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We've just created a new branch
based off of Symfony's master branch.

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And now, we're ready to create the
amazing new TargetPathHelper class.

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But... where should it live?

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It's related to Security...

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which means it could live in the
Security component or SecurityBundle.

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As a general rule, most code
should live in a component

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so that it's reusable even
outside of the framework.

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But, sometimes, you'll write code that's
really integrated with the framework.

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That code will live in the bundle.

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My best advice...

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don't over-think it: it usually becomes pretty
obvious if you put something in the wrong spot.

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Press Shift+Shift and search for
a file that's closely related

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to our new feature: TargetPathTrait.

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Ok, this lives in the Security component.

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I'll double click on the
directory to move there.

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At first, it seems like TargetPathHelper
should live right here.

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And that's where I would put it at first.

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I say "at first" because, if you
started coding, you'd notice a problem.

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What problem?

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This new class will ultimately use the
FirewallMap class internally to do its work.

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There are two FirwallMap classes:
one lives in the Security component,

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and the other lives in SecurityBundle.

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After digging a little bit, you'd find out that
we will need to use the one from SecurityBundle.

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And here's why that's important:
a class in a component can depend

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on classes from other components.

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But, it can never depend
on a class from a bundle.

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Because our new TargetPathHelper
needs a class from SecurityBundle,

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it can't live in the component:
it must live in the bundle.

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If you get this wrong, no big deal: someone
will help you out on your pull request.

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Go find SecurityBundle and look inside Security.

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Hey! Here are the FirewallMap and
FirewallConfig classes we'll be using!

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That's a good sign!

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Create the new PHP class: TargetPathHelper.

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Add our first public function, how about
just savePath() with a string $uri argument:

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Symfony 4.0 and above requires PHP 7.1, so you
should use scalar type-hints and return types.

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But, Symfony does not use the void return type.

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Because this is a public function, we
should add some PHPDoc to describe it.

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Add a clear, but short description
above this: Actually,

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Symfony does not have a lot of PHPDoc...

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which might seem weird at first.

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The reason is that we don't want to maintain
too much documentation inside the code -

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we use a separate repository for documentation.

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Oh, and, thanks to the string type-hint,

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the @param documentation is totally
redundant and should be removed...

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unless there's some valuable extra
info that you want to say about it.

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I'll keep it and add some extra notes...

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even though it doesn't add a lot of
extra context: Also every PHP file

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in Symfony should have a
copyright header on top.

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Grab that from another file and paste it here:
Don't worry too much about these details:

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it's easy to add them later if you forget.

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To make life nicer, use TargetPathTrait
on top of the class:

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Then all we need to do is say
$this->saveTargetPath(): But...

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hmm... this needs 3 arguments:
the session, provider key -

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which is the firewall name - and the URI.

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We know that we can get the firewall
name by using the FirewallMap service.

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Let's add some constructor arguments:
SessionInterface $session and FirewallMap -

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the one from SecurityBundle - $firewallMap: I'll
press Alt+Enter and select initialize fields

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to create those properties and set them: Make
sure to remove the PHPDoc above each property:

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this is redundant thanks to
the constructor type-hints.

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To calculate the provider key,
create a new private function:

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getProviderKey() that will return a string.

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For now, just put a TODO: Back up in
setTargetPath(), pass $this->session,

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$this->getProviderKey() and the $uri: Awesome!

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Look back at getProviderKey():
I didn't add any PHPDoc

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to this function, but that's
not because I'm lazy.

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Or... not entirely because I'm lazy.

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Really, it's for two reasons.

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First, this is a private function, and those
are typically not documented inside Symfony.

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And second, we already have the return
type - no reason to duplicate it!

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Lets finish this function.

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To get the firewall name, we
need to use the FirewallMap,

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call getFirewallConfig()
and pass it the request.

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Ok: $firewallConfig =
$this->firewallMap->getFirewallConfig().

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But, hmm...

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we don't have the Request object!

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No problem: add a third constructor
arg: RequestStack $requestStack.

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I'll hit Alt+Enter again to
create that property and set it.

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Clean off the PHPDoc, then head back
down: Normally, when you use RequestStack,

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you call its getCurrentRequest()
method to get the request.

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But, in this case, I'm going to use another
method: $this->requestStack->getMasterRequest():

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I'm not 100% sure that this is correct.

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The whole topic of requests and
sub-requests is pretty complex.

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But, basically, Symfony's security firewall
only operates on the outer, "master" request.

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So, to find the active firewall,
that's what we should use.

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If I'm wrong, hopefully someone
will tell me on the pull request.

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Next, if you look at the
getFirewallConfig() method,

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it's possible that this will return null.

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Code for that: if null === $firewallConfig:

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This is another Symfony coding
convention: we use Yoda conditionals!

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Mysterious, Symfony's coding conventions are.

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Herh herh herh.

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But hey! If the force isn't strong with you
today, don't worry: Symfony has a magic way

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of fixing coding convention
problems that I'll show you later.

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If there is no firewall config for
some reason, throw a new LogicException

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with as clear a message as
possible: Why a LogicException?

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Well, it seems to make sense
- something went wrong...

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logically.

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And usually, the exact exception
class won't matter.

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If it does matter, someone will
tell you when reviewing your PR.

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Finally, at the bottom, return
$firewallConfig->getName(): That should be it!

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While we're here, let's add one more
function: getPath() that will return a string.

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Inside, return $this->getTargetPath() with
$this->session and $this->getProviderKey():

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This time, I will add some PHPDoc.

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I don't need @return - that's
redundant - but I will add a description

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about what this method does: And...

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we're done!

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Yea, we still need to write a test
& add some config to register this

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as a service: we'll do that next.

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But, this class should work!

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However...

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I am going to make one, ahem, final change:
add final: Making this class final means

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that nobody is allowed to subclass it.

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Why are we doing this?

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Because, in the future, if the class is
final, it will be easier to make changes to it

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without breaking backwards compatibility.

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Basically, if you allow a
class to be sub-classed,

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you have to be a bit more careful
when making certain changes.

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Making classes final is a good
"default" for new Symfony classes.

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Of course, if there is a legitimate
use-case for some to sub-class this,

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then you don't need to make it final.

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But, while we can easily remove final
later, we can't add final in the future,

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at least not without jumping through a few extra
hoops to avoid breaking backwards compatibility.

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Ok, let's add some service config & a test!