Creating custom feature and related params

[frabert]

How do I define a custom feature in my server, and make it so that the request params are deserialized correctly? Currently I have something like the following, but params is still passed as a plain Object:

server = LanguageServer()

@attrs.define
def FooParams:
    foo_bar: str = attrs.field()

@server.feature("$/custom/feature")
def on_custom_feature(ls: LanguageServer, params: FooParams):
    # I'd like `params` to get deserialized correctly here
    pass

[alcarney]

In the current version of pygls there is the LanguageServerProtocol class, which among other things, provides the type definition that corresponds to a given message. (See here)

Subclassing LanguageServerProtocol allows you to add support for your own messages

from pygls.protocol import LanguageServerProtocol


@attrs.define
def FooParams:
    foo_bar: str = attrs.field()


# NOTE: This assumes that `$/custom/feature` is a notification message - rather than a request.
@attrs.define
class FooNotification:
    params: FooParams = attrs.field()
    method: str = "$/custom/feature"
    jsonrpc: str = attrs.field(default="2.0")


class MyProtocol(LanguageServerProtocol):

    def get_message_type(self, method: str) -> Optional[Type]:

        if method == FooNotification.method:
            return FooNotification

        return super().get_message_type(method)

    # NOTE: You might not need this method.
    #       This only applies if `$/custom/feature` is a request and that you want
    #       to be able to `return MyCustomResult()` from the handler.
    def get_result_type(self, method: str) -> Optional[Type]:
        return super().get_result_type(method)

To enable it, you need to then pass it as an argument to the LanguageServer

server = LanguageServer("my-server", "v1", protocol_cls=MyProtocol)

See also this guide if you need to extend pygls' default converter object to help it (de)serialize your types

Hope that helps! :)