go - The Go way to implement the Observer design pattern

Question

My experience is with OOP languages but I've started trying out Go. I'm having trouble working out the best way to implement the Observer design pattern in Go.

I've organised my project as follows where everything in the observers folder is part of package observers and everything in the subjects folder is part of package subjects. The attaching of observers to subjects is done in main.go.

my-project/
  main.go
  observers/
    observer.go
    observer_one.go
    observer_two.go
  subjects/
    subject.go
    subject_one.go

I've seen this section in the Go wiki about interfaces:

Go interfaces generally belong in the package that uses values of the interface type, not the package that implements those values. The implementing package should return concrete (usually pointer or struct) types: that way, new methods can be added to implementations without requiring extensive refactoring.

Keeping the comment from the Go Wiki in mind. I've implemented like this (left out function implementations):

subject.go:

type ObserverInterface interface {
    Update(subject *Subject, updateType string)
}

type Subject struct {
    observers map[string][]ObserverInterface
}

func (s *Subject) Attach(observer ObserverInterface, updateType string) {}

func (s *Subject) Detach(observer ObserverInterface, updateType string) {}

func (s *Subject) notify(updateType string) {}

observer.go:

type SubjectInterface interface {
   Attach(observer Observer, updateType string)
   Detach(observer Observer, updateType string)
   notify(updateType string)
}

type Observer struct {
    uuid uuid.UUID
}

observer_one.go

type ObserverOne struct {
    Observer
}

func (o *ObserverOne) Update(subject *SubjectInterface, updateType string) {}

main.go

subjectOne := &SubjectOne{}
observerOne := &ObserverOne{Observer{uuid: uuid.New()}}
subjectOne.Attach(observerOne, "update_type")

I expect to be able to use SubjectInterface for the argument to the Update() method in ObserverOne so that I can avoid having dependencies between my subject package and my observer package but I get the following compile-time error.

observers/observer_one.go:29:35: cannot use &observer (type *ObserverOne) as type subjects.ObserverInterface in argument to SubjectOne.Subject.Attach:
    *ObserverOne does not implement subjects.ObserverInterface (wrong type for Update method)
        have Update(*SubjectInterface, string)
        want Update(*subjects.Subject, string)

If I replace the definition of Update() in observer_one.go with the following it compiles fine but i thought the idea was to decouple the packages using interfaces:

func (o *ObserverOne) Update(subject *subjects.Subject, updateType string) {}

标签： godesign-patternsobserver-pattern

Answer 1

First, don't use pointers to interfaces.

func (o *ObserverOne) Update(subject *SubjectInterface, updateType string) {}

should be

func (o *ObserverOne) Update(subject SubjectInterface, updateType string) {}

Second, you've defined your interface to require a concrete type:

type ObserverInterface interface {
    Update(subject *Subject, updateType string)
}

Instead, make it accept an interface:

type ObserverInterface interface {
    Update(subject SubjectInterface, updateType string)
}