Get / read / modify response content from within a middleware

[DevilXD]

Hello,

I'm trying to make a middleware that would modify the current response's content. I know I can replace the current response in a request by just creating a new response and sending that instead, but to modify a response, I need to know the current content first, and it seems that there's currently no way to obtain that. I've searched through the documentation and the code, and it seems that AsyncWebServerResponse stores it's response content under a _content private attribute, so there's no way to read this, at least currently.

Something like this would be cool to have:

class AddFooterMiddleware : public AsyncMiddleware
{
    public:
        void run(AsyncWebServerRequest *request, ArMiddlewareNext next) override {
            next();  // continue middleware chain
            String content = request->getResponse()->getContent();  // this currently doesn't exist
            content.concat(
                "\n<footer style=\"text-align: center;\">"
                "<p>Powered by the ESPAsyncWebServer library</p>"
                "</footer>"
            );
            // NOTE: Alternatively, one would be able to call `setContent`, to avoid overwriting the code and content type
            request->send(200, "text/html", content);
        }
};

On a similar note, there's a way to set content type, but not get content type. The response code has both, thankfully. And the headers seem to have their respective methods too:

Ability to:	Get	Set
Code	✅	✅
Headers	✅	✅
Content Type	❌	✅
Content	❌	❌

[mathieucarbou]

and it seems that there's currently no way to obtain that

True. What can be override is NOT the response content but the response object itself.

Depending on the response type, the response content can be a string, binary, callback, json object etc, all handled by different response implementations.

What you are seeking for it much more templating than middleware.

[DevilXD]

Hmm, well... The code I provided was just an example. Maybe providing the actual code I'm working on will explain it better.

This is what I've managed to come up with over the past few days:

class PrettyResponses : public AsyncMiddleware
{
    public:
        void run(AsyncWebServerRequest *request, ArMiddlewareNext next) override {
            next();  // continue middleware chain
            AsyncWebServerResponse *response = request->getResponse();
            int code = response->code();
            if (code == 200) return;  // shortcut
            switch (code)
            {
                case 401:  // User hasn't logged in, missing credentials
                    response = request->beginResponse(
                        401, "text/plain", "401: Unauthorized"
                    );
                    response->addHeader("Refresh", "4; url=/login");
                    request->send(response);
                    break;
                case 403:  // Credentials are present, but they are not valid
                    response = request->beginResponse(
                        403, "text/plain", "403: Forbidden"
                    );
                    response->addHeader("Refresh", "4; url=/login");
                    request->send(response);
                    break;
                case 400:
                case 404:
                case 500:
                case 503:
                    String content = String(code);
                    content.concat(": ");
                    content.concat(response->responseCodeToString(code));
                    request->send(code, "text/plain", content);
                    break;
            }
        }
};

The purpose of this is to act as a "postprocessor" for all responses being sent back from the server. Before, every time I needed to return a 401 or 403, I'd need to state the content type and content to it. Now, I can just request->send(401) anywhere in the code, and the middleware will take care of providing the content to the response. I plan to expand the plain text responses into proper HTML later on.

What I've ran into, was me trying to provide custom messages in the responses, in addition to stating just the response code. So the plan was to use for ex. request->send(401, "text/plain", "Incorrect password"), and then let the middleware format that message into a string for ex. "401: Incorrect password" or "401: Unauthorized: Incorrect password". But to be able to do that, I need to get/read the response content within the middleware.

So, instead of this, I'd need to remove the middleware, and use a function that'd template in those messages (or use the default message)?

[mathieucarbou]

Yes, or you can develop your own response handler which will accumulate a buffer that you can access and send it later, you can look at the source code to see how it is done for a simple string response.

[DevilXD]

I see. Alright, I'll go with that. Thank you for your help =)