Add cron expression form validation (alexjustesen#279)

Author: jocmp

app/Filament/Pages/Settings/GeneralPage.php

@@ -2,6 +2,7 @@
 
 namespace App\Filament\Pages\Settings;
 
+use App\Rules\ValidCronExpression;
 use App\Settings\GeneralSettings;
 use Filament\Forms\Components\Card;
 use Filament\Forms\Components\Grid;
@@ -64,6 +65,7 @@ protected function getFormSchema(): array
                         Section::make('Speedtest Settings')
                             ->schema([
                                 TextInput::make('speedtest_schedule')
+                                    ->rules([new ValidCronExpression()])
                                     ->helperText('Leave empty to disable the schedule. You can also use the cron expression generator [HERE](https://crontab.cronhub.io/) to help you make schedules.')
                                     ->nullable()
                                     ->columnSpan(1),

app/Rules/ValidCronExpression.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace App\Rules;
+
+use Cron\CronExpression;
+use Illuminate\Contracts\Validation\InvokableRule;
+
+class ValidCronExpression implements InvokableRule
+{
+    /**
+     * Validates a string cron expression is correct
+     *
+     * @param  string  $attribute
+     * @param  mixed  $value
+     * @param  \Closure(string): \Illuminate\Translation\PotentiallyTranslatedString  $fail
+     * @return void
+     */
+    public function __invoke($attribute, $value, $fail)
+    {
+        $is_valid = CronExpression::isValidExpression($value);
+
+        if (! $is_valid) {
+            $fail('Cron expression is not valid');
+        }
+    }
+}