Add support for displaying function signature information

[bwilkerson]

Add a new request, something like the following:

analysis.getSignature
request: {
  "id": String
  "method": "analysis.getSignature"
  "params": {
    "file": FilePath
    "offset": int
  }
}

response: {
  "id": String
  "error": optional RequestError
  "result": {
    "name": String
    "dartdoc": String
    "parameters": List<ParameterInfo>
  }
}

Return the signature information associated with the given location in the given file. If the signature information for the given file has not yet been computed, or the most recently computed signature information for the given file is out of date, then the response for this request will be delayed until it has been computed. If the content of the file changes after this request was received but before a response could be sent, then an error of type CONTENT_MODIFIED will be generated.

If a request is made for a file which does not exist, or which is not currently subject to analysis (e.g. because it is not associated with any analysis root specified to analysis.setAnalysisRoots), an error of type GET_SIGNATURE_INVALID_FILE will be generated.

If the location given is not inside the argument list for a function (including method and constructor) invocation, then an error of type GET_SIGNATURE_INVALID_OFFSET will be generated. If the location is inside an argument list but the function is not defined or cannot be determined (such as a method invocation where the target has type 'dynamic') then an error of type GET_SIGNATURE_UNKNOWN_FUNCTION will be generated.

Parameters

file ( FilePath )
The file in which signature information is being requested.

offset ( int )
The location for which signature information is being requested.

Returns

name ( String )
The name of the function being invoked at the given offset.

dartdoc ( String )
The dartdoc associated with the function being invoked. Other than the removal of the comment delimiters, including leading asterisks in the case of a block comment, the dartdoc is unprocessed markdown. This data is omitted if there is no referenced element, or if the element has no dartdoc.

parameters ( List )
A list of information about each of the parameters of the function being invoked.

Where ParameterInfo contains the name, type and kind (required, optional or named) of the parameter.

[DanTup]

This is almost perfect! Missing only one thing for me - the parameter index for where the cursor currently is. In Code this will be underlined (and presumably if it existed, it'd display the docs for that param, though for Dart that might not apply unless we have to put optional/name status there):

[DanTup]

@bwilkerson

In analyzer-discuss you said

Do let us know if it turns out to be too hard to implement it using another request, and we'll see whether we can increase the priority.

I did spend some time trying to make this work using getHover, but being able to figure out the correct location to send the request seemed complicated without a good AST. I was originally going to just walk back to the previous ( but obviously that failed quite quickly:

myFunc((1 * 2), (fr|ed));

I was also going to have a go at implementing this (by copying getHover) but I spent several days trying to find a way to make changes to the analysis server and build on my machine but failed (seemed like it needed an entire build of the SDK and it quickly seemed like more work than I really had time for, and many of the analyzer tests in the SDK wouldn't pass for me on Windows).

So, if there's any possibility of someone looking at this; it would be greatly appreciated. It's one of my biggest annoyances with Dart Code (and @sethladd noticed it missing pretty quick upon first trying out Dart Code!).

Alternatively; if you can think of a decent way I can get the correct position to send a getHover request from the data already available, I'm happy to give it another shot!

[sethladd]

I'd imagine this would be for more than Visual Code? Would all IDEs be able to utilize this?

[DanTup]

@sethladd I would've thought so, but nobody else appears to have asked for it! @devoncarew said atom doesn't have it so I'm surprised nobody has asked - maybe things work differently there, like you can still see the method you completed? I'm not sure (never used Atom).

There is an IDE that has this (IntelliJ IDEA) but that parses Dart itself so is able to figure this out more easily.

[Porl91]

@sethladd Are there any plans to add support for this? I've been running into the same issue recently.

[sethladd]

Sorry, I'm not sure what the plans are.

[DanTup]

@bwilkerson @scheglov How complicated do you think this would be for an outsider (me!) to implement if I can get the analyzer code building/running tests locally? I think it'd be a big improvement for Dart Code and I hope I'll be done with Flutter integration in the next few weeks so maybe can find some time.