1 files changed, 64 insertions, 0 deletions

diff --git a/subprojects/frontend/src/utils/ConditionVariable.ts b/subprojects/frontend/src/utils/ConditionVariable.ts
new file mode 100644
index 00000000..0910dfa6
--- /dev/null
+++ b/subprojects/frontend/src/utils/ConditionVariable.ts
@@ -0,0 +1,64 @@
+import { getLogger } from './logger';
+import { PendingTask } from './PendingTask';
+
+const log = getLogger('utils.ConditionVariable');
+
+export type Condition = () => boolean;
+
+export class ConditionVariable {
+  condition: Condition;
+
+  defaultTimeout: number;
+
+  listeners: PendingTask<void>[] = [];
+
+  constructor(condition: Condition, defaultTimeout = 0) {
+    this.condition = condition;
+    this.defaultTimeout = defaultTimeout;
+  }
+
+  async waitFor(timeoutMs: number | null = null): Promise<void> {
+    if (this.condition()) {
+      return;
+    }
+    const timeoutOrDefault = timeoutMs || this.defaultTimeout;
+    let nowMs = Date.now();
+    const endMs = nowMs + timeoutOrDefault;
+    while (!this.condition() && nowMs < endMs) {
+      const remainingMs = endMs - nowMs;
+      const promise = new Promise<void>((resolve, reject) => {
+        if (this.condition()) {
+          resolve();
+          return;
+        }
+        const task = new PendingTask(resolve, reject, remainingMs);
+        this.listeners.push(task);
+      });
+      // We must keep waiting until the update has completed,
+      // so the tasks can't be started in parallel.
+      // eslint-disable-next-line no-await-in-loop
+      await promise;
+      nowMs = Date.now();
+    }
+    if (!this.condition()) {
+      log.error('Condition still does not hold after', timeoutOrDefault, 'ms');
+      throw new Error('Failed to wait for condition');
+    }
+  }
+
+  notifyAll(): void {
+    this.clearListenersWith((listener) => listener.resolve());
+  }
+
+  rejectAll(error: unknown): void {
+    this.clearListenersWith((listener) => listener.reject(error));
+  }
+
+  private clearListenersWith(callback: (listener: PendingTask<void>) => void) {
+    // Copy `listeners` so that we don't get into a race condition
+    // if one of the listeners adds another listener.
+    const { listeners } = this;
+    this.listeners = [];
+    listeners.forEach(callback);
+  }
+}