Liskov: What Changes *Are* Allowed?
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Login SubscribeCalculating how long it takes for the parent score() method to execute will be easy. But then... what do we do with that number? This method returns a BigFootSightingScore instance.... so we can't suddenly change this to return an int for the duration. How can this method return both the BigFootSightingScore and info about how long it took for the score to calculate?
Creating a Subclass for the Return Value
The answer is: create another subclass! A subclass of BigFootSightingScore that holds the extra info. BigFootSightingScore lives in the src/Model/ directory: there it is. Right next to it, add a new class called, how about, DebuggableBigFootSightingScore. Make it extend the normal BigFootSightingScore.
| // ... lines 1 - 4 | |
| class DebuggableBigFootSightingScore extends BigFootSightingScore | |
| { | |
| } |
Now we have two subclasses to play with! This time, override the constructor: do that by going to Code -> Generate - or Command + N on a Mac. Override __construct().
This calls the parent constructor with the score, which is great! Add a new argument: float $calculationTime. I'll hit Alt + Enter and go to "Initialize properties"... select just $calculationTime... to create that property and set it. To make the $calculationTime accessible, at the bottom, go back to Code -> Generate and make a "getter" method for this!
| // ... lines 1 - 4 | |
| class DebuggableBigFootSightingScore extends BigFootSightingScore | |
| { | |
| private float $calculationTime; | |
| public function __construct(int $score, float $calculationTime) | |
| { | |
| parent::__construct($score); | |
| $this->calculationTime = $calculationTime; | |
| } | |
| public function getCalculationTime(): float | |
| { | |
| return $this->calculationTime; | |
| } | |
| } |
Wait: Does __construct need to Follow Liskov's Rules?
By the way, adding a required argument to a method that you are overriding - like we're doing in __construct - is normally another way to violate Liskov's principle. Let's think about it using a different example: SightingScorer. When we use this, we can normally call score() and pass it a single argument. If we suddenly substituted in a different class whose score() method required two arguments... well, that would make our code explode. That new class would not be substitutable for the old one.
However, the constructor does not need to follow Liskov's principle... which took me a minute to wrap my head around. Why not? Because if you are instantiating a DebuggableBigFootSightingScore - with new DebuggableBigFootSightingScore - then you know exactly which class you are instantiating. And so, you can figure out exactly which arguments you need to pass.
This is different than being passed a BigFootSightingScore object... where the true class might be a subclass. In that situation, you need any of the methods that you call on that object to behave like the original class's methods. Since the constructor is never called on an object, that's not an issue.
Anyways, back in DebuggableSightingScorer, let's return our new DebuggableBigFootSightingScore class with a dummy duration. Say $bfScore = parent::score()... and then return a new DebuggableBigFootSightingScore passing the int score - $bfScore->getScore() - and 100 for a fake duration. Let's also advertise that we return this new class: DebuggableBigFootSightingScore
| // ... lines 1 - 6 | |
| use App\Model\DebuggableBigFootSightingScore; | |
| // ... line 8 | |
| class DebuggableSightingScorer extends SightingScorer | |
| { | |
| public function score(BigFootSighting $sighting): DebuggableBigFootSightingScore | |
| { | |
| $bfsScore = parent::score($sighting); | |
| return new DebuggableBigFootSightingScore( | |
| $bfsScore->getScore(), | |
| 100 | |
| ); | |
| } | |
| } |
But wait: we just changed the return-type to something different than our parent class! Is that allowed?
Narrower Return Types are Allowed
Find your browser, refresh and... PHP totally does allow this! That's because this does follow Liskov's principle: we are making the return type more narrow... or more specific.
But why is making a return type more narrow allowed? Look at BigFootSightingController: the class that uses the SightingScorer. This code requires a SightingScorer instance. And so, when we call the score() method later, we know that it will return a BigFootSightingScore object. We know that because, if we jump into the SightingScorer class, yep! The score() method returns a BigFootSightingScore.
And so, we know the $bfsScore variable is an instance of BigFootSightingScore... and we know that that class has a getScore() method on it. I'll, once again, jump into the class. This is the original BigFootSightingScore and here is its getScore() method. We use that in our controller to get the integer score and... everything is happy!
But now we know that we have substituted the SightingScorer for a DebuggableSightingScorer... and we know that its score() method returns a DebuggableBigFootSightingScore. But that's okay! Why? Because DebuggableBigFootSightingScore extends BigFootSightingScore. So we are still returning a BigFootSightingScore instance, which, of course, still has a getScore() method. The fact that we return a subclass... that potentially has extra methods on it, does not break its substitutability.
But if we had changed its return type to something less specific, like any object, then there would be no guarantee that what we return from this method has a getScore() method. And so, that would break Liskov's principle. PHP would be so mad at us, that it would generate a syntax error. Let's undo that.
We won't talk about it in detail, but the same philosophy can be applied to argument types, but in the opposite direction. It's okay to change an argument type as long as you support at least the original type. It's not okay to be more restrictive with the type you allow, but it is okay to be less specific: I am allowed to say that the score() method supports any object. Well, in this example, that would be problematic because we're passing the argument to the parent class... which still does require a BigFootSighting... but in general, allowing for a less specific, or wider argument type is allowed by Liskov. And you can see this if we refresh: no syntax error from PHP.
Let's change that back.
Next: it's time to celebrate our new system by using the new duration value, tweaking a few things in Symfony's config and listing the takeaways from Liskov's principle.
