Support custom struct tag

[mtt0]

Description

Many golang packages have their own struct type and tags, we can not bind their type by binding directly. For example:

go-gitlab use url as struct field tag:
https://github.com/xanzy/go-gitlab/blob/3ce8ea8933fa3f8cdbd0ec1205392417d4ba4701/pipelines.go#L149-L162
so, we can not do this:

func ListHandler(s *Service) func(ctx *gin.Context) {
	return func(ctx *gin.Context) {
		var opt gitlab.ListProjectPipelinesOptions
		// we can't get form data cause missing `form` tag
		err := ctx.BindQuery(&opt)
		if err != nil {
			_ = ctx.Error(err)
			return
		}
		// ...
	}
}

How to solve

• Add function bind.MapFormWithTag
  form_mapping.go:

func MapFormWithTag(ptr interface{}, form map[string][]string, tag string) error {
	return mapFormByTag(ptr, form, tag)
}

Now we can do this:

const (
	customerTag = "url"
	defaultMemory = 32 << 20
)

type customerBinding struct {}

func (customerBinding) Name() string {
	return "form"
}

func (customerBinding) Bind(req *http.Request, obj interface{}) error {
	if err := req.ParseForm(); err != nil {
		return err
	}
	if err := req.ParseMultipartForm(defaultMemory); err != nil {
		if err != http.ErrNotMultipart {
			return err
		}
	}
	if err := binding.MapFormWithTag(obj, req.Form, customerTag); err != nil {
		return err
	}
	return validate(obj)
}

func validate(obj interface{}) error {
	if binding.Validator == nil {
		return nil
	}
	return binding.Validator.ValidateStruct(obj)
}

// Now we can do this !
func ListHandler(s *Service) func(ctx *gin.Context) {
	return func(ctx *gin.Context) {
                var urlBinding = customerBinding{}
		var opt gitlab.ListProjectPipelinesOptions
		err := ctx.MustBindWith(&opt, urlBinding)
		if err != nil {
			_ = ctx.Error(err)
			return
		}
		// ...
	}
}