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+import { innerFrom } from '../observable/innerFrom';
+import { MonoTypeOperatorFunction, ObservableInput } from '../types';
+import { operate } from '../util/lift';
+import { noop } from '../util/noop';
+import { createOperatorSubscriber } from './OperatorSubscriber';
+
+/**
+ * Emits the most recently emitted value from the source Observable whenever
+ * another Observable, the `notifier`, emits.
+ *
+ * <span class="informal">It's like {@link sampleTime}, but samples whenever
+ * the `notifier` `ObservableInput` emits something.</span>
+ *
+ * ![](sample.png)
+ *
+ * Whenever the `notifier` `ObservableInput` emits a value, `sample`
+ * looks at the source Observable and emits whichever value it has most recently
+ * emitted since the previous sampling, unless the source has not emitted
+ * anything since the previous sampling. The `notifier` is subscribed to as soon
+ * as the output Observable is subscribed.
+ *
+ * ## Example
+ *
+ * On every click, sample the most recent `seconds` timer
+ *
+ * ```ts
+ * import { fromEvent, interval, sample } from 'rxjs';
+ *
+ * const seconds = interval(1000);
+ * const clicks = fromEvent(document, 'click');
+ * const result = seconds.pipe(sample(clicks));
+ *
+ * result.subscribe(x => console.log(x));
+ * ```
+ *
+ * @see {@link audit}
+ * @see {@link debounce}
+ * @see {@link sampleTime}
+ * @see {@link throttle}
+ *
+ * @param notifier The `ObservableInput` to use for sampling the
+ * source Observable.
+ * @return A function that returns an Observable that emits the results of
+ * sampling the values emitted by the source Observable whenever the notifier
+ * Observable emits value or completes.
+ */
+export function sample<T>(notifier: ObservableInput<any>): MonoTypeOperatorFunction<T> {
+  return operate((source, subscriber) => {
+    let hasValue = false;
+    let lastValue: T | null = null;
+    source.subscribe(
+      createOperatorSubscriber(subscriber, (value) => {
+        hasValue = true;
+        lastValue = value;
+      })
+    );
+    innerFrom(notifier).subscribe(
+      createOperatorSubscriber(
+        subscriber,
+        () => {
+          if (hasValue) {
+            hasValue = false;
+            const value = lastValue!;
+            lastValue = null;
+            subscriber.next(value);
+          }
+        },
+        noop
+      )
+    );
+  });
+}