How can I provide a custom message for a failed JFormRule validation?

Question

When validating a custom field, and the test fails, Joomla returns the following default error message:

**Warning** Invalid field: My Field

How can I specify the message that is returned to explain why it is invalid?

Answer 1

Add the message attribute, but better like this:

<field name="my_code" type="text" size="15" label="My Code"
  validate="myrule" message="Field may only contain A-z or 0-9" />

:)

Answer 2

For an message that may vary, add the message attribute like this:

public function test(SimpleXMLElement $element, $value, $group = null, JRegistry $input = null, JForm $form = null)
{
    if(!preg_match('/^[a-z0-9]+$/i', $value)) {
        $element->addAttribute('message', 'Field may only contain A-z or 0-9');
        return false;
    }elseif(!$somethingelse) {
        $element->addAttribute('message', 'Something else is wrong');
        return false;
    }