Serialize JSON as response to HTTP request

Question

I'd like my ESP8266 to provide some JSON endpoints. I want to use ArduinoJson for serialisation. While this works in general if I serialize into a String and return this String afterwards, I was wondering if there is really no way to do this without wasting so much memory. Shouldn't it be possible to do something like this? Any hints?

ESP8266WebServer server;
StaticJsonDocument<768> response;
response["night"] = _playback.isNight();
response["shuffle"] = _playback.isShuffle();
server.send_P(200, "application/json", NULL);
serializeJson(response, server.client());

(this gives the following error:)

no instance of overloaded function "serializeJson" matches the argument list -- argument types are: (ArduinoJson6141_0000010::StaticJsonDocument<768U>, WiFiServer::ClientType)

Answer 1

serializeJson() accepts a Print&, i.e, a reference to a class that implements the Print interface.

WiFiClient does implement the Print interface, but serializeJson() cannot make a reference to it because ESP8266WebServer::client() returns a temporary. Indeed, C++ forbids references to temporary variables, only const reference to temporaries are allowed.

To fix this compilation error, you must extract a variable:

WiFiClient client = server.client();
serializeJson(doc, client);

The way I see it, ESP8266WebServer::client() should return a WiFiClient& instead of a WiFiClient, just as HTTPClient::getStream() does. I opened issue #7075 about that.

Anyway, even it this fixes the compilation, I'm not sure it will work as you intend because ESP8266WebServer may not allow writing directly to the WiFiClient.