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Computed values

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  1. PHP 8.4 virtual properties

Earlier we saw how default values can be set for a data object, sometimes you want to set a default value based on other properties. For example, you might want to set a full_name property based on a first_name and last_name property. You can do this by using a computed property:

use Spatie\LaravelData\Attributes\Computed;

class SongData extends Data
{
    #[Computed]
    public string $full_name;

    public function __construct(
        public string $first_name,
        public string $last_name,
    ) {
        $this->full_name = "{$this->first_name} {$this->last_name}";
    }
}

You can now do the following:

SongData::from(['first_name' => 'Ruben', 'last_name' => 'Van Assche']);

Please notice: the computed property won't be reevaluated when its dependencies change. If you want to update a computed property, you'll have to create a new object.

Again there are a few conditions for this approach:

  • You must always use a sole property, a property within the constructor definition won't work
  • Computed properties cannot be defined in the payload, a CannotSetComputedValue will be thrown if this is the case
  • If the ignore_exception_when_trying_to_set_computed_property_value configuration option is set to true, the computed property will be silently ignored when trying to set it in the payload and no CannotSetComputedValue exception will be thrown.

If you're using PHP 8.4 or higher, you can use property hooks to define virtual properties. These are automatically detected as computed properties without needing the #[Computed] attribute:

class SongData extends Data
{
    public string $full_name {
        get => "{$this->first_name} {$this->last_name}";
    }

    public function __construct(
        public string $first_name,
        public string $last_name,
    ) {
    }
}

Virtual properties behave exactly like #[Computed] properties: they cannot be set from the payload and are excluded from serialization. Backed properties with hooks (properties that have a get hook but also store a value) are not treated as computed.

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