the confusion between accessibilityComponentType and accessibilityTraits, we decided to merge them into a single property. This made sense because they technically had the same intended functionality and by merging them, developers no longer had to worry about platform specific intricacies when building accessibility features.

Background

On iOS, UIAccessibilityTraits is a property that can be set on any NSObject. Each of the 17 traits passed in through the javascript property to native is mapped to a UIAccessibilityTraits element in Objective-C. Traits are each represented by a long int, and every trait that is set is ORed together.

On Android however, AccessibilityComponentType is a concept that was made up by React Native, and doesn't directly map to any properties in Android. Accessibility is handled by an accessibility delegate. Each view has a default accessibility delegate. If you want to customize any accessibility actions, you have to create a new accessibility delegate, override specific methods you want to customize, and then set the accessibility delegate of the view you are handling to be associated with the new delegate. When a developer set AccessibilityComponentType, the native code created a new delegate based off of the component that was passed in, and set the view to have that accessibility delegate.

Changes Made

For our new property, we wanted to create a superset of the two properties. We decided to keep the new property modeled mostly after the existing property accessibilityTraits, since accessibilityTraits has significantly more values. The functionality of Android for these traits would be polyfilled in by modifying the Accessibility Delegate.

There are 17 values of UIAccessibilityTraits that accessibilityTraits on iOS can be set to. However, we didn't include all of them as possible values to our new property. This is because the effect of setting some of these traits is actually not very well known, and many of these values are virtually never used.

The values UIAccessibilityTraits were set to generally took on one of two purposes. They either described a role that UI element had, or they described the state a UI element was in. Most uses of the previous properties we observed usually used one value that represented a role and combined it with either “state selected,” “state disabled,” or both. Therefore, we decided to create two new accessibility properties: accessibilityRole and accessibilityState.

accessibilityRole

The new property, accessibilityRole, is used to tell Talkback or Voiceover the role of a UI Element. This new property can take on one of the following values:

• none
• button
• link
• search
• image
• keyboardkey
• text
• adjustable
• header
• summary
• imagebutton

This property only allows one value to be passed in because UI elements generally don't logically take on more than one of these. The exception is image and button, so we've added a role imagebutton that is a combination of both.

accessibilityStates

The new property, accessibilityStates, is used to tell Talkback or Voiceover the state a UI Element is in. This property takes on an Array containing one or both of the following values:

• selected
• disabled

Solution Two: Adding Accessibility Hints​

For this, we added a new property, accessibilityHint. Setting this property will allow Talkback or Voiceover to recite the hint to users.

accessibilityHint

This property takes in the accessibility hint to be read in the form of a String.

On iOS, setting this property will set the corresponding native property AccessibilityHint on the view. The hint will then be read by Voiceover if Accessibility Hints are turned on in the iPhone.

On Android, setting this property appends the value of the hint to the end of the accessibility label. The upside to this implementation is that it mimics the behavior of hints on iOS, but the downside to this implementation is that these hints cannot be turned off in the settings on Android the way they can be on iOS.

The reason we made this decision on Android is because normally, accessibility hints correspond with a specific action (e.g. click), and we wanted to keep behaviors consistent across platforms.

Solution to Problem Three​

accessibilityIgnoresInvertColors

We exposed Apple's api AccessibilityIgnoresInvertColors to JavaScript, so now when you have a view where you don't want colors to be inverted (e.g image), you can set this property to true, and it won't be inverted.

New Usage​

These new properties will become available in the React Native 0.57 release.

How to Upgrade​

If you are currently using accessibilityComponentType and accessibilityTraits, here are the steps you can take to upgrade to the new properties.

1. Using jscodeshift​

The most simple use cases can be replaced by running a jscodeshift script.

This script replaces the following instances:

accessibilityTraits=“trait”
accessibilityTraits={[“trait”]}

With

accessibilityRole= “trait”

This script also removes instances of AccessibilityComponentType (assuming everywhere you set AccessibilityComponentType, you would also set AccessibilityTraits).

2. Using a manual codemod​

For the cases that used AccessibilityTraits that don't have a corresponding value for AccessibilityRole, and the cases where multiple traits were passed into AccessibilityTraits, a manual codemod would have to be done.

In general,

accessibilityTraits= {[“button”, “selected”]}

would be manually replaced with

accessibilityRole=“button”
accessibilityStates={[“selected”]}

These properties are already being used in Facebook's codebase. The codemod for Facebook was surprisingly simple. The jscodeshift script fixed about half of our instances, and the other half was fixed manually. Overall, the entire process took less than a few hours.

