Go中Mock实现无法匹配ClientProvider接口的问题求助

问题描述 #

在Go应用中定义了MyClient接口、MyClients结构体（持有MyClient类型的映射，可根据key获取指定客户端），为测试创建了Mock版本，认为Mock应满足ClientProvider接口，但Go不认可该Mock实现——原因是Mock的Client方法返回MyClientMock（已实现MyClient接口）而非MyClient类型。

相关代码 #

client.go #

type ClientProvider interface {
    Client(model string) MyClient
}

type MyClients struct {
    Clients map[string]MyClient
}

func (mc *MyClients) Client(client string) MyClient {
    return mc.Clients[client]
}

type MyClient interface {
    Get()
}

client_mock.go #

type MyClientsMock struct {
    Clients map[string]MyClientMock
}

func (mcm *MyClientsMock) Client(client string) MyClientMock {
    println("I an satisfying the ClientProvider Interface")
    return mcm.Clients[client]
}

type MyClientMock struct {
    MyClient
    Response string
    Err      error
}

func (mcm MyClientMock) Get() {
    println("I an satisfying the MyClient Interface")
}

func NewMyClientsMock() MyClientsMock {
    return MyClientsMock{
        Clients: map[string]MyClientMock{
            "client-a": {Response: "hello from client a"},
            "client-b": {Response: "hello from client b"},
        },
    }
}

main.go #

func main() {
    myClientsMock := NewMyClientsMock()

    // WHY CAN'T I DO THIS
    test(myClientsMock)
}

func test(clients ClientProvider) {

}

错误原因 #

Go的接口实现要求方法签名完全一致，包括返回值类型：

1. ClientProvider接口规定Client方法必须返回MyClient接口类型
2. MyClientsMock的Client方法返回的是具体类型MyClientMock，即使该类型实现了MyClient接口，返回类型不匹配就无法满足接口要求
3. 额外问题：MyClientsMock的Client方法使用指针接收器，但NewMyClientsMock返回的是值类型，值类型无法实现带指针接收器的接口，这也是导致无法传入test函数的原因之一

解决方案 #

修改Mock实现以匹配接口要求 #

调整MyClientsMock的Client方法返回类型为MyClient，同时让NewMyClientsMock返回指针类型：

type MyClientsMock struct {
    Clients map[string]MyClientMock
}

// 修改返回类型为MyClient，MyClientMock实现了该接口，可直接返回
func (mcm *MyClientsMock) Client(client string) MyClient {
    println("I am satisfying the ClientProvider Interface")
    return mcm.Clients[client]
}

type MyClientMock struct {
    Response string
    Err      error
}

// 移除冗余的MyClient嵌入，因为已经实现了Get方法
func (mcm MyClientMock) Get() {
    println("I am satisfying the MyClient Interface")
}

func NewMyClientsMock() *MyClientsMock {
    return &MyClientsMock{
        Clients: map[string]MyClientMock{
            "client-a": {Response: "hello from client a"},
            "client-b": {Response: "hello from client b"},
        },
    }
}

调整main.go调用 #

此时main.go中的调用即可正常运行：

func main() {
    myClientsMock := NewMyClientsMock()
    test(myClientsMock)
}

func test(clients ClientProvider) {

}

内容的提问来源于stack exchange，提问作者mike73