Determining context at a position in file using ANTLR4 - parsing

Question

I'm trying to write a Language Extension for VS Code in JavaScript and I seem to be missing something.
I have a Lexer.g4 and Parser.g4 for my language and can generate a tree using them.
My issue is that the VS Code API gives me a document and a position in that document (line #, character #). From any of the examples I've looked at for ANTLR4 I can't seem to find any functions generated that take a position in the file and give back the nodes of a tree at that position.
I want to know, for example that the cursor is placed on the name of a function.
Am I supposed to be walking the entire tree and checking the position of tokens to see if they enclose the position I'm in in the editor? Or maybe I'm not using the right tool for the job? I feel like I'm probably missing something more fundamental.

Answer 1

Yes, you have to walk the parse tree to find the context at a given position. This is a pretty simple task and you can see it in action in my ANLTR4 exension for vscode. There are multiple functions returning something useful for a given position. For instance this one:
/**
* Returns the parse tree which covers the given position or undefined if none could be found.
*/
function parseTreeFromPosition(root: ParseTree, column: number, row: number): ParseTree | undefined {
// Does the root node actually contain the position? If not we don't need to look further.
if (root instanceof TerminalNode) {
let terminal = (root as TerminalNode);
let token = terminal.symbol;
if (token.line != row)
return undefined;
let tokenStop = token.charPositionInLine + (token.stopIndex - token.startIndex + 1);
if (token.charPositionInLine <= column && tokenStop >= column) {
return terminal;
}
return undefined;
} else {
let context = (root as ParserRuleContext);
if (!context.start || !context.stop) { // Invalid tree?
return undefined;
}
if (context.start.line > row || (context.start.line == row && column < context.start.charPositionInLine)) {
return undefined;
}
let tokenStop = context.stop.charPositionInLine + (context.stop.stopIndex - context.stop.startIndex + 1);
if (context.stop.line < row || (context.stop.line == row && tokenStop < column)) {
return undefined;
}
if (context.children) {
for (let child of context.children) {
let result = parseTreeFromPosition(child, column, row);
if (result) {
return result;
}
}
}
return context;
}
}