Hopefully you will find the updated API useful! And please continue making apps accessible! #inclusion

The long-awaited 0.56 version of React Native is now available 🎉. This blog post highlights some of the changes introduced in this new release. We also want to take the opportunity to explain what has kept us busy since March.

The breaking changes dilemma, or, "when to release?"​

The Contributor's Guide explains the integration process that all changes to React Native go through. The project has is composed by many different tools, requiring coordination and constant support to keep everything working properly. Add to this the vibrant open source community that contributes back to the project, and you will get a sense of the mind-bending scale of it all.

With React Native's impressive adoption, breaking changes must be made with great care, and the process is not as smooth as we'd like. A decision was made to skip the April and May releases to allow the core team to integrate and test a new set of breaking changes. Dedicated community communication channels were used along the way to ensure that the June 2018 (0.56.0) release is as hassle-free as possible to adopt by those who patiently waited for the stable release.

Is 0.56.0 perfect? No, as every piece of software out there: but we reached a point where the tradeoff between "waiting for more stability" versus "testing led to successful results so we can push forward" that we feel ready to release it. Moreover, we are aware of a few issues that are not solved in the final 0.56.0 release. Most developers should have no issues upgrading to 0.56.0. For those that are blocked by the aforementioned issues, we hope to see you around in our discussions and we are looking forward to working with you on a solution to these issues.

You might consider 0.56.0 as a fundamental building block towards a more stable framework: it will take probably a week or two of widespread adoption before all the edge cases will be sanded off, but this will lead to an even better July 2018 (0.57.0) release.

We'd like to conclude this section by thanking all the 67 contributors who worked on a total of 818 commits (!) that will help make your apps even better 👏.

And now, without further ado...

The Big Changes​

Babel 7​

As you may know, the transpiler tool that allows us all to use the latest and greatest features of JavaScript, Babel, is moving to v7 soon. Since this new version brings along some important changes, we felt that now it would be a good time to upgrade, allowing Metro to leverage on its improvements.

If you find yourself in trouble with upgrading, please refer to the documentation section related to it.

Modernizing Android support​

On Android, much of the surrounding tooling has changed. We've updated to Gradle 3.5, Android SDK 26, Fresco to 1.9.0, and OkHttp to 3.10.0 and even the NDK API target to API 16. These changes should go without issue and result in faster builds. More importantly, it will help developers comply with the new Play Store requirements coming into effect next month.

Related to this, we'd like to particularly thank Dulmandakh for the many PRs submitted in order to make it possible 👏.

There are some more steps that need to be taken in this direction, and you can follow along with the future planning and discussion of updating the Android support in the dedicated issue (and a side one for the JSC).

New Node, Xcode, React, and Flow – oh my!​

Node 8 is now the standard for React Native. It was actually already being tested already, but we've put both feet forward as Node 6 entered maintenance mode. React was also updated to 16.4, which brings a ton of fixes with it.

We're dropping support for iOS 8, making iOS 9 the oldest iOS version that can be targeted. We do not foresee this being a problem, as any device that can run iOS 8, can be upgraded to iOS 9. This change allowed us to remove rarely-used code that implemented workarounds for older devices running iOS 8.

The continuous integration toolchain has been updated to use Xcode 9.4, ensuring that all iOS tests are run on the latest developer tools provided by Apple.

We have upgraded to Flow 0.75 to use the new error format that many devs appreciate. We've also created types for many more components. If you're not yet enforcing static typing in your project, please consider using Flow to identify problems as you code instead of at runtime.

And a lot of other things...​

For instance, YellowBox was replaced with a new implementation that makes debugging a lot better.

For the complete release notes, please reference the full changelog here. And remember to keep an eye on the upgrading guide to avoid issues moving to this new version.

A final note: starting this week, the React Native core team will resume holding monthly meetings. We'll make sure to keep everyone up-to-date with what's covered, and ensure to keep your feedback at hand for future meetings.

Happy coding everyone!

Lorenzo, Ryan, and the whole React Native core team

PS: as always, we'd like to remind everyone that React Native is still in 0.x versioning because of the many changes still undergoing - so remember when upgrading that yes, probably, something may still crash or be broken. Be helpful towards each other in the issues and when submitting PRs - and remember to follow the CoC enforced: there's always a human on the other side of the screen.

