Replace repetitive blurhash code with component (#14267)

This commit replaces all unnecessarily repeated code for decoding and
embedding blurhash canvases with separate component - <Blurhash>.

Under the hood Blurhash component will use effect dependent on its
props. This gives a few benefits: it will only be re-rendered whenever
the hash or width/height/dummy props update, and will not render if
canvas won't get to the final DOM, because then effect won't fire,
which prevents weird bugs like #14257.

2 files changed, 74 insertions, 33 deletions

diff --git a/app/javascript/mastodon/components/blurhash.js b/app/javascript/mastodon/components/blurhash.js
new file mode 100644
index 000000000..172f8c2f5
--- /dev/null
+++ b/app/javascript/mastodon/components/blurhash.js
@@ -0,0 +1,61 @@
+// @ts-check
+
+import { decode } from 'blurhash';
+import React, { useRef, useEffect } from 'react';
+import PropTypes from 'prop-types';
+
+/**
+ * @typedef BlurhashPropsBase
+ * @property {string} hash Hash to render
+ * @property {number} width
+ * Width of the blurred region in pixels. Defaults to 32
+ * @property {number} [height]
+ * Height of the blurred region in pixels. Defaults to width
+ * @property {boolean} [dummy]
+ * Whether dummy mode is enabled. If enabled, nothing is rendered
+ * and canvas left untouched
+ */
+
+/** @typedef {JSX.IntrinsicElements['canvas'] & BlurhashPropsBase} BlurhashProps */
+
+/**
+ * Component that is used to render blurred of blurhash string
+ *
+ * @param {BlurhashProps} param1 Props of the component
+ * @returns Canvas which will render blurred region element to embed
+ */
+function Blurhash({
+  hash,
+  width = 32,
+  height = width,
+  dummy = false,
+  ...canvasProps
+}) {
+  const canvasRef = /** @type {import('react').MutableRefObject<HTMLCanvasElement>} */ (useRef());
+
+  useEffect(() => {
+    const { current: canvas } = canvasRef;
+    canvas.width = canvas.width; // resets canvas
+
+    if (dummy) return;
+
+    const pixels = decode(hash, width, height);
+    const ctx = canvas.getContext('2d');
+    const imageData = new ImageData(pixels, width, height);
+
+    ctx.putImageData(imageData, 0, 0);
+  }, [dummy, hash, width, height]);
+
+  return (
+    <canvas {...canvasProps} ref={canvasRef} width={width} height={height} />
+  );
+}
+
+Blurhash.propTypes = {
+  hash: PropTypes.string.isRequired,
+  width: PropTypes.number,
+  height: PropTypes.number,
+  dummy: PropTypes.bool,
+};
+
+export default React.memo(Blurhash);
diff --git a/app/javascript/mastodon/components/media_gallery.js b/app/javascript/mastodon/components/media_gallery.js
index 0ec866138..0a8f42585 100644
--- a/app/javascript/mastodon/components/media_gallery.js
+++ b/app/javascript/mastodon/components/media_gallery.js
@@ -7,8 +7,8 @@ import { defineMessages, injectIntl, FormattedMessage } from 'react-intl';
 import { isIOS } from '../is_mobile';
 import classNames from 'classnames';
 import { autoPlayGif, cropImages, displayMedia, useBlurhash } from '../initial_state';
-import { decode } from 'blurhash';
 import { debounce } from 'lodash';
+import Blurhash from 'mastodon/components/blurhash';
 
 const messages = defineMessages({
   toggle_visible: { id: 'media_gallery.toggle_visible', defaultMessage: 'Hide {number, plural, one {image} other {images}}' },
@@ -74,36 +74,6 @@ class Item extends React.PureComponent {
     e.stopPropagation();
   }
 
-  componentDidMount () {
-    if (this.props.attachment.get('blurhash')) {
-      this._decode();
-    }
-  }
-
-  componentDidUpdate (prevProps) {
-    if (prevProps.attachment.get('blurhash') !== this.props.attachment.get('blurhash') && this.props.attachment.get('blurhash')) {
-      this._decode();
-    }
-  }
-
-  _decode () {
-    if (!useBlurhash) return;
-
-    const hash   = this.props.attachment.get('blurhash');
-    const pixels = decode(hash, 32, 32);
-
-    if (pixels) {
-      const ctx       = this.canvas.getContext('2d');
-      const imageData = new ImageData(pixels, 32, 32);
-
-      ctx.putImageData(imageData, 0, 0);
-    }
-  }
-
-  setCanvasRef = c => {
-    this.canvas = c;
-  }
-
   handleImageLoad = () => {
     this.setState({ loaded: true });
   }
@@ -166,7 +136,11 @@ class Item extends React.PureComponent {
       return (
         <div className={classNames('media-gallery__item', { standalone })} key={attachment.get('id')} style={{ left: left, top: top, right: right, bottom: bottom, width: `${width}%`, height: `${height}%` }}>
           <a className='media-gallery__item-thumbnail' href={attachment.get('remote_url') || attachment.get('url')} style={{ cursor: 'pointer' }} title={attachment.get('description')} target='_blank' rel='noopener noreferrer'>
-            <canvas width={32} height={32} ref={this.setCanvasRef} className='media-gallery__preview' />
+            <Blurhash
+              hash={attachment.get('blurhash')}
+              className='media-gallery__preview'
+              dummy={!useBlurhash}
+            />
           </a>
         </div>
       );
@@ -232,7 +206,13 @@ class Item extends React.PureComponent {
 
     return (
       <div className={classNames('media-gallery__item', { standalone })} key={attachment.get('id')} style={{ left: left, top: top, right: right, bottom: bottom, width: `${width}%`, height: `${height}%` }}>
-        <canvas width={32} height={32} ref={this.setCanvasRef} className={classNames('media-gallery__preview', { 'media-gallery__preview--hidden': visible && this.state.loaded })} />
+        <Blurhash
+          hash={attachment.get('blurhash')}
+          dummy={!useBlurhash}
+          className={classNames('media-gallery__preview', {
+            'media-gallery__preview--hidden': visible && this.state.loaded,
+          })}
+        />
         {visible && thumbnail}
       </div>
     );