Add registry for background promises

src/lib/settle.ts

@@ -0,0 +1,90 @@
+/**
+ * A registry for tracking background work and reacting once all work has
+ * settled.
+ */
+export class PromiseRegistry {
+  outstanding: Array<TrackedPromise<unknown>>
+  settleCallbacks: Array<() => void>
+
+  constructor() {
+    this.outstanding = []
+    this.settleCallbacks = []
+  }
+
+  track<T>(promise: Promise<T>, onSettle?: () => void) {
+    // Since built-in Promises don't have a way to synchronously check if
+    // they're settled, it cannot start out settled.
+    this.outstanding.push(
+      new TrackedPromise(promise, this.attemptCleanUp.bind(this))
+    )
+    if (onSettle) {
+      this.settleCallbacks.push(onSettle)
+    }
+  }
+
+  /**
+   * Returns a promise that resolves when all promises have settled.
+   */
+  waitForSettle(): Promise<void> {
+    return new Promise((resolve) => {
+      this.addSettleCallback(resolve)
+    })
+  }
+
+  /**
+   * Add a callback to be called when all promises have settled.
+   *
+   * @see waitForSettle for a Promise-based interface.
+   */
+  addSettleCallback(onSettle: () => void) {
+    if (this.isSettled()) {
+      // Already settled, so schedule the callback.
+      setTimeout(onSettle, 0)
+    } else {
+      this.settleCallbacks.push(onSettle)
+    }
+  }
+
+  isSettled(): boolean {
+    return this.outstanding.every((p) => p.settled)
+  }
+
+  private attemptCleanUp() {
+    if (this.outstanding.length === 0) {
+      return
+    }
+    // Garbage collect.
+    const unsettled = this.outstanding.filter((p) => !p.settled)
+    this.outstanding = unsettled
+    // Transition to settled.  We could move this into TrackedPromise.
+    // It's a trade-off between reducing latency and reducing overhead.
+    if (unsettled.length === 0) {
+      for (const cb of this.settleCallbacks) {
+        cb()
+      }
+      this.settleCallbacks = []
+    }
+  }
+}
+
+/**
+ * Native Promises don't have a way to synchronously detect if they're settled.
+ */
+class TrackedPromise<T> {
+  settled: boolean
+  inner: Promise<T>
+
+  constructor(promise: Promise<T>, onSettle: () => void) {
+    this.settled = false
+    this.inner = promise.finally(() => {
+      this.settled = true
+      // TODO: debounce?
+      setTimeout(onSettle, 0)
+    })
+  }
+}
+
+/**
+ * Singleton regsitry for the whole app.
+ */
+export const AppPromises = new PromiseRegistry()