It's been a while since we last published a status update about React Native.

At Facebook, we're using React Native more than ever and for many important projects. One of our most popular products is Marketplace, one of the top-level tabs in our app which is used by 800 million people each month. Since its creation in 2015, all of Marketplace has been built with React Native, including over a hundred full-screen views throughout different parts of the app.

We're also using React Native for many new parts of the app. If you watched the F8 keynote last month, you'll recognize Blood Donations, Crisis Response, Privacy Shortcuts, and Wellness Checks – all recent features built with React Native. And projects outside the main Facebook app are using React Native too. The new Oculus Go VR headset includes a companion mobile app that is fully built with React Native, not to mention React VR powering many experiences in the headset itself.

Naturally, we also use many other technologies to build our apps. Litho and ComponentKit are two libraries we use extensively in our apps; both provide a React-like component API for building native screens. It's never been a goal for React Native to replace all other technologies – we are focused on making React Native itself better, but we love seeing other teams borrow ideas from React Native, like bringing instant reload to non-JavaScript code too.

Architecture​

When we started the React Native project in 2013, we designed it to have a single “bridge” between JavaScript and native that is asynchronous, serializable, and batched. Just as React DOM turns React state updates into imperative, mutative calls to DOM APIs like document.createElement(attrs) and .appendChild(), React Native was designed to return a single JSON message that lists mutations to perform, like [["createView", attrs], ["manageChildren", ...]]. We designed the entire system to never rely on getting a synchronous response back and to ensure everything in that list could be fully serialized to JSON and back. We did this for the flexibility it gave us: on top of this architecture, we were able to build tools like Chrome debugging, which runs all the JavaScript code asynchronously over a WebSocket connection.

Over the last 5 years, we found that these initial principles have made building some features harder. An asynchronous bridge means you can't integrate JavaScript logic directly with many native APIs expecting synchronous answers. A batched bridge that queues native calls means it's harder to have React Native apps call into functions that are implemented natively. And a serializable bridge means unnecessary copying instead of directly sharing memory between the two worlds. For apps that are entirely built in React Native, these restrictions are usually bearable. But for apps with complex integration between React Native and existing app code, they are frustrating.

We're working on a large-scale rearchitecture of React Native to make the framework more flexible and integrate better with native infrastructure in hybrid JavaScript/native apps. With this project, we'll apply what we've learned over the last 5 years and incrementally bring our architecture to a more modern one. We're rewriting many of React Native's internals, but most of the changes are under the hood: existing React Native apps will continue to work with few or no changes.

To make React Native more lightweight and fit better into existing native apps, this rearchitecture has three major internal changes. First, we are changing the threading model. Instead of each UI update needing to perform work on three different threads, it will be possible to call synchronously into JavaScript on any thread for high-priority updates while still keeping low-priority work off the main thread to maintain responsiveness. Second, we are incorporating async rendering capabilities into React Native to allow multiple rendering priorities and to simplify asynchronous data handling. Finally, we are simplifying our bridge to make it faster and more lightweight; direct calls between native and JavaScript are more efficient and will make it easier to build debugging tools like cross-language stack traces.

Once these changes are completed, closer integrations will be possible. Today, it's not possible to incorporate native navigation and gesture handling or native components like UICollectionView and RecyclerView without complex hacks. After our changes to the threading model, building features like this will be straightforward.

We'll release more details about this work later this year as it approaches completion.

Community​

Alongside the community inside Facebook, we're happy to have a thriving population of React Native users and collaborators outside Facebook. We'd like to support the React Native community more, both by serving React Native users better and by making the project easier to contribute to.

Just as our architecture changes will help React Native interoperate more cleanly with other native infrastructure, React Native should be slimmer on the JavaScript side to fit better with the JavaScript ecosystem, which includes making the VM and bundler swappable. We know the pace of breaking changes can be hard to keep up with, so we'd like to find ways to have fewer major releases. Finally, we know that some teams are looking for more thorough documentation in topics like startup optimization, where our expertise hasn't yet been written down. Expect to see some of these changes over the coming year.

If you're using React Native, you're part of our community; keep letting us know how we can make React Native better for you.

React Native is just one tool in a mobile developer's toolbox, but it's one that we strongly believe in – and we're making it better every day, with over 2500 commits in the last year from 500+ contributors.

