feat(packages/excalidraw): state tracking, api hook, and others (#10870)

packages/common/src/versionedSnapshotStore.ts

@@ -0,0 +1,70 @@
+export type VersionedSnapshot<T> = Readonly<{
+  version: number;
+  value: T;
+}>;
+
+export class VersionedSnapshotStore<T> {
+  private version = 0;
+  private value: T;
+  private readonly waiters = new Set<
+    (snapshot: VersionedSnapshot<T>) => void
+  >();
+  private readonly subscribers = new Set<
+    (snapshot: VersionedSnapshot<T>) => void
+  >();
+
+  constructor(
+    initialValue: T,
+    private readonly isEqual: (prev: T, next: T) => boolean = Object.is,
+  ) {
+    this.value = initialValue;
+  }
+
+  public getSnapshot(): VersionedSnapshot<T> {
+    return { version: this.version, value: this.value };
+  }
+
+  public set(nextValue: T): boolean {
+    if (this.isEqual(this.value, nextValue)) {
+      return false;
+    }
+
+    this.value = nextValue;
+    this.version += 1;
+
+    const snapshot = this.getSnapshot();
+
+    for (const subscriber of this.subscribers) {
+      subscriber(snapshot);
+    }
+    for (const waiter of this.waiters) {
+      waiter(snapshot);
+    }
+    this.waiters.clear();
+
+    return true;
+  }
+
+  public update(updater: (prev: T) => T): boolean {
+    return this.set(updater(this.value));
+  }
+
+  public subscribe(
+    subscriber: (snapshot: VersionedSnapshot<T>) => void,
+  ): () => void {
+    this.subscribers.add(subscriber);
+    return () => {
+      this.subscribers.delete(subscriber);
+    };
+  }
+
+  public pull(sinceVersion = -1): Promise<VersionedSnapshot<T>> {
+    if (this.version !== sinceVersion) {
+      return Promise.resolve(this.getSnapshot());
+    }
+
+    return new Promise((resolve) => {
+      this.waiters.add(resolve);
+    });
+  }
+}