Wandering about for the first time in the Json classed of Berlin, I have a few questions from a noob perspective,…

Wandering about for the first time in the Json classed of Berlin, I have a few questions from a noob perspective, which I hope someone with experience can enlighten me on.

I am implementing the JsonRPC protocol, where a request basically looks like this:

{

“jsonrpc”:”2.0″,

“method”:”JSon.MethodName”,

“params”: {

“aIntParam”: 100,

“aStrParam”:”A string”

},

“Id”: “UniqueIdValue”

}

I.e. The method name may vary, and with it the “params” – or implicitly, the type of the JSon object that recides in “params”. The challenge here is that I don’t know the type, until I have identified the content of “method”.

Using TJSon, it is trivial to get the value – but relatively costly, since it will also parse “params” – which can be large and complex.

// property fields omitted for brevity

class TJSonRPC = class

public

property jsonrpc: String;

property method: String;

property params: String;

property id: String;

end;

And if I parse

var

rpc: TJsonRPC;

begin

rpc := TJSon.JsonToObject(ReceivedJsonString);

“rpc” will contain the correct string values in the “jsonrpc”, “method” and “id” fields – while “params” will be an empty string, since it was an unknown object type.

I use “method” to look up the associated “params” type, as well as the handler methods. Now I have to reparse the whole thing again – instead of avoiding the first parsing of params, and then only parsing params with the right type later.

So, after this long intro – my question is: Can I persuade TJSon to treat “params” as it was a string and not an object for the TJsonRPC class?

8 thoughts on “Wandering about for the first time in the Json classed of Berlin, I have a few questions from a noob perspective,…


  1. JSON is not very good at polymorphism in strongly typed languages (no surprise given it originated in java script).


    I would put a wrapper around the json object to get the fixed fields and return the params as json value to be able to deal with it.

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  2. Edit: Paul TOTH Since I don’t know the class at receiving the JsonRPC message, that’s not possible.


    Wait a sec – I think I misread you. I’ll have a look at the custom Converter.

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  3. Stefan Glienke Currently I use a class without params to parse the package, before parsing again with params when I know the object type, but the TJson class will always parse the entire structure.


    I could make a pseudo wrapper that just do a Pos(‘”method”:’, aJSonString) to find the method fast, but it is a bit dirty, and potentially easy to break with spacing or case.

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  4. If you use the mORMot serializer, you can just define the “Params” property as RawJSON type, and it will do what you expect. Or define it as a variant, and it will be filled with a TDocVariant custom type, which you may resolve easily with late-binding, typing directly Params.aIntParam or Params.aStrParam in your code – so Stefan Glienke you can have a weakly typed fields in Delphi. But not with the RTL JSON library, for sure.

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