bvaughn

Infinite lists and reflow

In my experience, infinite lists use two basic layout strategies. The first uses absolute positioning to control where visible items are rendered. The second uses relative positioning (with top/left padding to offset for unrendered items).

In both cases, the list abstraction caches some metadata about the size of items once they have been rendered– so that it knows where to position the items that come after them.

Both of these strategies need to handle reflow. For example, changing the width of a list often affects the height of its itesm. Generally speaking, only the "window" of rendered (visible) items are remeasured in this case (because it would be too slow to rerender and remeasure all of the items before). But once a user scrolls backwards (up/left)– the list needs to account for the reflowed sizes. If it didn't, items would appear to jump up or down (depending on the delta between the previous, cached sizes and the new/reflowed sizes).

How the list deals with new sizes

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The MIT License (MIT)

Copyright © <year> <copyright holders>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell

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Pre-rendering virtual rows

Example list with ten total rows and a viewport large enough to display two rows.

Legend

█ - Visible row ▓ - Hidden/display-locked row ░ - Empty space (nothing rendered here)

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function setFocusIfFocusable(node) {
  if (node.nodeType !== Node.ELEMENT_NODE) {
    // Text and comment nodes aren't focusable.
    return false;
  }

  if (node.disabled === true) {
    // Disabled elements can't be focused.
    return false;
  }

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RFC Test Selectors

Owner: Brian Vaughn

---

As of facebook/react/pull/22760, the experimental Test Selector API is now available in the experimental release channel.

To test the API, first install the experimental release:

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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:

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// Simplistic (probably most common) approach.
//
// This approach assumes either that:
// 1) passive effects are always run asynchronously, after paint, or
// 2) passive effects never attach handlers for bubbling events
//
// If both of the above are wrong (as can be the case) then problems might occur!
useEffect(() => {
  const handleDocumentClick = (event: MouseEvent) => {
    // It's possible that a "click" event rendered the component with this effect,

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import { CustomConsole } from '@jest/console';

function formatter(type, message) {
  switch(type) {
    case 'error':
      return "\x1b[31m" + message + "\x1b[0m";
    case 'warn':
      return "\x1b[33m" + message + "\x1b[0m";
    case 'log':
    default:

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const COLORS = {
  background: {
    bgBlack: {
      browser: [0, 0, 0],
      node: "\x1b[40m",
    },
    bgRed: {
      browser: [255, 0, 0],
      node: "\x1b[41m",
    },

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/** Measures framerate for the time between start() and stop() calls */
function FramerateMeasurer () {
  this.start = () => {
    this._beginTime = ( performance || Date ).now()
    this._frames = 0
    this._animationFrameId = requestAnimationFrame(this._loop)
  }

  this.stop = () => {
    const endTime = ( performance || Date ).now()