JavaScript! We all love it. But some of us also love types. Luckily, options exist to add stronger types to JavaScript. My favourite is TypeScript, but React Native supports Flow out of the box. Which you prefer is a matter of preference, they each have their own approach on how to add the magic of types to JavaScript. Today, we're going to look at how to use TypeScript in React Native apps.

This post uses Microsoft's TypeScript-React-Native-Starter repo as a guide.

Update: Since this blog post was written, things have gotten even easier. You can replace all the set up described in this blog post by running just one command:

npx react-native init MyAwesomeProject --template react-native-template-typescript

However, there are some limitations to Babel's TypeScript support, which the blog post above goes into in detail. The steps outlined in this post still work, and Artsy is still using react-native-typescript-transformer in production, but the fastest way to get up and running with React Native and TypeScript is using the above command. You can always switch later if you have to.

In any case, have fun! The original blog post continues below.

Prerequisites​

Because you might be developing on one of several different platforms, targeting several different types of devices, basic setup can be involved. You should first ensure that you can run a plain React Native app without TypeScript. Follow the instructions on the React Native website to get started. When you've managed to deploy to a device or emulator, you'll be ready to start a TypeScript React Native app.

You will also need Node.js, npm, and Yarn.

Initializing​

Once you've tried scaffolding out an ordinary React Native project, you'll be ready to start adding TypeScript. Let's go ahead and do that.

react-native init MyAwesomeProject
cd MyAwesomeProject

Adding TypeScript​

The next step is to add TypeScript to your project. The following commands will:

• add TypeScript to your project
• add React Native TypeScript Transformer to your project
• initialize an empty TypeScript config file, which we'll configure next
• add an empty React Native TypeScript Transformer config file, which we'll configure next
• adds typings for React and React Native

Okay, let's go ahead and run these.

yarn add --dev typescript
yarn add --dev react-native-typescript-transformer
yarn tsc --init --pretty --jsx react
touch rn-cli.config.js
yarn add --dev @types/react @types/react-native

The tsconfig.json file contains all the settings for the TypeScript compiler. The defaults created by the command above are mostly fine, but open the file and uncomment the following line:

{
  /* Search the config file for the following line and uncomment it. */
  // "allowSyntheticDefaultImports": true,  /* Allow default imports from modules with no default export. This does not affect code emit, just typechecking. */
}

The rn-cli.config.js contains the settings for the React Native TypeScript Transformer. Open it and add the following:

module.exports = {
  getTransformModulePath() {
    return require.resolve('react-native-typescript-transformer');
  },
  getSourceExts() {
    return ['ts', 'tsx'];
  }
};

Migrating to TypeScript​

Rename the generated App.js and __tests_/App.js files to App.tsx. index.js needs to use the .js extension. All new files should use the .tsx extension (or .ts if the file doesn't contain any JSX).

If you tried to run the app now, you'd get an error like object prototype may only be an object or null. This is caused by a failure to import the default export from React as well as a named export on the same line. Open App.tsx and modify the import at the top of the file:

-import React, { Component } from 'react';
+import React from 'react'
+import { Component } from 'react';

Some of this has to do with differences in how Babel and TypeScript interoperate with CommonJS modules. In the future, the two will stabilize on the same behaviour.

At this point, you should be able to run the React Native app.

Adding TypeScript Testing Infrastructure​

React Native ships with Jest, so for testing a React Native app with TypeScript, we'll want to add ts-jest to our devDependencies.

yarn add --dev ts-jest

Then, we'll open up our package.json and replace the jest field with the following:

{
  "jest": {
    "preset": "react-native",
    "moduleFileExtensions": [
      "ts",
      "tsx",
      "js"
    ],
    "transform": {
      "^.+\\.(js)$": "<rootDir>/node_modules/babel-jest",
      "\\.(ts|tsx)$": "<rootDir>/node_modules/ts-jest/preprocessor.js"
    },
    "testRegex": "(/__tests__/.*|\\.(test|spec))\\.(ts|tsx|js)$",
    "testPathIgnorePatterns": [
      "\\.snap$",
      "<rootDir>/node_modules/"
    ],
    "cacheDirectory": ".jest/cache"
  }
}

This will configure Jest to run .ts and .tsx files with ts-jest.

Installing Dependency Type Declarations​

To get the best experience in TypeScript, we want the type-checker to understand the shape and API of our dependencies. Some libraries will publish their packages with .d.ts files (type declaration/type definition files), which can describe the shape of the underlying JavaScript. For other libraries, we'll need to explicitly install the appropriate package in the @types/ npm scope.

