Added "when" method for validation at condition

src/Rule.php

@@ -17,6 +17,11 @@ abstract class Rule
     private bool $skipOnEmpty = false;
     private bool $skipOnError = true;
 
+    /**
+     * @var callable|null
+     */
+    private $when = null;
+
     /**
      * Validates the value
      *
@@ -31,7 +36,10 @@ final public function validate($value, DataSetInterface $dataSet = null, bool $p
             return new Result();
         }
 
-        if ($this->skipOnError && $previousRulesErrored) {
+        if (
+          ($this->skipOnError && $previousRulesErrored) ||
+          (is_callable($this->when) && !call_user_func($this->when, $value, $dataSet))
+        ) {
             return new Result();
         }
 
@@ -82,6 +90,32 @@ public function translateMessage(string $message, array $arguments = []): string
         );
     }
 
+    /**
+     * Add a PHP callable whose return value determines whether this rule should be applied.
+     * By default rule will be always applied.
+     *
+     * The signature of the callable should be `function ($value, DataSetInterface $dataSet): bool`, where $value and $dataSet
+     * refer to the value validated and the data set in which context it is validated. The callable should return
+     * a boolean value.
+     *
+     * The following example will enable the validator only when the country currently selected is USA:
+     *
+     * ```php
+     * function ($value, DataSetInterface $dataSet) {
+         return $dataSet->getAttributeValue('country') === Country::USA;
+     }
+     * ```
+     *
+     * @param callable $callback
+     * @return $this
+     */
+    public function when(callable $callback): self
+    {
+        $new = clone $this;
+        $new->when = $callback;
+        return $new;
+    }
+
     public function skipOnError(bool $value): self
     {
         $new = clone $this;

tests/RulesTest.php

@@ -56,6 +56,22 @@ static function ($value): Result {
         $this->assertCount(1, $result->getErrors());
     }
 
+    public function testWhenValidate()
+    {
+        $rules = new Rules(
+            [
+                (new Number())->min(10),
+                (new Number())->min(10)->when(fn() => false)->skipOnError(false),
+                (new Number())->min(10)->skipOnError(false)
+            ]
+        );
+
+        $result = $rules->validate(1);
+
+        $this->assertFalse($result->isValid());
+        $this->assertCount(2, $result->getErrors());
+    }
+
     public function testSkipOnError()
     {
         $rules = new Rules(