IdHttp : how to deal with json request and response - json

Question

I have encountered a few sites that uses JSON for request and response
I come across two types :
1- application/x-www-form-urlencoded as request and return a response application/json content type
2- application/json content type for both request and response
in type 1 i tried changing the the response content type using
mIdHttp.Response.ContentType := 'application/json';
but using http analyzer i can see that it doesn't change and its still text/html
now i do not know if the problem is with the fact that i can't change content type or not but i do not know how to deal with json !
a few question regarding json :
1- do i have to encode json data when posting ? how ?
2- how can i parse the json response code ? how to get it ? does it need some kind of encode or special convertion ?
3- what kind of idhttp setting for json changes with each site and needs configuring ?
I understand my questions sound a bit general, but all the other questions are very specific and don't explain the basics when dealing with 'application/json' content type.
Edit 1 :
thanks to Remy Lebeau answer i was able to successfully work with type 1
but i still have trouble sending JSON request, can someone please share an working example, this is one of the sites posting information please use this for your example :
One important note : this particular site's post and request content are exactly like each other ! and it baffles me because at the site, i specify an start date and an end date then click on a folder like icon and this post is sent (the one you can see above), and the result should be links (and it is) but instead of appearing only in request content they also appear in post ! (also i'm trying to get the links but at the post the links, the thing i want, are also being sent, how can i post something i don't have !!?)
just for more clarity here is the place i fill the date and the icon i mentioned :

Answer 1

You cannot specify the format of the response, unless the requested resource offers an explicit input parameter or dedicated URL for that exact purpose (ie, to request a response be sent as html, xml, json, etc). Setting the TIdHTTP.Response.ContentType property is useless. It will be overwritten by the actual Content-Type header of the response.
To send JSON in a request, you must post it as a TStream, like TMemoryStream or TStringStream, and set the TIdHTTP.Request.ContentType as needed, eg:
var
ReqJson: TStringStream;
begin
ReqJson := TStringStream.Create('json content here', TEncoding.UTF8);
try
IdHTTP1.Request.ContentType := 'application/json';
IdHTTP1.Post(URL, ReqJson);
finally
ReqJson.Free;
end;
end;
To receive JSON, TIdHTTP can either
return it as a String (decoded using a server-reported charset):
var
ReqJson: TStringStream;
RespJson: String;
begin
ReqJson := TStringStream.Create('json content here', TEncoding.UTF8);
try
IdHTTP1.Request.ContentType := 'application/json';
RespJson := IdHTTP1.Post(URL, ReqJson);
finally
ReqJson.Free;
end;
// use RespJson as needed...
end;
write the raw bytes to an output TStream of your choosing:
var
ReqJson: TStringStream;
RespJson: TMemoryStream;
begin
RespJson := TMemoryStream.Create;
try
ReqJson := TStringStream.Create('json content here', TEncoding.UTF8);
try
IdHTTP1.Request.ContentType := 'application/json';
RespJson := IdHTTP1.Post(URL, ReqJson, RespJson);
finally
ReqJson.Free;
end;
RespJson.Position := 0;
// use RespJson as needed...
finally
RespJson.Free;
end;
end;
The HTTP response code is available in the TIdHTTP.Response.ResponseCode (and TIdHTTP.ResponseCode) property.