Gin允許我們自定義參數驗證器，[參考1](https://github.com/gin-gonic/gin/blob/master/examples/custom-validation/server.go)，[參考2](https://github.com/go-playground/validator/releases/tag/v8.7)，[參考3](https://github.com/gin-gonic/gin/tree/master/examples/struct-lvl-validations) ``` package main import ( "net/http" "reflect" "time" "github.com/gin-gonic/gin" "github.com/gin-gonic/gin/binding" "gopkg.in/go-playground/validator.v8" ) // Booking contains binded and validated data. 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() if v, ok := binding.Validator.Engine().(*validator.Validate); ok { v.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()}) } } ``` ``` $ curl "localhost:8085/bookable?check_in=2018-04-16&check_out=2018-04-17" {"message":"Booking dates are valid!"} $ curl "localhost:8085/bookable?check_in=2018-03-08&check_out=2018-03-09" {"error":"Key: 'Booking.CheckIn' Error:Field validation for 'CheckIn' failed on the 'bookabledate' tag"} ```