react-reconciler

What is react-reconciler?

The react-reconciler package is a framework for building custom React renderers. It provides the core algorithms that React uses for reconciliation, allowing developers to create their own renderers for different target environments beyond what React supports out of the box (e.g., the web or native mobile). This package is an under-the-hood tool primarily intended for advanced use cases where you need to create a custom rendering environment.

What are react-reconciler's main functionalities?

Creating a Custom Renderer

This code sample demonstrates how to start creating a custom renderer using react-reconciler. You define methods like `createInstance` for creating instances of components and `createTextInstance` for handling text. The host config object passed to `ReactReconciler` includes a variety of methods for managing instances, updates, and more.

const MyRenderer = ReactReconciler({
  now: Date.now,
  createInstance: (type, props) => {
    // Create instance of a component
  },
  createTextInstance: (text) => {
    // Create text instance
  },
  // Other host config methods...
});

Scheduling Updates

This example shows how to schedule updates to the container using your custom renderer. The `updateContainer` method is used to apply updates to the container, which in this case is a custom object you manage. This method is part of what allows React's declarative API to work with your custom rendering logic.

const container = {}; // Your container object
MyRenderer.updateContainer(<App />, container, null, () => {
  console.log('Rendered with MyRenderer');
});

Other packages similar to react-reconciler

preact

Preact is a fast 3kB alternative to React with the same modern API. While not a direct alternative to react-reconciler, it offers a different approach to building user interfaces with a smaller footprint. Preact can be used to build custom renderers, but its primary use case is as a React alternative for building web applications.

inferno

Inferno is another high-performance alternative to React that focuses on speed and performance. Similar to Preact, Inferno is not a direct alternative to react-reconciler but offers a different approach to building UIs with an emphasis on performance. Inferno includes its own mechanisms for creating custom renderers, making it a potential alternative for performance-critical applications.

README

react-reconciler

This is an experimental package for creating custom React renderers.

Its API is not as stable as that of React, React Native, or React DOM, and does not follow the common versioning scheme.

Use it at your own risk.

Usage

const Reconciler = require('react-reconciler');

const HostConfig = {
  // You'll need to implement some methods here.
  // See below for more information and examples.
};

const MyRenderer = Reconciler(HostConfig);

const RendererPublicAPI = {
  render(element, container, callback) {
    // Call MyRenderer.updateContainer() to schedule changes on the roots.
    // See ReactDOM, React Native, or React ART for practical examples.
  }
};

module.exports = RendererPublicAPI;

Practical Examples

A "host config" is an object that you need to provide, and that describes how to make something happen in the "host" environment (e.g. DOM, canvas, console, or whatever your rendering target is). It looks like this:

const HostConfig = {
  createInstance(type, props) {
    // e.g. DOM renderer returns a DOM node
  },
  // ...
  supportsMutation: true, // it works by mutating nodes
  appendChild(parent, child) {
    // e.g. DOM renderer would call .appendChild() here
  },
  // ...
};

For an introduction to writing a very simple custom renderer, check out this article series:

• Building a simple custom renderer to DOM
• Building a simple custom renderer to native

The full list of supported methods can be found here. For their signatures, we recommend looking at specific examples below.

The React repository includes several renderers. Each of them has its own host config.

The examples in the React repository are declared a bit differently than a third-party renderer would be. In particular, the HostConfig object mentioned above is never explicitly declared, and instead is a module in our code. However, its exports correspond directly to properties on a HostConfig object you'd need to declare in your code:

• React ART and its host config
• React DOM and its host config
• React Native and its host config

If these links break please file an issue and we’ll fix them. They intentionally link to the latest versions since the API is still evolving. If you have more questions please file an issue and we’ll try to help!

An (Incomplete!) Reference

At the moment, we can't commit to documenting every API detail because the host config still changes very often between the releases. The documentation below is provided in the spirit of making our best effort rather than an API guarantee. It focuses on the parts that don't change too often. This is a compromise that strikes a balance between the need for a fast-paced development of React itself, and the usefulness of this package to the custom renderer community. If you notice parts that are out of date or don't match how the latest stable update is behaving, please file an issue or send a pull request, although a response might take time.

Modes

The reconciler has two modes: mutation mode and persistent mode. You must specify one of them.

