如何对一个调度事件的方法进行单元测试，该事件会更改被测方法的行为？

Question

I have a custom event that has a public property:

class MyCustomEvent
{
    public $allowAction = false;
}

I have a class that creates this event, and dispatches an event with the event object, allowing event listeners/subscribers to change the property on the object.

class MyBizLogic
{
    private $dispatcher;

    public function __construct(EventDispatcher $dispatcher)
    {
        $this->dispatcher = $dispatcher;
    }

    public function doSomething()
    {
        $event = new MyCustomEvent();
        $dispatcher = $this->dispatcher->dispatch('my_custom_event', $event);
        if ($event->allowAction) {
            // do action
        } else {
            // do something else
        }
    }
}

How can I unit test doSomething ? I need a way to control the properties of the event object, but the event object is not a dependency I can mock. It's created within the method I'm testing.

I don't think this is a design smell, since this is how imagine most developers dispatch events. What can I do here to properly test the different outcomes that doSomething should deal with?

Answer 1

Answer: Just create a configurable listener that you will use for the tests, and make it behave the way you want in each case.

---

don't do that! The design smell here is that event receivers should not be able to alter the logic of the method that emits the event. What if there were 2 listeners? One of them can set a value and the other set another value? The last one will win and the first listener does not know that.

Events are a way of notifying another objects without the emitting entity knowing who will listen (maybe no other object). The emitter should work the same way regarding how many listeners there are. If you need to have some other object taking control over some aspect of the logic, do it explicitly (if in doubt, write another question and we'll try to help)

Answer 2

You aren't able to control the properties of the event object because you are creating the object within your function.

There are a couple of ways to get around this.

1) Have your doSomething method take a MyCustomEvent object as its argument. Then you would be able to pass in a mock object and control it that way.

2) Don't create the event in doSomething instead have the dispatcher return a MyCustomEvent with the properties that you want. So in your test, you would have a mockDispatcher that would return the event object from the dispatch method.

3) Pass in an event factory object that you can use to get instances of the proper event. Then you can mock this and have it return a mock event object for you.

4) You can use a callback function for the dispatch method for the event dispatcher. Your function can then set the MyCustomEvent::$allowAction property to what ever you want.

$allowAction = 'foo';
$mockEventDispatcher->expects($this->once())
    ->method('dispatch')
    ->with('my_custom_event', $this->isInstanceOf('MyCustomEvent')
    ->will($this->returnCallback(function($string, $event) use ($allowAction) {
         $event->allowAction = $allowAction
         // Return whatever the dispatcher is supposed to return.
       }));

IMO, the last two options have the test smell of mock objects returning mock objects which isn't ideal. But depending on the surrounding architecture, it may be the direction that you have to go.

Creating objects for use in a method is always a code smell and makes testing very difficult. Most event handling methods take the event as the argument.