yield example

    public function testGetFromIterator()
    {
        $messages = [
            (object) ['id' => 'first'],
            (object) ['id' => 'second'],
        ];
        $generator = $this->createMock(MessageGeneratorInterface::class);
        $generator->method('getMessages')->willReturnCallback(function D) use ($messages): \Generator {
            $trigger = $this->createMock(TriggerInterface::class);
            $triggerAt = new \DateTimeImmutable('2020-02-20T02:00:00', new \DateTimeZone('UTC'));
            yield (new MessageContext('default', 'id1', $trigger, $triggerAt)) => $messages[0];
            yield (new MessageContext('default', 'id2', $trigger, $triggerAt)) => $messages[1];
        });
        $transport = new SchedulerTransport($generator);

        foreach ($transport->get() as $i => $envelope) {
            $this->assertInstanceOf(Envelope::class, $envelope);
            $this->assertNotNull($stamp = $envelope->last(ScheduledStamp::class));
            $this->assertSame(array_shift($messages), $envelope->getMessage());
            $this->assertSame('default', $stamp->messageContext->name);
            $this->assertSame('id'.$i + 1, $stamp->messageContext->id);
        }