Validator::formatMessage: support dynamic error messages

This is useful when you want to show user an informative error message, whose text
depends on some external data that depends on the value of some other field.

For example, my form allows user to choose a category of a team. Each category has a different
limit for number of allowed members, which is specified in config.neon. I want to show the user
the limit as part of the error message when she manages to exceed the limit.

src/Forms/Helpers.php

@@ -102,6 +102,9 @@ public static function exportRules(Rules $rules): array
 				}
 				$op = Nette\Utils\Callback::toString($op);
 			}
+			if (is_callable($rule->message)) {
+				continue;
+			}
 			if ($rule->branch) {
 				$item = [
 					'op' => ($rule->isNegative ? '~' : '') . $op,

src/Forms/Rules.php

@@ -69,7 +69,7 @@ public function isRequired(): bool
 	/**
 	 * Adds a validation rule for the current control.
 	 * @param  callable|string  $validator
-	 * @param  string|object  $errorMessage
+	 * @param  string|callable|object  $errorMessage
 	 * @return static
 	 */
 	public function addRule($validator, $errorMessage = null, $arg = null)

src/Forms/Validator.php

@@ -51,6 +51,15 @@ class Validator
 	public static function formatMessage(Rule $rule, bool $withValue = true)
 	{
 		$message = $rule->message;
+		if (is_callable($message)) {
+			$params = Nette\Utils\Callback::toReflection($message)->getParameters();
+			if (isset($params[1])) {
+				$message = $message($rule->control, $rule->arg);
+			} else {
+				$message = $message($rule->control);
+			}
+		}
+
 		if ($message instanceof Nette\Utils\IHtmlString) {
 			return $message;
 

tests/Forms/Helpers.exportRules.phpt

@@ -6,6 +6,7 @@
 
 declare(strict_types=1);
 
+use Nette\Forms\Controls\TextInput;
 use Nette\Forms\Form;
 use Nette\Forms\Helpers;
 use Tester\Assert;
@@ -91,3 +92,13 @@ test(function () {
 		],
 	], Helpers::exportRules($input2->getRules()));
 });
+
+
+test(function () {
+	$form = new Form;
+	$input = $form->addText('text');
+	$input->addRule(Form::EMAIL, function (TextInput $input, $arg) {
+		return $input->getValue() . ' is not valid e-mail address.';
+	});
+	Assert::same([], Helpers::exportRules($input->getRules()));
+});

tests/Forms/Rules.formatMessage.phpt

@@ -6,6 +6,7 @@
 
 declare(strict_types=1);
 
+use Nette\Forms\Controls\TextInput;
 use Nette\Forms\Form;
 use Tester\Assert;
 
@@ -34,11 +35,36 @@ $form->addText('special', 'Label:')
 	->addRule(Form::EMAIL, '%label %value is invalid [field %name] %d', $form['special'])
 	->setDefaultValue('xyz');
 
+$form->addText('function', 'Label:')
+	->setRequired()
+	->addRule(function (TextInput $input, string $arg) {
+		return strpos($input->getValue(), $arg) === false;
+	}, function (TextInput $input, string $arg) {
+		return "String “{$input->getValue()}” contains a letter “{$arg}”, which is not allowed";
+	}, 'a')
+	->setDefaultValue('banana');
+
+$form->addText('functionWithoutArg', 'Label:')
+	->setRequired()
+	->addRule(function (TextInput $input) {
+		return strpos($input->getValue(), 'e') === false;
+	}, function (TextInput $input) {
+		return "String “{$input->getValue()}” contains a letter “e”, which is not allowed";
+	})
+	->setDefaultValue('orange');
+
 $form->validate();
 
 Assert::true($form->hasErrors());
 
-Assert::same(['1 5', '5 1', '1 ', 'Label xyz is invalid [field special] xyz'], $form->getErrors());
+Assert::same([
+	'1 5',
+	'5 1',
+	'1 ',
+	'Label xyz is invalid [field special] xyz',
+	'String “banana” contains a letter “a”, which is not allowed',
+	'String “orange” contains a letter “e”, which is not allowed',
+], $form->getErrors());
 
 Assert::same([], $form->getOwnErrors());