Add unfillable method to be able to create "ui fields"

[RomainMazB]

As of today, some Field already implements the Unfillable interface, like the Header to create a Vue component which doesn't hold any value that won't fill the model before saving it to the database.

The feature request is to expand this feature to field using an unfillable method instead of the interface.

Why this is needed

A basic use case is to be able to use a select dropdown where the value is looked from other fields using dependsOn feature to hide/show some fields.

I have a Loot model, where the amount is generated using 3 fields: amount_fixed, amount_min, amount_max.
amount_fixed is only used when I want the loot to be "fixed": always 3.
amount_min and amount_max are used when I want the loot amount to be random: between 1 and 5.

// This field doesn't exist in database, it's just live for UX improvement
Select::make('Amount mode', 'amount_mode')
    ->onlyOnForms()
    ->fillUsing(fn () => null)
    ->options([
        'random' => 'Random',
        'fixed' => 'Fixed',
    ])
    ->default(fn(NovaRequest $request) => is_null($request->resource->amount_fixed) ? 'random' : 'fixed'),

// This field is only displayed when the dropdown value is "fixed"
Number::make('Amount', 'amount_fixed')
    ->dependsOn(
        'quantities',
        fn (Number $field, NovaRequest $request, FormData $formData) =>
            $formData->quantities === 'fixed' ? $field->show() : null
    ),


// These fields are only displayed when the dropdown value is "random"
Number::make('Amount min')
    ->dependsOn(
        'quantities',
        fn (Number $field, NovaRequest $request, FormData $formData) =>
            $formData->quantities === 'random' ? $field->show() : null
    ),

Number::make('Amount max')
    ->dependsOn(
        'quantities',
        fn (Number $field, NovaRequest $request, FormData $formData) =>
            $formData->quantities === 'random' ? $field->show() : null
    )

All these three fields are nullable and I have a dropdown to select whether I want the Loot to be fixed or random.

Today, to not persist the amount mode value to the database using the ->fillUsing(fn () => null) and ->onlyOnForms() because there isn't no value to display on index/preview but it would be clearer if we could easily apply this trick using an ->unfillable() method.

[crynobone]

Released with Laravel Nova 5.0.0

Select::make('Amount mode', 'amount_mode')
-   ->onlyOnForms()
-   ->fillUsing(fn () => null)
+   ->computed()

Feel free to open up a new issue if you're still experiencing this problem on the latest version.