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04.

Optional type-hinting & Semantic Methods

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I need to show you something - so start another battle between some Jedi Star Fighters. It works... but if I refresh enough times... come on... yes! It blows up!

Argument 2 passed to BattleResult::__construct() must be an instance
of Ship, null given.

In BattleResult - because we're good programmers - we type-hinted the two Ship arguments. Buuuuut, if you look at the battle() function, there's a case where the ships can destroy each other. And when that happens, there is no winning or losing ship - they're both null. Since - news flash null is not a Ship object, PHP gets angry and casts down this big error.

When you type-hint an argument, the value must be that class - not even null is ok. But sometimes you do have a spot where an argument might be a specific object, or it might be null. To support this, make the argument optional - add an = null after it:

55 lines | lib/BattleResult.php
// ... lines 1 - 2
class BattleResult
{
// ... lines 5 - 13
public function __construct($usedJediPowers, Ship $winningShip = null, Ship $losingShip = null)
{
$this->usedJediPowers = $usedJediPowers;
$this->winningShip = $winningShip;
$this->losingShip = $losingShip;
}
// ... lines 20 - 53
}

I don't have to, but I'll update @return on the methods to be Ship|null:

55 lines | lib/BattleResult.php
// ... lines 1 - 2
class BattleResult
{
// ... lines 5 - 36
/**
* @return Ship|null
*/
public function getLosingShip()
{
return $this->losingShip;
}
/**
* Was there a winner? Or did everybody die :(
*
* @return bool
*/
public function isThereAWinner()
{
return $this->getWinningShip() !== null;
}
}

PhpStorm will still give me auto-completion - but this is a signal to other developers not to blindly call this method and always assume it will return a Ship object. We're already coding safely in battle.php: we check to make sure getWinningShip() returns something before calling a method on it. Cool.

Adding a Semantic isThereAWinner Method

To check if a BattleResult has a winner, you can see if getWinningShip() returns null. But we can do even better. Go to BattleResult and make a new public method called isThereAWinner(). Here, return $this->getWinningShip != null:

55 lines | lib/BattleResult.php
// ... lines 1 - 2
class BattleResult
{
// ... lines 5 - 43
/**
* Was there a winner? Or did everybody die :(
*
* @return bool
*/
public function isThereAWinner()
{
return $this->getWinningShip() !== null;
}
}

There's at least two great things about this. First, code outside of this class doesn't need to know how to figure out whether or not there was a winner: that code can be dumb and just call this method. Second, if something happens in the future and the logic used to figure out if there is a winner changes, we only need to update the code in this one spot: no need to run around the code base trying to figure out where we have the old logic for seeing if there was a winner.

Update battle.php to use this. The first if statement is really trying to figure out whether or not there was a winner. Update this to $battleResult->isThereAWinner(). Use that again right below:

99 lines | battle.php
// ... lines 1 - 70
<?php if ($battleResult->isThereAWinner()): ?>
// ... line 72
<?php else: ?>
// ... line 74
<?php endif; ?>
// ... lines 76 - 77
<?php if (!$battleResult->isThereAWinner()): ?>
// ... line 79
<?php else: ?>
// ... lines 81 - 86
<?php endif; ?>
// ... lines 88 - 99

Go back and refresh! You'll have to trust me that if we refresh this 1000 times, it'll always work - our bug is gone - and we have a nifty new helper method in BattleResult.