Simplify/improve katapult.Client methods

I'd like to propose a change to the katapult.Client type with a goal of simplifying it, and making it easier to mock with an interface.

Currently the client has two methods:

NewRequestWithContext: Create a new *http.Request object and populate headers as needed, and marshal body to JSON.
Do: Performs a *http.Request against the Katapult API, under the assumption that the given *http.Request has come from NewRequestWithContext so it has all the right headers set.

This means to mock the Client in other packages, you need to create an interface with both methods, and also implement both in your fakeClient, ideally in such a way that it can verify if the request object is one that came from the client.

Instead, I would like to propose the Client type only has one method:

func (c *Client) Do(r *Request, v interface {}) (*Response, error)

This requires the introduction of a new katapult.Request type, which would contain a few pieces of additional Katapult-specific metadata, as well as what a http.Request struct already contains. And then the Do method becomes responsible for setting various headers, like user-agent, authorization, etc.

This means a fakeClient in other packages only needs to implement the Do method, and simply verify that the correct attributes are set on the given *katapult.Request argument.

I would expect the Request struct to be something like this:

type Request struct {
    http.Request

    // Authenticated determines if the "Authorization" header should be sent.
    // Set to false for un-authenticated API endpoints.
    Authenticated bool

    // Body is any object which can be marshaled to JSON with json.Marshal().
    Body interface{}
}

func NewRequestWithContext(
    ctx context.Context,
    method string,
    url string,
    body interface{},
) *Request {
    return &Request{
        Request: http.NewRequestWithContext(ctx, method, url, nil),
        Body:    body,
    }
}

(I'm not sure embedding http.Request is the best approach, but good enough as a demo here)

krystal / go-katapult

Simplify/improve katapult.Client methods #99