React Native — A Bridge To Fabric 

Now, let’s explain about all the new words you see here: JSI, Fabric, Turbo Modules

JSI is gonna replace the Bridge

JSI (Will replace the Bridge) — Its purpose is to make the JS and Native are aware of each other. There won’t be any need to serialize JSON through the bridge and even more than that — there won’t be a bridge! It will allow to expose native object as js objects and vice-versa. It will also expose an API for synchronous calling on this object on both sides. Actually, all the rest of the architecture will be built on top of that (Fabric, Turbo Modules which will be explained next).

Fabric — the new name for the UIManager which will be responsible for the native side. The biggest difference now that instead of communicating the JS side by the bridge, it will expose its native function using the JSI so the JS side and vice-versa can communication directly those ref functions. Better and efficient performance and passing data between sides.

Remember the native modules from the last post? Text, Image, View. So their new name is Turbo Modules. They have the same purpose but implemented and acting differently. First, they are lazy-loaded (only when the app needs them) comparing to loading all of them on the launch time. In addition, they are also exposed using the JSI so JS holds a ref to use them on the React Native JS lib side. Result => better performance especially on launch time.

CodeGen — Suppose to make the JS side a Single Source Of Truth. Can let you create static types of the js so the native side (Fabric and Turbo Modules) will be aware to them and avoid validating the data each time => better performance and less place for mistakes when passing data.

Lean Core — A change in React native repository structure. The purpose is to make the lib lighter and help the community resolve more pull requests faster than before. Facebook splits some of its parts to an external repo. It is already a process in progress that you can follow in their Github here.

We now go over the flow from the previous post and will show the differences. There isn’t a bridge anymore

1. User clicks on the app icon
2. Fabric loads the native side (no native modules)
3. It tells the JS thread that it is ready and now the JS side loads all the main.bundle.js which contains all js and react logic + components
4. JS called through the ref native function (the one that was exposed as an object using the JSI API) to Fabric and the shadow node creates the tree as before
5. Yoga does the layout calculation converting from flexbox-based style to host layout
6. Fabric does its thing and shows the UI