创建使用指针接收器方法包装现有类型的接口

Question

I need to test an app which uses Google Cloud Pubsub, and so must wrap its types pubsub.Client and pubsub.Subscriber for testing purposes. However, despite several attempts I can't get an interface around them which compiles.

The definitions of the methods I'm trying to wrap are:

func (s *Subscription) Receive(
    ctx context.Context, f func(context.Context, *Message)) error

func (c *Client) Subscription(id string) *Subscription

Here is the current code. The Receiver interface (wrapper around Subscriber) seems to work, but I suspect it may need to change in order to fix SubscriptionMaker, so I've include both.

Note: I've tried several variations of where to reference and dereference pointers, so please don't tell me to change that unless you have an explanation of why your suggested configuration is the correct one or you've personally verified it compiles.

import (
    "context"

    "cloud.google.com/go/pubsub"
)

type Receiver interface {
    Receive(context.Context, func(ctx context.Context, msg *pubsub.Message)) (err error)
}

// Pubsub subscriptions implement Receiver
var _ Receiver = &pubsub.Subscription{}

type SubscriptionMaker interface {
    Subscription(name string) (s Receiver)
}

// Pubsub clients implement SubscriptionMaker
var _ SubscriptionMaker = pubsub.Client{}

Current error message:

common_types.go:21:5: cannot use "cloud.google.com/go/pubsub".Client literal (type "cloud.google.com/go/pubsub".Client) as type SubscriptionMaker in assignment:
    "cloud.google.com/go/pubsub".Client does not implement SubscriptionMaker (wrong type for Subscription method)
        have Subscription(string) *"cloud.google.com/go/pubsub".Subscription
        want Subscription(string) Receiver

Answer 1

It can't be done.

When defining the type signature of a method on an interface, it must match exactly. func (c *Client) Subscription(id string) *Subscription returns a *Subscription, and a *Subscription is a valid Receiver, but it does not count as conforming to the interface method Subscription(string) Receiver. Go requires precise matching for function signatures, not the duck-typing style that it usually uses for interfaces.