State Machines

BaseInvoiceState.php

<?php

use Exception;

class BaseInvoiceState
{
    function __construct(public Invoice $invoice) { }

    function finalize() { throw new Exception(); }

    function pay() { throw new Exception(); }

    function void() { throw new Exception(); }

    function cancel() { throw new Exception(); }
}

DraftInvoiceState.php

<?php

class DraftInvoiceState extends BaseInvoiceState
{
    function finalize() {
        $this->invoice->update(['status' => 'open']);
        Mail::send(new InvoiceDue($this->invoice));
    }
}

FinalizeInvoiceController.php

<?php

class FinalizeInvoiceController extends Controller
{
    public function __invoke(Request $request, Invoice $invoice)
    {
        $invoice->state()->finalize();
        
        return view('invoice.show', compact('invoice'));
    }
}

Invoice.php

<?php

class Invoice extends Model {
    protected $attributes = [
        'status' => 'draft',
    ];
    
    public function state(): BaseInvoiceState
    {
        return match ($this->status) {
            'draft' => new DraftInvoiceState($this),
            'open' => new OpenInvoiceState($this),
            'paid' => new PaidInvoiceState($this),
            'void' => new VoidInvoiceState($this),
            'uncollectable' => new UncollectableInvoiceState($this),
            default => throw new InvalidArgumentException('Invalid Status'),
        };
    }
}