feat: custom validation implementation

Related to #1162

packages/form-js-viewer/src/core/Validator.js

@@ -81,6 +81,18 @@ export class Validator {
 
     errors = [...errors, ...runPresetValidation(field, evaluatedValidation, value)];
 
+    const evaluateExpression = (expression) =>
+      runExpressionEvaluation(this._expressionLanguage, expression, { ...expressionContextInfo, value });
+
+    if ('custom' in evaluatedValidation && value && evaluatedValidation.custom.length) {
+      const { custom } = evaluatedValidation;
+      custom.forEach(({ condition, message }) => {
+        if (condition && message && !evaluateExpression(condition)) {
+          errors = [...errors, evaluateExpression(message)];
+        }
+      });
+    }
+
     return errors;
   }
 }

packages/form-js-viewer/src/util/expressions.js

@@ -22,13 +22,13 @@ export function buildExpressionContext(context) {
  * If the string is not an expression, it is returned as is.
  *
  * @param {any} expressionLanguage - The expression language to use.
- * @param {string} value - The string to evaluate.
+ * @param {string} expression - The string expression to evaluate.
  * @param {Object} expressionContextInfo - The context information to use.
  * @returns {any} - Evaluated value or the original value if not an expression.
  */
-export function runExpressionEvaluation(expressionLanguage, value, expressionContextInfo) {
-  if (expressionLanguage && expressionLanguage.isExpression(value)) {
-    return expressionLanguage.evaluate(value, buildExpressionContext(expressionContextInfo));
+export function runExpressionEvaluation(expressionLanguage, expression, expressionContextInfo) {
+  if (expressionLanguage && expressionLanguage.isExpression(expression)) {
+    return expressionLanguage.evaluate(expression, buildExpressionContext(expressionContextInfo));
   }
-  return value;
+  return expression;
 }