尝试使用 go 接口实现多态性

Question

我正在尝试在第三方库之上创建一个层，在本例中为 libchan。这是我定义的接口：

type ReceiverStream interface {
    Receive(msg interface{}) error
}

type InboundTransport interface {
    WaitReceiveChannel() (ReceiverStream, error)
}

InboundTransport 是 Transport 类型的替代：

// libchan.go
type Transport interface {
    // NewSendChannel creates and returns a new send channel.  The receive
    // end will get picked up on the remote end of the transport through
    // the remote calling WaitReceiveChannel.
    NewSendChannel() (Sender, error)

    // WaitReceiveChannel waits for a new channel be created by the
    // remote end of the transport calling NewSendChannel.
    WaitReceiveChannel() (Receiver, error)
}

仅作为上下文，这是libchan.Receiver 定义（请注意它与我的ReceiverStream 匹配：

// libchan.go
type Receiver interface {
    // Receive receives a message sent across the channel from
    // a sender on the other side of the underlying transport.
    // Receive is expected to receive the same object that was
    // sent by the Sender, any differences between the
    // receive and send type should be handled carefully.  It is
    // up to the application to determine type compatibility, if
    // the receive object is incompatible, Receiver will
    // throw an error.
    Receive(message interface{}) error
}

Transport 由 libchan 库返回 here：

// libchan/session.go:62
func NewTransport(provider StreamProvider) libchan.Transport {
...
}

由于libchan.Transport 和InboundTransport 共享WaitReceiveChannel() (ReceiverStream, error) 方法，我想我应该可以将一个替换为另一个，如下所示：

func (ln SpdyListener) Accept(addr string) InboundTransport {
    var listener net.Listener
    var err error
    listener, err = net.Listen("tcp", addr)
    if err != nil {
        log.Fatal(err)
    }
    c, err := listener.Accept()
    if err != nil {
        log.Fatal(err)
    }
    p, err := spdy.NewSpdyStreamProvider(c, true)
    if err != nil {
        log.Fatal(err)
    }

    return spdy.NewTransport(p)
}

但我得到一个错误：

cannot use spdy.NewTransport(p) (type libchan.Transport) as type InboundTransport in return argument:
    libchan.Transport does not implement InboundTransport (wrong type for WaitReceiveChannel method)
        have WaitReceiveChannel() (libchan.Receiver, error)
        want WaitReceiveChannel() (ReceiverStream, error)

我假设这个错误的意思是ReceiverStream的类型与libchan.Receiver不匹配，但我认为golang接口是隐式的，这意味着只要返回类型实现与预期接口相同的方法，它将通过编译。有什么我可以更改的，以便我可以将自定义接口叠加到第三方库返回的接口上？

TLDR：第三方库正在返回接口Transport 的对象。 Transport 接口指定了一个方法WaitReceiveChannel()。我有一个自定义接口InboundTransport，它还指定了WaitReceiveChannel()。我正在调用的第三方方法返回一个通过方法WaitReceiveChannel() 实现Transport 的对象。我假设它也会实现InboundTransport，因为后者还指定了相同类型的WaitReceiveChannel()。这是行不通的。为什么不呢？

Answer 1

如您所知，Go 中的接口是隐式满足的。

但是，正如错误所述，

WaitReceiveChannel() (libchan.Receiver, error)

和

WaitReceiveChannel() (ReceiverStream, error)

是两种不同的方法类型，导致libchan.Transport 没有隐式实现InboundTransport。

要解决此问题，您必须围绕 libchan.Transport 编写一个瘦包装器，以正确实现 InboundTransport。

type TransportWrapper struct {
    t *libchan.Transport
}

func (w *TransportWrapper) WaitReceiveChannel() (Receiver, error) {
    return w.t.WaitReceiveChannel()
}

// ...

func (ln SpdyListener) Accept(addr string) InboundTransport {
    var listener net.Listener
    var err error
    listener, err = net.Listen("tcp", addr)
    if err != nil {
        log.Fatal(err)
    }
    c, err := listener.Accept()
    if err != nil {
        log.Fatal(err)
    }
    p, err := spdy.NewSpdyStreamProvider(c, true)
    if err != nil {
        log.Fatal(err)
    }

    return &TransportWrapper{spdy.NewTransport(p)}
}