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+import { Observable } from '../Observable';
+import { ObservedValueOf, ObservableInput } from '../types';
+import { innerFrom } from './innerFrom';
+
+/**
+ * Creates an Observable that, on subscribe, calls an Observable factory to
+ * make an Observable for each new Observer.
+ *
+ * <span class="informal">Creates the Observable lazily, that is, only when it
+ * is subscribed.
+ * </span>
+ *
+ * ![](defer.png)
+ *
+ * `defer` allows you to create an Observable only when the Observer
+ * subscribes. It waits until an Observer subscribes to it, calls the given
+ * factory function to get an Observable -- where a factory function typically
+ * generates a new Observable -- and subscribes the Observer to this Observable.
+ * In case the factory function returns a falsy value, then EMPTY is used as
+ * Observable instead. Last but not least, an exception during the factory
+ * function call is transferred to the Observer by calling `error`.
+ *
+ * ## Example
+ *
+ * Subscribe to either an Observable of clicks or an Observable of interval, at random
+ *
+ * ```ts
+ * import { defer, fromEvent, interval } from 'rxjs';
+ *
+ * const clicksOrInterval = defer(() => {
+ *   return Math.random() > 0.5
+ *     ? fromEvent(document, 'click')
+ *     : interval(1000);
+ * });
+ * clicksOrInterval.subscribe(x => console.log(x));
+ *
+ * // Results in the following behavior:
+ * // If the result of Math.random() is greater than 0.5 it will listen
+ * // for clicks anywhere on the "document"; when document is clicked it
+ * // will log a MouseEvent object to the console. If the result is less
+ * // than 0.5 it will emit ascending numbers, one every second(1000ms).
+ * ```
+ *
+ * @see {@link Observable}
+ *
+ * @param observableFactory The Observable factory function to invoke for each
+ * Observer that subscribes to the output Observable. May also return any
+ * `ObservableInput`, which will be converted on the fly to an Observable.
+ * @return An Observable whose Observers' subscriptions trigger an invocation of the
+ * given Observable factory function.
+ */
+export function defer<R extends ObservableInput<any>>(observableFactory: () => R): Observable<ObservedValueOf<R>> {
+  return new Observable<ObservedValueOf<R>>((subscriber) => {
+    innerFrom(observableFactory()).subscribe(subscriber);
+  });
+}