Modify encrypted images loading for better UX

Adds proper react css transition to encrypted images placeholders, so
that we can animate it's entrance and exit properly.
In addition, adds simple css animations file to properly handle all of
the animations in one place, so that it's much easier to properly handle
prefers-reduced-motion media query.

res/css/_animations.scss

@@ -0,0 +1,57 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/**
+ * React Transition Group animations are prefixed with 'mx_rtg--' so that we
+ * know they should not be used anywhere outside of React Transition Groups.
+*/
+
+.mx_rtg--fade {
+    &-enter {
+        opacity: 0;
+        &-active {
+            opacity: 1;
+            transition: opacity 300ms ease;
+        }
+    }
+    &-exit {
+        opacity: 1;
+        &-active {
+            opacity: 0;
+            transition: opacity 300ms ease;
+        }
+    }
+}
+
+
+@keyframes mx--anim-pulse {
+    0% { opacity: 1; }
+    50% { opacity: 0.7; }
+    100% { opacity: 1; }
+}
+
+
+@media (prefers-reduced-motion) {
+    @keyframes mx--anim-pulse {
+        // Override all keyframes in reduced-motion
+    }
+    .mx_rtg--fade-enter-active {
+        transition: none;
+    }
+    .mx_rtg--fade-exit-active {
+        transition: none;
+    }
+}

res/css/_common.scss

@@ -18,6 +18,7 @@ limitations under the License.
 
 @import "./_font-sizes.scss";
 @import "./_font-weights.scss";
+@import "./_animations.scss";
 
 $hover-transition: 0.08s cubic-bezier(.46, .03, .52, .96); // quadratic
 

res/css/views/messages/_MImageBody.scss

@@ -16,6 +16,12 @@ limitations under the License.
 
 $timelineImageBorderRadius: 4px;
 
+.mx_MImageBody_thumbnail--blurhash {
+    position: absolute;
+    left: 0;
+    top: 0;
+}
+
 .mx_MImageBody_thumbnail {
     object-fit: contain;
     border-radius: $timelineImageBorderRadius;
@@ -23,8 +29,11 @@ $timelineImageBorderRadius: 4px;
     display: flex;
     justify-content: center;
     align-items: center;
+    height: 100%;
+    width: 100%;
 
-    > div > canvas {
+    .mx_Blurhash > canvas {
+        animation: mx--anim-pulse 1.75s infinite cubic-bezier(.4, 0, .6, 1);
         border-radius: $timelineImageBorderRadius;
     }
 }

src/components/views/messages/MImageBody.tsx

@@ -25,12 +25,14 @@ import SettingsStore from "../../../settings/SettingsStore";
 import MatrixClientContext from "../../../contexts/MatrixClientContext";
 import InlineSpinner from '../elements/InlineSpinner';
 import { replaceableComponent } from "../../../utils/replaceableComponent";
-import { mediaFromContent } from "../../../customisations/Media";
+import { Media, mediaFromContent } from "../../../customisations/Media";
 import { BLURHASH_FIELD } from "../../../ContentMessages";
 import { IMediaEventContent } from '../../../customisations/models/IMediaEventContent';
 import ImageView from '../elements/ImageView';
 import { SyncState } from 'matrix-js-sdk/src/sync.api';
 import { IBodyProps } from "./IBodyProps";
+import classNames from 'classnames';
+import { CSSTransition, SwitchTransition } from 'react-transition-group';
 
 interface IState {
     decryptedUrl?: string;
@@ -157,19 +159,21 @@ export default class MImageBody extends React.Component<IBodyProps, IState> {
             // this is only used as a fallback in case content.info.w/h is missing
             loadedImageDimensions = { naturalWidth, naturalHeight };
         }
-
         this.setState({ imgLoaded: true, loadedImageDimensions });
     };
 
     protected getContentUrl(): string {
-        const media = mediaFromContent(this.props.mxEvent.getContent());
-        if (media.isEncrypted) {
+        if (this.media.isEncrypted) {
             return this.state.decryptedUrl;
         } else {
-            return media.srcHttp;
+            return this.media.srcHttp;
         }
     }
 