If your target platform is similar to the DOM and has methods similar to appendChild, removeChild, and so on, you'll want to use the mutation mode. This is the same mode used by React DOM, React ART, and the classic React Native renderer.

const HostConfig = {
  // ...
  supportsMutation: true,
  // ...
}

If your target platform has immutable trees, you'll want the persistent mode instead. In that mode, existing nodes are never mutated, and instead every change clones the parent tree and then replaces the whole parent tree at the root. This is the node used by the new React Native renderer, codenamed "Fabric".

const HostConfig = {
  // ...
  supportsPersistence: true,
  // ...
}

Depending on the mode, the reconciler will call different methods on your host config.

If you're not sure which one you want, you likely need the mutation mode.

Core Methods

createInstance(type, props, rootContainer, hostContext, internalHandle)

This method should return a newly created node. For example, the DOM renderer would call document.createElement(type) here and then set the properties from props.

You can use rootContainer to access the root container associated with that tree. For example, in the DOM renderer, this is useful to get the correct document reference that the root belongs to.

The hostContext parameter lets you keep track of some information about your current place in the tree. To learn more about it, see getChildHostContext below.

The internalHandle data structure is meant to be opaque. If you bend the rules and rely on its internal fields, be aware that it may change significantly between versions. You're taking on additional maintenance risk by reading from it, and giving up all guarantees if you write something to it.

This method happens in the render phase. It can (and usually should) mutate the node it has just created before returning it, but it must not modify any other nodes. It must not register any event handlers on the parent tree. This is because an instance being created doesn't guarantee it would be placed in the tree — it could be left unused and later collected by GC. If you need to do something when an instance is definitely in the tree, look at commitMount instead.

createTextInstance(text, rootContainer, hostContext, internalHandle)

Same as createInstance, but for text nodes. If your renderer doesn't support text nodes, you can throw here.

appendInitialChild(parentInstance, child)

This method should mutate the parentInstance and add the child to its list of children. For example, in the DOM this would translate to a parentInstance.appendChild(child) call.

This method happens in the render phase. It can mutate parentInstance and child, but it must not modify any other nodes. It's called while the tree is still being built up and not connected to the actual tree on the screen.

finalizeInitialChildren(instance, type, props, rootContainer, hostContext)

In this method, you can perform some final mutations on the instance. Unlike with createInstance, by the time finalizeInitialChildren is called, all the initial children have already been added to the instance, but the instance itself has not yet been connected to the tree on the screen.

This method happens in the render phase. It can mutate instance, but it must not modify any other nodes. It's called while the tree is still being built up and not connected to the actual tree on the screen.

There is a second purpose to this method. It lets you specify whether there is some work that needs to happen when the node is connected to the tree on the screen. If you return true, the instance will receive a commitMount call later. See its documentation below.

If you don't want to do anything here, you should return false.

prepareUpdate(instance, type, oldProps, newProps, rootContainer, hostContext)

React calls this method so that you can compare the previous and the next props, and decide whether you need to update the underlying instance or not. If you don't need to update it, return null. If you need to update it, you can return an arbitrary object representing the changes that need to happen. Then in commitUpdate you would need to apply those changes to the instance.

This method happens in the render phase. It should only calculate the update — but not apply it! For example, the DOM renderer returns an array that looks like [prop1, value1, prop2, value2, ...] for all props that have actually changed. And only in commitUpdate it applies those changes. You should calculate as much as you can in prepareUpdate so that commitUpdate can be very fast and straightforward.

See the meaning of rootContainer and hostContext in the createInstance documentation.

shouldSetTextContent(type, props)

Some target platforms support setting an instance's text content without manually creating a text node. For example, in the DOM, you can set node.textContent instead of creating a text node and appending it.

If you return true from this method, React will assume that this node's children are text, and will not create nodes for them. It will instead rely on you to have filled that text during createInstance. This is a performance optimization. For example, the DOM renderer returns true only if type is a known text-only parent (like 'textarea') or if props.children has a 'string' type. If you return true, you will need to implement resetTextContent too.

If you don't want to do anything here, you should return false.

This method happens in the render phase. Do not mutate the tree from it.

getRootHostContext(rootContainer)

This method lets you return the initial host context from the root of the tree. See getChildHostContext for the explanation of host context.

