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+import { Subscriber } from '../Subscriber';
+import { ObservableInput, OperatorFunction } from '../types';
+import { operate } from '../util/lift';
+import { noop } from '../util/noop';
+import { createOperatorSubscriber } from './OperatorSubscriber';
+import { innerFrom } from '../observable/innerFrom';
+
+/**
+ * Buffers the source Observable values, using a factory function of closing
+ * Observables to determine when to close, emit, and reset the buffer.
+ *
+ * <span class="informal">Collects values from the past as an array. When it
+ * starts collecting values, it calls a function that returns an Observable that
+ * tells when to close the buffer and restart collecting.</span>
+ *
+ * ![](bufferWhen.svg)
+ *
+ * Opens a buffer immediately, then closes the buffer when the observable
+ * returned by calling `closingSelector` function emits a value. When it closes
+ * the buffer, it immediately opens a new buffer and repeats the process.
+ *
+ * ## Example
+ *
+ * Emit an array of the last clicks every [1-5] random seconds
+ *
+ * ```ts
+ * import { fromEvent, bufferWhen, interval } from 'rxjs';
+ *
+ * const clicks = fromEvent(document, 'click');
+ * const buffered = clicks.pipe(
+ *   bufferWhen(() => interval(1000 + Math.random() * 4000))
+ * );
+ * buffered.subscribe(x => console.log(x));
+ * ```
+ *
+ * @see {@link buffer}
+ * @see {@link bufferCount}
+ * @see {@link bufferTime}
+ * @see {@link bufferToggle}
+ * @see {@link windowWhen}
+ *
+ * @param closingSelector A function that takes no arguments and returns an
+ * Observable that signals buffer closure.
+ * @return A function that returns an Observable of arrays of buffered values.
+ */
+export function bufferWhen<T>(closingSelector: () => ObservableInput<any>): OperatorFunction<T, T[]> {
+  return operate((source, subscriber) => {
+    // The buffer we keep and emit.
+    let buffer: T[] | null = null;
+    // A reference to the subscriber used to subscribe to
+    // the closing notifier. We need to hold this so we can
+    // end the subscription after the first notification.
+    let closingSubscriber: Subscriber<T> | null = null;
+
+    // Ends the previous closing notifier subscription, so it
+    // terminates after the first emission, then emits
+    // the current buffer  if there is one, starts a new buffer, and starts a
+    // new closing notifier.
+    const openBuffer = () => {
+      // Make sure to finalize the closing subscription, we only cared
+      // about one notification.
+      closingSubscriber?.unsubscribe();
+      // emit the buffer if we have one, and start a new buffer.
+      const b = buffer;
+      buffer = [];
+      b && subscriber.next(b);
+
+      // Get a new closing notifier and subscribe to it.
+      innerFrom(closingSelector()).subscribe((closingSubscriber = createOperatorSubscriber(subscriber, openBuffer, noop)));
+    };
+
+    // Start the first buffer.
+    openBuffer();
+
+    // Subscribe to our source.
+    source.subscribe(
+      createOperatorSubscriber(
+        subscriber,
+        // Add every new value to the current buffer.
+        (value) => buffer?.push(value),
+        // When we complete, emit the buffer if we have one,
+        // then complete the result.
+        () => {
+          buffer && subscriber.next(buffer);
+          subscriber.complete();
+        },
+        // Pass all errors through to consumer.
+        undefined,
+        // Release memory on finalization
+        () => (buffer = closingSubscriber = null!)
+      )
+    );
+  });
+}