1 files changed, 52 insertions, 0 deletions
diff --git a/packages/backend/src/misc/loader.ts b/packages/backend/src/misc/loader.ts
new file mode 100644
index 0000000000..25f7b54d31
--- /dev/null
+++ b/packages/backend/src/misc/loader.ts
@@ -0,0 +1,52 @@
+export type FetchFunction<K, V> = (key: K) => Promise<V>;
+
+type ResolveReject<V> = Parameters<ConstructorParameters<typeof Promise<V>>[0]>;
+
+type ResolverPair<V> = {
+	resolve: ResolveReject<V>[0];
+	reject: ResolveReject<V>[1];
+};
+
+export class DebounceLoader<K, V> {
+	private resolverMap = new Map<K, ResolverPair<V>>();
+	private promiseMap = new Map<K, Promise<V>>();
+	private resolvedPromise = Promise.resolve();
+	constructor(private loadFn: FetchFunction<K, V>) {}
+
+	public load(key: K): Promise<V> {
+		const promise = this.promiseMap.get(key);
+		if (typeof promise !== 'undefined') {
+			return promise;
+		}
+
+		const isFirst = this.promiseMap.size === 0;
+		const newPromise = new Promise<V>((resolve, reject) => {
+			this.resolverMap.set(key, { resolve, reject });
+		});
+		this.promiseMap.set(key, newPromise);
+
+		if (isFirst) {
+			this.enqueueDebouncedLoadJob();
+		}
+
+		return newPromise;
+	}
+
+	private runDebouncedLoad(): void {
+		const resolvers = [...this.resolverMap];
+		this.resolverMap.clear();
+		this.promiseMap.clear();
+
+		for (const [key, { resolve, reject }] of resolvers) {
+			this.loadFn(key).then(resolve, reject);
+		}
+	}
+
+	private enqueueDebouncedLoadJob(): void {
+		this.resolvedPromise.then(() => {
+			process.nextTick(() => {
+				this.runDebouncedLoad();
+			});
+		});
+	}
+}