add callback validator.

README.md

@@ -190,6 +190,7 @@ type ValidatorResult = null | Errors;
 | pattern          | `pattern: RegExp` |
 | email            | - |
 | equalToControl   | `controlName: string` |
+| callback         | `(value: any, control: Control) => boolean` |
 
 Options are passed via props
 ```jsx

__tests__/form.test.tsx

@@ -624,3 +624,38 @@ test('onChange callback', () => {
       bar: 'qux'
     });
 });
+
+test('callback validator', () => {
+  const wrapper = mount(
+    <Form>
+      { _ => (
+        <span>
+          <Control name="foo" callback={(value: string) => value !== 'qux'}>
+            { control => (
+              <input 
+                type="text" value={control.value} 
+                onChange={event => control.onChange(event.target.value)}
+              />
+            ) }
+          </Control> 
+        </span>
+      ) }
+    </Form>
+  );
+
+  const instance = wrapper.instance() as Form;
+
+  instance.setValue('foo', 'foobar');
+
+  expect(instance.errors)
+    .toEqual({
+      foo: {},
+    });
+
+  instance.setValue('foo', 'qux');
+
+  expect(instance.errors)
+    .toEqual({
+      foo: { callback: true },
+    });
+});

__tests__/validators.test.ts

@@ -76,3 +76,19 @@ test('equalToControl', () => {
   expect(Validators.get('equalToControl')('bar', 'foo', mockControl as any))
     .toBeNull();
 });
+
+test('callback', () => {
+  const mockControl = {
+    get form() { 
+      return { 
+        getValue() { return 'bar'; } 
+      } 
+    }
+  };
+
+  expect(Validators.get('callback')('bar', (value: any) => value !== 'bar', mockControl as any))
+    .toEqual({ callback: true });
+
+  expect(Validators.get('callback')('baz', (value: any) => value !== 'bar', mockControl as any))
+    .toBeNull();
+});

src/control.ts

@@ -15,7 +15,7 @@ function isValidationRulesChanged(
   }
 
   for (const rule of oldRules) {
-    if (currentProps[rule] !== nextProps[rule]) {
+    if ('function' !== typeof currentProps[rule] && currentProps[rule] !== nextProps[rule]) {
       return true;
     }
   }

src/validators.ts

@@ -58,6 +58,14 @@ validators.set('equalToControl', (val: any, controlName: string, control: Contro
   return val === control.form.getValue(controlName) ? null : { equalToControl: true };
 });
 
+validators.set('callback', (val: any, fn: (value: any, control: Control) => boolean, control: Control) => {
+  if (isEmpty(val)) {
+    return null;
+  }
+
+  return fn(val, control) ? null : { callback: true };
+});
+
 export default {
   delete(v: string) { return validators.delete(v); },
   add(v: string, fn: Validator) { return validators.set(v, fn); },