feat(binding): add support for custom validator / validation tags (#1068)

* feat(binding): Add support for custom validation tags

* docs: Add example for custom validation tag

* test(binding): Add test for registering custom validation

binding/validate_test.go

@@ -6,9 +6,12 @@ package binding
 
 import (
 	"bytes"
+	"reflect"
 	"testing"
 	"time"
 
+	validator "gopkg.in/go-playground/validator.v8"
+
 	"github.com/stretchr/testify/assert"
 )
 
@@ -190,3 +193,42 @@ func TestValidatePrimitives(t *testing.T) {
 	assert.NoError(t, validate(&str))
 	assert.Equal(t, str, "value")
 }
+
+// structCustomValidation is a helper struct we use to check that
+// custom validation can be registered on it.
+// The `notone` binding directive is for custom validation and registered later.
+type structCustomValidation struct {
+	Integer int `binding:"notone"`
+}
+
+// notOne is a custom validator meant to be used with `validator.v8` library.
+// The method signature for `v9` is significantly different and this function
+// would need to be changed for tests to pass after upgrade.
+// See https://github.com/gin-gonic/gin/pull/1015.
+func notOne(
+	v *validator.Validate, topStruct reflect.Value, currentStructOrField reflect.Value,
+	field reflect.Value, fieldType reflect.Type, fieldKind reflect.Kind, param string,
+) bool {
+	if val, ok := field.Interface().(int); ok {
+		return val != 1
+	}
+	return false
+}
+
+func TestRegisterValidation(t *testing.T) {
+	// This validates that the function `notOne` matches
+	// the expected function signature by `defaultValidator`
+	// and by extension the validator library.
+	err := Validator.RegisterValidation("notone", notOne)
+	// Check that we can register custom validation without error
+	assert.Nil(t, err)
+
+	// Create an instance which will fail validation
+	withOne := structCustomValidation{Integer: 1}
+	errs := validate(withOne)
+
+	// Check that we got back non-nil errs
+	assert.NotNil(t, errs)
+	// Check that the error matches expactation
+	assert.Error(t, errs, "", "", "notone")
+}