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The Wonder of Class Constants

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Hey friends! I'm so glad you're here for part 4 of "Baking Delicious Chocolate Chip Cookies". Wait, they're telling me that's not right. Oh, ok, I'm so glad you're here for part 4 of our Object Oriented Programming series!

After the first 3 parts, you guys are already dangerous, so I'm impressed you're still showing up and aren't off coding something cool. You made the right choice: in this course we're going to really have fun with some of the coolest parts of OO, showing off features that we haven't mentioned yet. This is packed with the final pieces that will let you recognize all the different OO things that you see in other people's code. There's lots to get through, so let's go!

Get the Starting Code!

If you're serious about getting really good at this stuff, code along with me. To do that, download the source code from this page, unzip it, and move into the start directory. When you do that, you'll have the same code that I have here. Open up the README file and follow the instructions inside to get things setup.

When that's done, open your favorite terminal application, move into the directory, and - like we've done in the previous courses - start the built in php web server by running:

php -S localhost:8000

This is a great server to use for development. Then, in our browser, we can go to http://localhost:8000. Here is our beautiful Battles app!

New Feature! Battle Types

People have been clamoring for a new feature: a way to battle that forces Jedi powers to be used or completely avoided. Let's add this - it'll show off a new cool thing: class constants.

Open index.php and scroll down. Right after the ship select boxes, but before the submit button, I'll paste some HTML for a new select box:

138 lines index.php
... lines 1 - 29
<html>
... lines 31 - 55
<body>
<div class="container">
... lines 58 - 92
<div class="battle-box center-block border">
<div>
<form method="POST" action="/battle.php">
... lines 96 - 119
<div class="text-center">
<label for="battle_type">Battle Type</label>
<select name="battle_type" id="battle_type" class="form-control drp-dwn-width center-block">
<option value="normal">Normal</option>
<option value="no_jedi">No Jedi Powers</option>
<option value="only_jedi">Only Jedi Powers</option>
</select>
</div>
<br/>
<button class="btn btn-md btn-danger center-block" type="submit">Engage</button>
</form>
</div>
</div>
</div>
</body>
</html>

Let's refresh and see what it looks like. Ok, it's a new drop-down called "Battle Type" with option for "Normal", "No Jedi Powers" and "Only Jedi Powers". If you look at the code, this is a single select field that has a name of battle_type.

Here's the idea: each type will cause the BattleManager to battle these two ships in slightly different ways. Let's hook this up as simply as possible.

Since the field is named battle_type, open battle.php - the file that handles the submit. Right before calling the battle() method, create a new variable called $battleType set to $_POST['battle_type']. Then, pass $battleType as a new fifth argument to the battle() method:

110 lines battle.php
... lines 1 - 33
$battleType = $_POST['battle_type'];
$battleResult = $battleManager->battle($ship1, $ship1Quantity, $ship2, $ship2Quantity, $battleType);
... lines 36 - 110

Hooking up the Logic: No Magic Yet

Let's add that! Open BattleManager and find battle(). Give this a new fifth argument: $battleType:

76 lines lib/Service/BattleManager.php
... lines 1 - 2
class BattleManager
{
... lines 5 - 9
public function battle(AbstractShip $ship1, $ship1Quantity, AbstractShip $ship2, $ship2Quantity, $battleType)
{
... lines 12 - 66
}
... lines 68 - 74
}

Great! We know that this will be one of three special strings, either normal, no_jedi or only_jedi. We can use those to change the behavior.

First, the two blocks near the top should only be run if Jedi powers are being used. Add to the if statement: if $battleType != 'no_jedi', then we can run this. Copy that and add it to the second block:

76 lines lib/Service/BattleManager.php
... lines 1 - 2
class BattleManager
{
... lines 5 - 9
public function battle(AbstractShip $ship1, $ship1Quantity, AbstractShip $ship2, $ship2Quantity, $battleType)
{
... lines 12 - 17
while ($ship1Health > 0 && $ship2Health > 0) {
// first, see if we have a rare Jedi hero event!
if ($battleType != 'no_jedi' && $this->didJediDestroyShipUsingTheForce($ship1)) {
... lines 21 - 24
}
if ($battleType != 'no_jedi' && $this->didJediDestroyShipUsingTheForce($ship2)) {
... lines 27 - 30
}
... lines 32 - 44
}
... lines 46 - 66
}
... lines 68 - 74
}

Perfect! If the battle type is normal or only_jedi, these blocks will execute.

