[7.x] Add when() method to InteractsWithInput trait

[pascalbaljet]

This PR adds a when method to the InteractsWithInput trait. It's inspired by the when method of the Collection class. The method checks if the request contains the given input item key, and then calls the callback with the input value.

$request->when('city', function($city) {
    //
});

The method returns the Request instance itself so you can easily chain it. One use-case is to apply scopes and constraints to a Query Builder or a similar repository:

$request->when('city', function ($city) {
    $this->feed->city($city);
})->when('minimum_price', function ($minimumPrice) {
    $this->feed->minimumPrice($minimumPrice);
})->when('maximum_price', function ($maximumPrice) {
    $this->feed->maximumPrice($maximumPrice);
});

This is even prettier with the arrow functions introduced in PHP 7.4 😃

$request->instance()
    ->when('city', fn ($city) => $feed->city($city))
    ->when('minimum_price', fn ($minimumPrice) => $feed->minimumPrice($minimumPrice))
    ->when('maximum_price', fn ($maximumPrice) => $feed->maximumPrice($maximumPrice));

[rodrigopedra]

Like the idea, but wonder if it would better to name the method ->whenHas() instead of when() to make the intent clearer.

For instance ->when() in a collection and in the query builder expect a truthy value as their first parameter.

->whenHas() would resemble the query builder's ->whereHas() which expects a name of a relation.

Just my two cents, overall I can see a lot of simplification on my code base if this PR gets merged.

[derekmd]

Triggering the callback when Request::has() is true instead of Request::filled(), the docs will have to be clear this considers the parameter existing on the payload but a value may not be present. The framework's other when() methods (Collection::when(), query Builder::when(), JsonResource@when(), etc.) all consider "truthiness" of the passed value when choosing to invoke the callback.

Some examples to consider:

1. GET query parameters ?is_published=&foo=bar

    request()->when('is_published', fn () => doSomePublishAction())

   triggers that function when the parameter's value is null (assuming ConvertEmptyStringsToNull middleware is enabled.)

2. POST <input type="text" name="title" value="">

    request()->when('title', fn () => saveOldTitle())

   Leaving the field empty also triggers the arrow function on a null 'title' value.

However still using Request::has() I can see this being useful for implementing RESTful PATCH endpoints that handle partial model updates. e.g., that may clear a field so you would be sending null values to the database.

[pascalbaljet]

Maybe we could introduce two new methods: whenHas() and whenFilled().

[taylorotwell]

Yeah I think we would maybe want two methods here for completeness.

[pascalbaljet]

I've updated the PR with a whenHas() and whenFilled() method.