How to access Keys of gin.Context?

Hi, I set the user infomation in a filter of gin, the filter is as follows:
```go
func Authentication() gin.HandlerFunc {
    return func(c *gin.Context){
        userId := "123"
        c.Set("userId", userId)
        c.Next()
    }
}
```

How should I get the value of userId?

```go

func (h *Handler) current(c droplet.Context) (interface{}, error){
    userId := ????
}
```