Next, the last two lines are when the two ships battle each other normally. If we're on only_jedi mode, this shouldn't happen. Surround them with an if statement: if ($battleType != 'only_jedi') then run these lines:

76 lines lib/Service/BattleManager.php
... lines 1 - 2
class BattleManager
{
... lines 5 - 9
public function battle(AbstractShip $ship1, $ship1Quantity, AbstractShip $ship2, $ship2Quantity, $battleType)
{
... lines 12 - 17
while ($ship1Health > 0 && $ship2Health > 0) {
... lines 19 - 32
// now battle them normally
if ($battleType != 'only_jedi') {
$ship1Health = $ship1Health - ($ship2->getWeaponPower() * $ship2Quantity);
$ship2Health = $ship2Health - ($ship1->getWeaponPower() * $ship1Quantity);
}
... lines 38 - 44
}
... lines 46 - 66
}
... lines 68 - 74
}

Awesome! Now, there's just one little last detail: if two ships are fighting in only_jedi mode, and both have zero Jedi powers, they'll get caught in this loop and fight forever! To prevent that, above the while, add a new $i = 0 variable:

76 lines lib/Service/BattleManager.php
... lines 1 - 2
class BattleManager
{
... lines 5 - 9
public function battle(AbstractShip $ship1, $ship1Quantity, AbstractShip $ship2, $ship2Quantity, $battleType)
{
... lines 12 - 16
$i = 0;
while ($ship1Health > 0 && $ship2Health > 0) {
... lines 19 - 44
}
... lines 46 - 66
}
... lines 68 - 74
}

Then, at the bottom, if $i = 100, we're probably stuck in a loop. Just set $ship1Health = 0; and $ship2Health = 0 and increment $i below that:

76 lines lib/Service/BattleManager.php
... lines 1 - 2
class BattleManager
{
... lines 5 - 9
public function battle(AbstractShip $ship1, $ship1Quantity, AbstractShip $ship2, $ship2Quantity, $battleType)
{
... lines 12 - 16
$i = 0;
while ($ship1Health > 0 && $ship2Health > 0) {
... lines 19 - 38
// prevent 2 non-jedi ships from fighting forever in only_jedi mode
if ($i == 100) {
$ship1Health = 0;
$ship2Health = 0;
}
$i++;
}
... lines 46 - 66
}
... lines 68 - 74
}

Done!

Give it a try!. Select one Jedi Starfighter, one CloakShape fighter, and choose "Only Jedi Powers". Hit engage and ... the Jedi Starfighter used its Jedi powers for a stunning victory! If we refresh, one of the ships will use its Jedi powers every single time.

Magic Strings Make Kittens Cry

Feature complete! And it was easy. So... what's the problem? Look at these strings: normal, no_jedi and only_jedi:

138 lines index.php
... lines 1 - 29
<html>
... lines 31 - 55
<body>
<div class="container">
... lines 58 - 92
<div class="battle-box center-block border">
<div>
<form method="POST" action="/battle.php">
... lines 96 - 119
<div class="text-center">
... line 121
<select name="battle_type" id="battle_type" class="form-control drp-dwn-width center-block">
<option value="normal">Normal</option>
<option value="no_jedi">No Jedi Powers</option>
<option value="only_jedi">Only Jedi Powers</option>
</select>
</div>
... lines 128 - 131
</form>
</div>
</div>
</div>
</body>
</html>

They're kind of magic. I mean, we chose them randomly and if you misspell one somewhere, you won't get an error, but things won't work right.

To make things worse, in BattleManager, when you see these strings, it's not clear what other strings might be possible. Are there other battle types we're forgetting to handle? And if we wanted to add or remove a battle type, what other files would we need to change? It's really common to have "magic strings" like these, but they can become hard to keep track of: you end up referencing these exact little strings in many places.

Class Constants to the Rescue

Of course, object-oriented code has an answer! It's called "class constants", and it works like this. Inside any class, you can use a special keyword called const followed by a word - which is usually in all uppercase - like TYPE_NORMAL and equals a value - normal. Repeat this for const TYPE_NO_JEDI = 'no_jedi' and const TYPE_ONLY_JEDI = 'only_jedi':

80 lines lib/Service/BattleManager.php
... lines 1 - 2
class BattleManager
{
const TYPE_NORMAL = 'normal';
const TYPE_NO_JEDI = 'no_jedi';
const TYPE_ONLY_JEDI = 'only_jedi';
... lines 8 - 78
}

Constants are like variables, except they can never be changed. You can call the constants anything - by adding TYPE_ before each one, it helps me remember what these are used for - battle types. You can also add these to any class. I choice BattleManager because these types are used here.

