CallbackValidator: Cache callback results by default

Since Icinga/ipl-html#97 form elements don't cache
the validation results anymore. This isn't a problem for most validators.
Though, custom callbacks written before this change were written with the
cache in mind. This change allows to control this when creating the validator.
It caches by default for historical reasons. (To not break compatibility)

src/CallbackValidator.php

@@ -25,21 +25,38 @@ class CallbackValidator extends BaseValidator
     /** @var callable Validation callback */
     protected $callback;
 
+    /** @var bool Whether to cache the callback's result */
+    protected $cacheResult;
+
+    /** @var ?bool The result, if {@see self::$cacheResult} is `true` (default) */
+    protected $result;
+
     /**
      * Create a new callback validator
      *
      * @param callable $callback Validation callback
+     * @param boolean $cacheResult Whether to cache the callback's result
      */
-    public function __construct(callable $callback)
+    public function __construct(callable $callback, bool $cacheResult = true)
     {
         $this->callback = $callback;
+        $this->cacheResult = $cacheResult;
     }
 
     public function isValid($value)
     {
+        if ($this->result !== null) {
+            return $this->result;
+        }
+
         // Multiple isValid() calls must not stack validation messages
         $this->clearMessages();
 
-        return call_user_func($this->callback, $value, $this);
+        $valid = (bool) call_user_func($this->callback, $value, $this);
+        if ($this->cacheResult) {
+            $this->result = $valid;
+        }
+
+        return $valid;
     }
 }

tests/CallbackValidatorTest.php

@@ -22,4 +22,34 @@ public function testWhetherValidationCallbackIsOnlyExecutedWhenIsValidIsCalled()
 
         $this->assertSame($messages, $validator->getMessages());
     }
+
+    public function testWhetherCallbackIsOnlyCalledAgainIfNotCached()
+    {
+        $count = 0;
+
+        $validator = new CallbackValidator(function ($value, CallbackValidator $validator) use (&$count) {
+            $count++;
+            return true;
+        });
+
+        $validator->isValid(true);
+        $validator->isValid(true);
+
+        $this->assertEquals(1, $count, 'The callback is called again even if the cache is enabled');
+    }
+
+    public function testWhetherCallbackIsCalledAgainIfCached()
+    {
+        $count = 0;
+
+        $validator = new CallbackValidator(function ($value, CallbackValidator $validator) use (&$count) {
+            $count++;
+            return true;
+        }, false);
+
+        $validator->isValid(true);
+        $validator->isValid(true);
+
+        $this->assertEquals(2, $count, 'The callback is not called again even if the cache is disabled');
+    }
 }