+    private get media(): Media {
+        return mediaFromContent(this.props.mxEvent.getContent());
+    }
+
     protected getThumbUrl(): string {
         // FIXME: we let images grow as wide as you like, rather than capped to 800x600.
         // So either we need to support custom timeline widths here, or reimpose the cap, otherwise the
@@ -225,7 +229,7 @@ export default class MImageBody extends React.Component<IBodyProps, IState> {
                     info.w > thumbWidth ||
                     info.h > thumbHeight
                 );
-                const isLargeFileSize = info.size > 1*1024*1024; // 1mb
+                const isLargeFileSize = info.size > 1 * 1024 * 1024; // 1mb
 
                 if (isLargeFileSize && isLargerThanThumbnail) {
                     // image is too large physically and bytewise to clutter our timeline so
@@ -374,23 +378,39 @@ export default class MImageBody extends React.Component<IBodyProps, IState> {
             gifLabel = <p className="mx_MImageBody_gifLabel">GIF</p>;
         }
 
+        const classes = classNames({
+            'mx_MImageBody_thumbnail': true,
+            'mx_MImageBody_thumbnail--blurhash': this.props.mxEvent.getContent().info[BLURHASH_FIELD],
+        });
+
+        // This has incredibly broken types.
+        const C = CSSTransition as any;
         const thumbnail = (
             <div className="mx_MImageBody_thumbnail_container" style={{ maxHeight: maxHeight, maxWidth: maxWidth, aspectRatio: `${infoWidth}/${infoHeight}` }}>
-                { showPlaceholder &&
-                    <div
-                        className="mx_MImageBody_thumbnail"
-                        style={{
-                            // Constrain width here so that spinner appears central to the loaded thumbnail
-                            maxWidth: `min(100%, ${infoWidth}px)`,
-                        }}
+                <SwitchTransition mode="out-in">
+                    <C
+                        classNames="mx_rtg--fade"
+                        key={`img-${showPlaceholder}`}
+                        timeout={300}
                     >
-                        { placeholder }
-                    </div>
-                }
+                        <div>
+                            { showPlaceholder && <div
+                                className={classes}
+                                style={{
+                                    // Constrain width here so that spinner appears central to the loaded thumbnail
+                                    maxWidth: `min(100%, ${infoWidth}px)`,
+                                    maxHeight: maxHeight,
+                                    aspectRatio: `${infoWidth}/${infoHeight}`,
+                                }}
+                            >
+                                { placeholder }
+                            </div> }
+                        </div>
+                    </C>
+                </SwitchTransition>
 
                 <div style={{
-                    display: !showPlaceholder ? undefined : 'none',
-                    height: '100%', // Also force to size of a parent to prevent scroll-jumps (see above)
+                    height: '100%',
                 }}>
                     { img }
                     { gifLabel }
@@ -413,7 +433,7 @@ export default class MImageBody extends React.Component<IBodyProps, IState> {
     // Overidden by MStickerBody
     protected getPlaceholder(width: number, height: number): JSX.Element {
         const blurhash = this.props.mxEvent.getContent().info[BLURHASH_FIELD];
-        if (blurhash) return <Blurhash hash={blurhash} width={width} height={height} />;
+        if (blurhash) return <Blurhash className="mx_Blurhash" hash={blurhash} width={width} height={height} />;
         return (
             <InlineSpinner w={32} h={32} />
         );
@@ -455,10 +475,12 @@ export default class MImageBody extends React.Component<IBodyProps, IState> {
         const thumbnail = this.messageContent(contentUrl, thumbUrl, content);
         const fileBody = this.getFileBody();
 
-        return <div className="mx_MImageBody">
-            { thumbnail }
-            { fileBody }
-        </div>;
+        return (
+            <div className="mx_MImageBody">
+                { thumbnail }
+                { fileBody }
+            </div>
+        );
     }
 }