在实现 actix 中间件时，我应该如何理解复杂的 where 子句？

Question

我正在查看 actix examples 以了解如何在 actix_web 中实现中间件。他们实现了这样的特征：

impl<S, B> Transform<S> for SayHi
where
    S: Service<Request = ServiceRequest, Response = ServiceResponse<B>, Error = Error>,
    S::Future: 'static,
    B: 'static,
{
    type Request = ServiceRequest;
    type Response = ServiceResponse<B>;
    type Error = Error;
    type InitError = ();
    type Transform = SayHiMiddleware<S>;
    type Future = FutureResult<Self::Transform, Self::InitError>;

    fn new_transform(&self, service: S) -> Self::Future {
        ok(SayHiMiddleware { service })
    }
}

我在理解这行代码时遇到了一些问题：

S: Service<Request = ServiceRequest, Response = ServiceResponse<B>, Error = Error>

是说 S 受 Service 特征的约束，并且在该特征中有一个名为 Request 的类型，在这种情况下 Request 实际上是一个ServiceRequest?

如果是这样，为什么需要在块中指定 type Request = ServiceRequest;？

Answer 1

Is it saying that S is bound by the Service trait and in that trait there is a type named Request and in THIS case Request is actually a ServiceRequest?

有两个特征，Transform 和 Service，它们都有一个名为 Request 的关联类型。这些没有关系。

If so, why is there a need to specify type Request = ServiceRequest; in the block?

通过为 SayHi 编写 Transform 的实现，您实际上是在创建这种关系。您已经定义了这些关联类型是什么，用于仅在满足 where 子句中的所有条件时才有效的实现。

在实现 actix 中间件时，我应该如何理解复杂的 where 子句？

How should I understand a compilcated where clause when implementing actix middleware?

traits

rust