If you don't intend to use host context, you can return null.

This method happens in the render phase. Do not mutate the tree from it.

getChildHostContext(parentHostContext, type, rootContainer)

Host context lets you track some information about where you are in the tree so that it's available inside createInstance as the hostContext parameter. For example, the DOM renderer uses it to track whether it's inside an HTML or an SVG tree, because createInstance implementation needs to be different for them.

If the node of this type does not influence the context you want to pass down, you can return parentHostContext. Alternatively, you can return any custom object representing the information you want to pass down.

If you don't want to do anything here, return parentHostContext.

This method happens in the render phase. Do not mutate the tree from it.

getPublicInstance(instance)

Determines what object gets exposed as a ref. You'll likely want to return the instance itself. But in some cases it might make sense to only expose some part of it.

If you don't want to do anything here, return instance.

prepareForCommit(containerInfo)

This method lets you store some information before React starts making changes to the tree on the screen. For example, the DOM renderer stores the current text selection so that it can later restore it. This method is mirrored by resetAfterCommit.

Even if you don't want to do anything here, you need to return null from it.

resetAfterCommit(containerInfo)

This method is called right after React has performed the tree mutations. You can use it to restore something you've stored in prepareForCommit — for example, text selection.

You can leave it empty.

preparePortalMount(containerInfo)

This method is called for a container that's used as a portal target. Usually you can leave it empty.

scheduleTimeout(fn, delay)

You can proxy this to setTimeout or its equivalent in your environment.

cancelTimeout(id)

You can proxy this to clearTimeout or its equivalent in your environment.

noTimeout

This is a property (not a function) that should be set to something that can never be a valid timeout ID. For example, you can set it to -1.

supportsMicrotask

Set this to true to indicate that your renderer supports scheduleMicrotask. We use microtasks as part of our discrete event implementation in React DOM. If you're not sure if your renderer should support this, you probably should. The option to not implement scheduleMicrotask exists so that platforms with more control over user events, like React Native, can choose to use a different mechanism.

scheduleMicrotask(fn)

Optional. You can proxy this to queueMicrotask or its equivalent in your environment.

isPrimaryRenderer

This is a property (not a function) that should be set to true if your renderer is the main one on the page. For example, if you're writing a renderer for the Terminal, it makes sense to set it to true, but if your renderer is used on top of React DOM or some other existing renderer, set it to false.

getCurrentEventPriority

To implement this method, you'll need some constants available on the special react-reconciler/constants entry point:

import {
  DiscreteEventPriority,
  ContinuousEventPriority,
  DefaultEventPriority,
} from 'react-reconciler/constants';

const HostConfig = {
  // ...
  getCurrentEventPriority() {
    return DefaultEventPriority;
  },
  // ...
}

const MyRenderer = Reconciler(HostConfig);

The constant you return depends on which event, if any, is being handled right now. (In the browser, you can check this using window.event && window.event.type).

• Discrete events: If the active event is directly caused by the user (such as mouse and keyboard events) and each event in a sequence is intentional (e.g. click), return DiscreteEventPriority. This tells React that they should interrupt any background work and cannot be batched across time.

• Continuous events: If the active event is directly caused by the user but the user can't distinguish between individual events in a sequence (e.g. mouseover), return ContinuousEventPriority. This tells React they should interrupt any background work but can be batched across time.

• Other events / No active event: In all other cases, return DefaultEventPriority. This tells React that this event is considered background work, and interactive events will be prioritized over it.

You can consult the getCurrentEventPriority() implementation in ReactDOMHostConfig.js for a reference implementation.

Mutation Methods

If you're using React in mutation mode (you probably do), you'll need to implement a few more methods.

appendChild(parentInstance, child)

This method should mutate the parentInstance and add the child to its list of children. For example, in the DOM this would translate to a parentInstance.appendChild(child) call.

Although this method currently runs in the commit phase, you still should not mutate any other nodes in it. If you need to do some additional work when a node is definitely connected to the visible tree, look at commitMount.

appendChildToContainer(container, child)

Same as appendChild, but for when a node is attached to the root container. This is useful if attaching to the root has a slightly different implementation, or if the root container nodes are of a different type than the rest of the tree.

insertBefore(parentInstance, child, beforeChild)

This method should mutate the parentInstance and place the child before beforeChild in the list of its children. For example, in the DOM this would translate to a parentInstance.insertBefore(child, beforeChild) call.