For example, here we'll need types for Jest, React, and React Native, and React Test Renderer.

yarn add --dev @types/jest @types/react @types/react-native @types/react-test-renderer

We saved these declaration file packages to our dev dependencies because this is a React Native app that only uses these dependencies during development and not during runtime. If we were publishing a library to NPM, we might have to add some of these type dependencies as regular dependencies.

You can read more here about getting .d.ts files.

Ignoring More Files​

For your source control, you'll want to start ignoring the .jest folder. If you're using git, we can just add entries to our .gitignore file.

# Jest
#
.jest/

As a checkpoint, consider committing your files into version control.

git init
git add .gitignore # import to do this first, to ignore our files
git add .
git commit -am "Initial commit."

Adding a Component​

Let's add a component to our app. Let's go ahead and create a Hello.tsx component. It's a pedagogical component, not something that you'd actually write in an app, but something nontrivial that shows off how to use TypeScript in React Native.

Create a components directory and add the following example.

// components/Hello.tsx
import React from 'react';
import { Button, StyleSheet, Text, View } from 'react-native';

export interface Props {
  name: string;
  enthusiasmLevel?: number;
}

interface State {
  enthusiasmLevel: number;
}

export class Hello extends React.Component<Props, State> {
  constructor(props: Props) {
    super(props);

    if ((props.enthusiasmLevel || 0) <= 0) {
      throw new Error(
        'You could be a little more enthusiastic. :D'
      );
    }

    this.state = {
      enthusiasmLevel: props.enthusiasmLevel || 1
    };
  }

  onIncrement = () =>
    this.setState({
      enthusiasmLevel: this.state.enthusiasmLevel + 1
    });
  onDecrement = () =>
    this.setState({
      enthusiasmLevel: this.state.enthusiasmLevel - 1
    });
  getExclamationMarks = (numChars: number) =>
    Array(numChars + 1).join('!');

  render() {
    return (
      <View style={styles.root}>
        <Text style={styles.greeting}>
          Hello{' '}
          {this.props.name +
            this.getExclamationMarks(this.state.enthusiasmLevel)}
        </Text>

        <View style={styles.buttons}>
          <View style={styles.button}>
            <Button
              title="-"
              onPress={this.onDecrement}
              accessibilityLabel="decrement"
              color="red"
            />
          </View>

          <View style={styles.button}>
            <Button
              title="+"
              onPress={this.onIncrement}
              accessibilityLabel="increment"
              color="blue"
            />
          </View>
        </View>
      </View>
    );
  }
}

// styles
const styles = StyleSheet.create({
  root: {
    alignItems: 'center',
    alignSelf: 'center'
  },
  buttons: {
    flexDirection: 'row',
    minHeight: 70,
    alignItems: 'stretch',
    alignSelf: 'center',
    borderWidth: 5
  },
  button: {
    flex: 1,
    paddingVertical: 0
  },
  greeting: {
    color: '#999',
    fontWeight: 'bold'
  }
});

Whoa! That's a lot, but let's break it down:

• Instead of rendering HTML elements like div, span, h1, etc., we're rendering components like View and Button. These are native components that work across different platforms.
• Styling is specified using the StyleSheet.create function that React Native gives us. React's stylesheets allow us to control our layout using Flexbox, and style using other constructs similar to those in CSS.

Adding a Component Test​

Now that we've got a component, let's try testing it.

We already have Jest installed as a test runner. We're going to write snapshot tests for our components, let's add the required add-on for snapshot tests:

yarn add --dev react-addons-test-utils

Now let's create a __tests__ folder in the components directory and add a test for Hello.tsx:

// components/__tests__/Hello.tsx
import React from 'react';
import renderer from 'react-test-renderer';

import { Hello } from '../Hello';

it('renders correctly with defaults', () => {
  const button = renderer
    .create(<Hello name="World" enthusiasmLevel={1} />)
    .toJSON();
  expect(button).toMatchSnapshot();
});

The first time the test is run, it will create a snapshot of the rendered component and store it in the components/__tests__/__snapshots__/Hello.tsx.snap file. When you modify your component, you'll need to update the snapshots and review the update for inadvertent changes. You can read more about testing React Native components here.

Next Steps​

Check out the official React tutorial and state-management library Redux. These resources can be helpful when writing React Native apps. Additionally, you may want to look at ReactXP, a component library written entirely in TypeScript that supports both React on the web as well as React Native.

Have fun in a more type-safe React Native development environment!