Using Class Constants

As soon as you do this, you can replace the random string with BattleManager::TYPE_NO_JEDI. Below that, use BattleManager::TYPE_ONLY_JEDI:

80 lines lib/Service/BattleManager.php
... lines 1 - 2
class BattleManager
{
... lines 5 - 13
public function battle(AbstractShip $ship1, $ship1Quantity, AbstractShip $ship2, $ship2Quantity, $battleType)
{
... lines 16 - 21
while ($ship1Health > 0 && $ship2Health > 0) {
... line 23
if ($battleType != BattleManager::TYPE_NO_JEDI && $this->didJediDestroyShipUsingTheForce($ship1)) {
... lines 25 - 28
}
if ($battleType != BattleManager::TYPE_NO_JEDI && $this->didJediDestroyShipUsingTheForce($ship2)) {
... lines 31 - 34
}
... lines 36 - 37
if ($battleType != BattleManager::TYPE_ONLY_JEDI) {
... lines 39 - 40
}
... lines 42 - 48
}
... lines 50 - 70
}
... lines 72 - 78
}

That will work the exact same way as before. In index.php, do the same thing: <?php echo BattleManager::TYPE_NORMAL. Copy that and replace it with TYPE_NO_JEDI and TYPE_ONLY_JEDI:

138 lines index.php
... lines 1 - 29
<html>
... lines 31 - 55
<body>
<div class="container">
... lines 58 - 92
<div class="battle-box center-block border">
<div>
<form method="POST" action="/battle.php">
... lines 96 - 119
<div class="text-center">
... line 121
<select name="battle_type" id="battle_type" class="form-control drp-dwn-width center-block">
<option value="<?php echo BattleManager::TYPE_NORMAL ?>">Normal</option>
<option value="<?php echo BattleManager::TYPE_NO_JEDI ?>">No Jedi Powers</option>
<option value="<?php echo BattleManager::TYPE_ONLY_JEDI ?>">Only Jedi Powers</option>
</select>
</div>
... lines 128 - 131
</form>
</div>
</div>
</div>
</body>
</html>

To prove it still works, refresh this page. Everything's still happy!

In a sense, nothing changed! But now, these magic strings have a single home: at the top of BattleManager. If we ever needed to change these strings, we can do it in just once place.

This also gives these strings some context - these are obviously related to BattleManager, and we can probably look here to see how they're used. We can also document what they mean by adding some details above each type:

83 lines lib/Service/BattleManager.php
... lines 1 - 2
class BattleManager
{
// normal battle mode
const TYPE_NORMAL = 'normal';
// don't allow jedi powers
const TYPE_NO_JEDI = 'no_jedi';
// you can *only* win with jedi powers
const TYPE_ONLY_JEDI = 'only_jedi';
... lines 11 - 81
}

Now, check this out. When some other developer looks inside index.php, instead of seeing some magic, meaningless strings like before, they'll see these constants and think:

Oh, BattleManager::TYPE_NORMAL. Let me go look in that class to see what this means. Oh hey, there's even some documentation!

So anytime you have a special string or other value that has some special meaning but will never change, make it a constant and stay happy.

Leave a comment!

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Jeffrey C. Avatar
Jeffrey C. Avatar Jeffrey C. | posted 4 years ago

Hi!

Just wondering do i need to download the course code or can i use the code from OO course 3 as we have left. Or are there things in this course code that i need to have in order to follow this code.

Cheers!

Reply
Victor Avatar

Hey Emin,

We recommend to download course code for each tutorial and work in start/ directory. The reason behind it is that most of the time we do minor changes and improvements after previous courses, like upgrading dependencies to the new latest versions, add more code examples, slightly tweak or add new code. Most of them are minor, but if you want to exactly match the video - better to start with new downloaded code. But it's not a requirement! You can still continue your project applying new changes to it. You even can commit with Git all your changes you have before start the new course, then download new course code, and copy the code from the start/ directory to your project and with "git diff" command you'll review any changes that were made by us. So, you can either leave or discard them. This is always a good strategy to continue your project but keep up to date with our changes at the same time.

I hope this helps

Cheers!

Reply
Max S. Avatar

Hey!
In the exercise I just tried $this::CONST instead of ClassName::CONST within the class method. It seemed to work, but is it a sensible thing to do? Thank you!

Reply
weaverryan Avatar

Yo Max!

You found the flexibility of PHP! This does work... but I wouldn't do it on purpose :) - mostly because it is kind of "unexpected" (like, wait, why is he/she doing that? Is something fancy happening here!?). The key thing is that constants are *static* - they're a "thing" that's attached to the *class* (e.g. ClassName) itself - it is not something that's attached to your object. When you use $this::CONST (you can also have $someObj::CONST) it makes it kind of look like the constant is somehow attached to the object instead of the class.

But, I like that you're poking at things to see if they work - all good for learning :).

Cheers!

Reply
boykodev Avatar

I can just add that there are two additional ways to get the value of a constant inside of a class:
- self::CONST
- static::CONST

PHP is indeed very flexible! :)

Reply
Victor Avatar

Hey Serge,

Yeah, thanks for mentioning it! Late static binding indeed could be useful in some cases, see docs: http://php.net/manual/en/la...

Cheers!

Reply
Max S. Avatar

Thought so... The image of attaching a constant to a class rather than to an instance of that class makes definitely sense...

Reply
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