Note that React uses this method both for insertions and for reordering nodes. Similar to DOM, it is expected that you can call insertBefore to reposition an existing child. Do not mutate any other parts of the tree from it.

insertInContainerBefore(container, child, beforeChild)

Same as insertBefore, but for when a node is attached to the root container. This is useful if attaching to the root has a slightly different implementation, or if the root container nodes are of a different type than the rest of the tree.

removeChild(parentInstance, child)

This method should mutate the parentInstance to remove the child from the list of its children.

React will only call it for the top-level node that is being removed. It is expected that garbage collection would take care of the whole subtree. You are not expected to traverse the child tree in it.

removeChildFromContainer(container, child)

Same as removeChild, but for when a node is detached from the root container. This is useful if attaching to the root has a slightly different implementation, or if the root container nodes are of a different type than the rest of the tree.

resetTextContent(instance)

If you returned true from shouldSetTextContent for the previous props, but returned false from shouldSetTextContent for the next props, React will call this method so that you can clear the text content you were managing manually. For example, in the DOM you could set node.textContent = ''.

If you never return true from shouldSetTextContent, you can leave it empty.

commitTextUpdate(textInstance, prevText, nextText)

This method should mutate the textInstance and update its text content to nextText.

Here, textInstance is a node created by createTextInstance.

commitMount(instance, type, props, internalHandle)

This method is only called if you returned true from finalizeInitialChildren for this instance.

It lets you do some additional work after the node is actually attached to the tree on the screen for the first time. For example, the DOM renderer uses it to trigger focus on nodes with the autoFocus attribute.

Note that commitMount does not mirror removeChild one to one because removeChild is only called for the top-level removed node. This is why ideally commitMount should not mutate any nodes other than the instance itself. For example, if it registers some events on some node above, it will be your responsibility to traverse the tree in removeChild and clean them up, which is not ideal.

The internalHandle data structure is meant to be opaque. If you bend the rules and rely on its internal fields, be aware that it may change significantly between versions. You're taking on additional maintenance risk by reading from it, and giving up all guarantees if you write something to it.

If you never return true from finalizeInitialChildren, you can leave it empty.

commitUpdate(instance, updatePayload, type, prevProps, nextProps, internalHandle)

This method should mutate the instance according to the set of changes in updatePayload. Here, updatePayload is the object that you've returned from prepareUpdate and has an arbitrary structure that makes sense for your renderer. For example, the DOM renderer returns an update payload like [prop1, value1, prop2, value2, ...] from prepareUpdate, and that structure gets passed into commitUpdate. Ideally, all the diffing and calculation should happen inside prepareUpdate so that commitUpdate can be fast and straightforward.

The internalHandle data structure is meant to be opaque. If you bend the rules and rely on its internal fields, be aware that it may change significantly between versions. You're taking on additional maintenance risk by reading from it, and giving up all guarantees if you write something to it.

hideInstance(instance)

This method should make the instance invisible without removing it from the tree. For example, it can apply visual styling to hide it. It is used by Suspense to hide the tree while the fallback is visible.

hideTextInstance(textInstance)

Same as hideInstance, but for nodes created by createTextInstance.

unhideInstance(instance, props)

This method should make the instance visible, undoing what hideInstance did.

unhideTextInstance(textInstance, text)

Same as unhideInstance, but for nodes created by createTextInstance.

clearContainer(container)

This method should mutate the container root node and remove all children from it.

Persistence Methods

If you use the persistent mode instead of the mutation mode, you would still need the "Core Methods". However, instead of the Mutation Methods above you will implement a different set of methods that performs cloning nodes and replacing them at the root level. You can find a list of them in the "Persistence" section listed in this file. File an issue if you need help.

Hydration Methods

You can optionally implement hydration to "attach" to the existing tree during the initial render instead of creating it from scratch. For example, the DOM renderer uses this to attach to an HTML markup.

To support hydration, you need to declare supportsHydration: true and then implement the methods in the "Hydration" section listed in this file. File an issue if you need help.