Added singleInstance operator

demo/share/singleInstance.php

@@ -0,0 +1,24 @@
+<?php
+
+require_once __DIR__ . '/../bootstrap.php';
+
+$interval = Rx\Observable::interval(1000);
+
+$source = $interval
+    ->take(2)
+    ->do(function () {
+        echo 'Side effect', PHP_EOL;
+    });
+
+$single = $source->singleInstance();
+
+// two simultaneous subscriptions, lasting 2 seconds
+$single->subscribe($createStdoutObserver('SourceA '));
+$single->subscribe($createStdoutObserver('SourceB '));
+
+\Rx\Observable::timer(5000)->subscribe(function () use ($single, &$createStdoutObserver) {
+    // resubscribe two times again, more than 5 seconds later,
+    // long after the original two subscriptions have ended
+    $single->subscribe($createStdoutObserver('SourceC '));
+    $single->subscribe($createStdoutObserver('SourceD '));
+});

demo/share/singleInstance.php.expect

@@ -0,0 +1,16 @@
+Side effect
+SourceA Next value: 0
+SourceB Next value: 0
+Side effect
+SourceA Next value: 1
+SourceB Next value: 1
+SourceA Complete!
+SourceB Complete!
+Side effect
+SourceC Next value: 0
+SourceD Next value: 0
+Side effect
+SourceC Next value: 1
+SourceD Next value: 1
+SourceC Complete!
+SourceD Complete!

src/Observable.php

@@ -11,7 +11,6 @@
 use Rx\Observable\ConnectableObservable;
 use Rx\Observable\EmptyObservable;
 use Rx\Observable\ErrorObservable;
-use Rx\Observable\FromPromiseObservable;
 use Rx\Observable\ForkJoinObservable;
 use Rx\Observable\IntervalObservable;
 use Rx\Observable\IteratorObservable;
@@ -1312,6 +1311,43 @@ public function share(): RefCountObservable
         return $this->publish()->refCount();
     }
 
+    /**
+     * Returns an observable sequence that shares a single subscription to the underlying sequence. This observable sequence
+     * can be resubscribed to, even if all prior subscriptions have ended.
+     *
+     * This operator behaves like share() in RxJS 5
+     *
+     * @return \Rx\Observable An observable sequence that contains the elements of a sequence
+     * produced by multicasting the source sequence.
+     *
+     * @demo share/singleInstance.php
+     * @operator
+     * @reactivex refcount
+     */
+    public function singleInstance(): Observable
+    {
+        $hasObservable = false;
+        $observable = null;
+        $source = $this;
+
+        $getObservable = function () use (&$hasObservable, &$observable, $source): Observable {
+            if (!$hasObservable) {
+                $hasObservable = true;
+                $observable = $source
+                    ->finally(function () use (&$hasObservable) {
+                        $hasObservable = false;
+                    })
+                    ->publish()
+                    ->refCount();
+            }
+            return $observable;
+        };
+
+        return new Observable\AnonymousObservable(function (ObserverInterface $o) use ($getObservable) {
+            return $getObservable()->subscribe($o);
+        });
+    }
+
     /**
      * Returns an observable sequence that shares a single subscription to the underlying sequence and starts with an
      * initialValue.

test/Rx/Functional/Operator/SingleInstanceTest.php

@@ -0,0 +1,120 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Rx\Functional\Operator;
+
+use Rx\Disposable\CompositeDisposable;
+use Rx\Disposable\SerialDisposable;
+use Rx\Functional\FunctionalTestCase;
+
+class SingleInstanceTest extends FunctionalTestCase
+{
+    /**
+     * @test
+     */
+    public function singleInstance_basic()
+    {
+        $xs = $this->createColdObservable([
+            onNext(100, 1),
+            onNext(150, 2),
+            onNext(200, 3),
+            onCompleted(250)
+        ]);
+
+        $ys = null;
+        $results1 = $this->scheduler->createObserver();
+        $results2 = $this->scheduler->createObserver();
+        $disposable = null;
+
+        $this->scheduler->scheduleAbsolute($this->scheduler::CREATED, function () use (&$ys, $xs) {
+            $ys = $xs->singleInstance();
+        });
+
+        $this->scheduler->scheduleAbsolute($this->scheduler::SUBSCRIBED, function () use (&$ys, &$disposable, $results1, $results2) {
+            $disposable = new CompositeDisposable([
+                $ys->subscribe($results1),
+                $ys->subscribe($results2)
+            ]);
+        });
+
+        $this->scheduler->scheduleAbsolute($this->scheduler::DISPOSED, function () use (&$disposable) {
+            $disposable->dispose();
+        });
+
+        $this->scheduler->start();
+
+        $this->assertMessages([
+            onNext(300, 1),
+            onNext(350, 2),
+            onNext(400, 3),
+            onCompleted(450)
+        ], $results1->getMessages());
+
+        $this->assertMessages([
+            onNext(300, 1),
+            onNext(350, 2),
+            onNext(400, 3),
+            onCompleted(450)
+        ], $results2->getMessages());
+
+        $this->assertSubscriptions([
+            subscribe(200, 450)
+        ], $xs->getSubscriptions());
+    }
+
+    /**
+     * @test
+     */
+    public function singleInstance_subscribe_after_stopped()
+    {
+        $xs = $this->createColdObservable([
+            onNext(100, 1),
+            onNext(150, 2),
+            onNext(200, 3),
+            onCompleted(250)
+        ]);
+
+        $ys = null;
+        $results1 = $this->scheduler->createObserver();
+        $results2 = $this->scheduler->createObserver();
+        $disposable = new SerialDisposable();
+
+        $this->scheduler->scheduleAbsolute(100, function () use (&$ys, $xs) {
+            $ys = $xs->singleInstance();
+        });
+
+        $this->scheduler->scheduleAbsolute(200, function () use (&$ys, $disposable, $results1) {
+            $disposable->setDisposable($ys->subscribe($results1));
+        });
+
+        $this->scheduler->scheduleAbsolute(600, function () use (&$ys, $disposable, $results2) {
+            $disposable->setDisposable($ys->subscribe($results2));
+        });
+
+        $this->scheduler->scheduleAbsolute(900, function () use (&$disposable) {
+            $disposable->dispose();
+        });
+
+        $this->scheduler->start();
+
+        $this->assertMessages([
+            onNext(300, 1),
+            onNext(350, 2),
+            onNext(400, 3),
+            onCompleted(450)
+        ], $results1->getMessages());
+
+        $this->assertMessages([
+            onNext(700, 1),
+            onNext(750, 2),
+            onNext(800, 3),
+            onCompleted(850)
+        ], $results2->getMessages());
+
+        $this->assertSubscriptions([
+            subscribe(200, 450),
+            subscribe(600, 850)
+        ], $xs->getSubscriptions());
+    }
+}