public function testObserverIsUpdated()
{
// Create a mock for the ProductPriceObserver class and only mock the update() method.
$productPriceObserver = $this->getMockBuilder(UsingSPL\ProductPriceObserver::class)->setMethods(array('update'))->getMock();
// Set up the expectation for the update() method (called only once
// and with an instance of SplSubject as its parameter).
$productPriceObserver->expects($this->once())->method('update')->with($this->isInstanceOf('SplSubject'));
// Create the Product, attach the ProductPriceObserver object and notify it.
$product = new UsingSPL\Product();
$product->attach($productPriceObserver);
$product->notify();
}
