MVC: service injecting fails

[zheeeng]

package parse

import (
	"github.com/kataras/iris/mvc"
)

type Parser interface {
	Parse() string
}

type parser struct {
	inner string
}

func InitParser() Parser {
	return &parser{
		inner: "hello",
	}
}

func (p *parser) Parse() string {
	return p.inner
}

func App(app *mvc.Application) {
	parser := InitParser()

	app.Register(parser)

	app.Party("/parse").Configure(ParseApp)
}

func ParseApp(app *mvc.Application) {
	service := NewService()
	app.Register(service)

	app.Handle(new(controller))
}

type Service interface {
}

type service struct{}

func NewService() Service {
	return &service{}
}

type controller struct {
	Service Service
	Parser  Parser
}

func (c *controller) Get() string {
	return c.Parser.Parse()
}

This code will throw Internal Server Error:

Trace: runtime error: invalid memory address or nil pointer dereference

cos the c.Parser is nil

If we change the controller to

type controller struct {
	Parser  Parser
}

There is no problems.

[kataras]

@zheeeng The problem here is that *parser is a Service too as Service does not contain any method and it's just an interface{} which binds to everything. So the *parser is bind to the Service field instead of the Parser.

[kataras]

This is the same as you posted before: #1342.

These work:

type controller struct {
	Parser  Parser
}

type controller struct {
	Parser  Parser
        Service Service // (or interface {}, it's the same in your case)
}

And that's because it tries to match it by order of registering, because the Service field accepts anything, and you bind the *parser so it does bind to it and stops there, all binding values are registered so it does not continue to the Parser field (in Iris controllers, a binding value cannot be binded to more than one field of struct or input parameter of a function).

What we can do here?

Here we can add "priorities" so the Parser field binds to the *parser as it has methods and completes the interface. But in your previous issue where two fields are expecting any value and two binding values are registered, we can't do that, all fields have the same priority because they are empty interface{} , so the order of registering those binding values matters. These all are not real-world scenarios though.

[kataras]

Hello @zheeeng,

With the recent commit we have added the ability to customize and change the order of fields and their valid available registered values (if more than one) through the mvc.Application.Sorter field. It defaults to nil but you can change it. We have a builtin sorter(SortByNumMethods) which does exactly what you want here.

func App(app *mvc.Application) {
        app.SortByNumMethods()

	parser := InitParser()
	app.Register(parser)
	app.Party("/parse").Configure(ParseApp)
}