Simple POC for moving download button to action bar

src/utils/LazyValue.ts

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+/*
+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.
+*/
+
+/**
+ * Utility class for lazily getting a variable.
+ */
+export class LazyValue<T> {
+    private val: T;
+    private prom: Promise<T>;
+    private done = false;
+
+    public constructor(private getFn: () => Promise<T>) {
+    }
+
+    /**
+     * Whether or not a cached value is present.
+     */
+    public get present(): boolean {
+        // we use a tracking variable just in case the final value is falsey
+        return this.done;
+    }
+
+    /**
+     * Gets the value without invoking a get. May be undefined until the
+     * value is fetched properly.
+     */
+    public get cachedValue(): T {
+        return this.val;
+    }
+
+    /**
+     * Gets a promise which resolves to the value, eventually.
+     */
+    public get value(): Promise<T> {
+        if (this.prom) return this.prom;
+        this.prom = this.getFn();
+
+        // Fork the promise chain to avoid accidentally making it return undefined always.
+        this.prom.then(v => {
+            this.val = v;
+            this.done = true;
+        });
+
+        return this.prom;
+    }
+}

src/utils/MediaEventHelper.ts

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+/*
+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.
+*/
+
+import { MatrixEvent } from "matrix-js-sdk/src";
+import { LazyValue } from "./LazyValue";
+import { Media, mediaFromContent } from "../customisations/Media";
+import { decryptFile } from "./DecryptFile";
+import { IMediaEventContent } from "../customisations/models/IMediaEventContent";
+import { IDestroyable } from "./IDestroyable";
+
+// TODO: We should consider caching the blobs. https://github.com/vector-im/element-web/issues/17192
+
+export class MediaEventHelper implements IDestroyable {
+    public readonly sourceUrl: LazyValue<string>;
+    public readonly thumbnailUrl: LazyValue<string>;
+    public readonly sourceBlob: LazyValue<Blob>;
+    public readonly thumbnailBlob: LazyValue<Blob>;
+    public readonly media: Media;
+
+    public constructor(private event: MatrixEvent) {
+        this.sourceUrl = new LazyValue(this.prepareSourceUrl);
+        this.thumbnailUrl = new LazyValue(this.prepareThumbnailUrl);
+        this.sourceBlob = new LazyValue(this.fetchSource);
+        this.thumbnailBlob = new LazyValue(this.fetchThumbnail);
+
+        this.media = mediaFromContent(this.event.getContent());
+    }
+
+    public destroy() {
+        if (this.media.isEncrypted) {
+            if (this.sourceUrl.present) URL.revokeObjectURL(this.sourceUrl.cachedValue);
+            if (this.thumbnailUrl.present) URL.revokeObjectURL(this.thumbnailUrl.cachedValue);
+        }
+    }
+
+    private prepareSourceUrl = async () => {
+        if (this.media.isEncrypted) {
+            const blob = await this.sourceBlob.value;
+            return URL.createObjectURL(blob);
+        } else {
+            return this.media.srcHttp;
+        }
+    };
+
+    private prepareThumbnailUrl = async () => {
+        if (this.media.isEncrypted) {
+            const blob = await this.thumbnailBlob.value;
+            return URL.createObjectURL(blob);
+        } else {
+            return this.media.thumbnailHttp;
+        }
+    };
+
+    private fetchSource = () => {
+        if (this.media.isEncrypted) {
+            return decryptFile(this.event.getContent<IMediaEventContent>().file);
+        }
+        return this.media.downloadSource().then(r => r.blob());
+    };
+
+    private fetchThumbnail = () => {
+        if (!this.media.hasThumbnail) return Promise.resolve(null);
+
+        if (this.media.isEncrypted) {
+            const content = this.event.getContent<IMediaEventContent>();
+            if (content.info?.thumbnail_file) {
+                return decryptFile(content.info.thumbnail_file);
+            } else {
+                // "Should never happen"
+                console.warn("Media claims to have thumbnail and is encrypted, but no thumbnail_file found");
+                return Promise.resolve(null);
+            }
+        }
+
+        return fetch(this.media.thumbnailHttp).then(r => r.blob());
+    };
+}