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

examples/custom-validation/server.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"net/http"
+	"reflect"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"github.com/gin-gonic/gin/binding"
+	validator "gopkg.in/go-playground/validator.v8"
+)
+
+type Booking struct {
+	CheckIn  time.Time `form:"check_in" binding:"required,bookabledate" time_format:"2006-01-02"`
+	CheckOut time.Time `form:"check_out" binding:"required,gtfield=CheckIn" time_format:"2006-01-02"`
+}
+
+func bookableDate(
+	v *validator.Validate, topStruct reflect.Value, currentStructOrField reflect.Value,
+	field reflect.Value, fieldType reflect.Type, fieldKind reflect.Kind, param string,
+) bool {
+	if date, ok := field.Interface().(time.Time); ok {
+		today := time.Now()
+		if today.Year() > date.Year() || today.YearDay() > date.YearDay() {
+			return false
+		}
+	}
+	return true
+}
+
+func main() {
+	route := gin.Default()
+	binding.Validator.RegisterValidation("bookabledate", bookableDate)
+	route.GET("/bookable", getBookable)
+	route.Run(":8085")
+}
+
+func getBookable(c *gin.Context) {
+	var b Booking
+	if err := c.ShouldBindWith(&b, binding.Query); err == nil {
+		c.JSON(http.StatusOK, gin.H{"message": "Booking dates are valid!"})
+	} else {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+	}
+}