How can a validator be created for field values in Typo3 when the record is saved in the database?

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To create a validator for field values in Typo3, you need to follow these steps:

1. Open the TypoScript setup.

2. Use "TCA" (Table Configuration Array) to define the fields and their validation rules. For example:

   tca.<table>.columns.<fieldname>.config.validation = <validator>

   Here, <validator> is the validation rule that you want to apply to the field.

3. Create a custom validation function that checks whether the field value is valid or not. You can do this using PHP code. For example:

   function myCustomValidator($value, $options) {
      if($value > $options['max']) {
          return 'Value too high';
      } elseif($value < $options['min']) {
          return 'Value too low';
      } else {
          return true;
      }
   }
   

   This function takes the field value as the first argument ($value) and an array of options as the second argument ($options). The function returns true if the value is valid, otherwise it returns an error message.

4. Register your custom validation function using the extbase_validation hook. For example:

   $GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['extbase']['validator']['<YourExtensionKey>']['<ValidatorName>'] = \Your\Extension\MyValidator::class;
   

   Here, <YourExtensionKey> is the key of your extension and <ValidatorName> is the name of your validator function.

5. Save the record in the database. Typo3 will automatically call your custom validation function and show error messages if the field values are invalid.

Note: The above steps are just a general guideline. The actual implementation may vary depending on your specific requirements.