Go 函数类型返回结构如何用于接口实现？

Go 函数类型返回接口实现的结构

考虑一个场景，其中包含一个包含耗时方法和工厂的包中的结构功能。为了有效地测试这个结构，最好在创建后同时使用一个假工厂函数和一个假结构。

以下代码举例说明了这一点：

package expensive

import "fmt"

type myStruct struct{}

func (m *myStruct) DoSomething() {
    fmt.Println("In Do something")
}

func (m *myStruct) DoSomethingElse() {
    fmt.Println("In Do something else")
}

// CreateInstance is expensive to call
func CreateInstance() *myStruct {
    return &myStruct{}
}

在依赖包中，我们接口像这样：

package main

import "play/expensive"

func main() {
    thing := structToConstruct{expensive.CreateInstance}
    thing.performAction()
}

type myInterface interface {
    DoSomething()
}

type structToConstruct struct {
    factoryFunction func() myInterface
}

func (s *structToConstruct) performAction() {
    instance := s.factoryFunction()
    instance.DoSomething()
}

但是，这段代码会引发一个错误：

.\main.go:6: cannot use expensive.CreateInstance (type func() *expensive.myStruct) as type func() myInterface in field value

尽管 *expense.myStruct 实现了 myInterface，Go 仍然抱怨类型安全。

根据 Go 团队的说法，这种行为是故意的：

Having a function type in a struct field that returns the underlying concrete type would violate a core principle of interfaces. That principle is that the concrete type can be replaced by any other implementation of the interface type. If the receiver type of a function were the underlying concrete type, the behavior of the function would change if called via an interface.

解决方案是包装工厂函数：

wrapper := func() myInterface {
    return expensive.CreateInstance()
}
thing := structToConstruct{wrapper}

这个包装器遵循 myInterface 并且代码可以编译。

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