User Story

As a developer, I want to register functions that access the Function evaluation context, in order to read sheet information. This is useful if the function need to provide a result based on the sheet structure itself. Excel includes such functions like "indirect" "cell" or the search functions.

Additionally, I want to be able to register a function from Java code instead of relying on the ELEvalFunction resolver to find a function from zul / EL context.

Acceptance Criteria

A per-spreadsheet and / or a global way to register a FreeRefFunction into the function resolver.
This could be used with an object or a lambda such as:
in a local spreadsheet:
spreadsheet.getFunctionResolver().putFunction(String name, FreeRefFunction function)
in a global utility:
FunctionResolvers.putFunction(String name, FreeRefFunction function)

Details

Put document or specification reference
currently the org.zkoss.poi.ss.formula.udf.AggregatingUDFFinder will check for functions in the available _usedToolPacks when executing org.zkoss.poi.ss.formula.udf.AggregatingUDFFinder.findFunction(String)
The custom functions could be added in a new toolpack for a global registration.

The current UserDefined functions restrict the context passed to the function itself.
io.keikaiex.formula.ELEvalFunction.evaluate(ValueEval[], OperationEvaluationContext)
This method recieve the full EvaluationContext, but restrict it to only arguments, row and col when invoking the user defined function.

Workaround - mockup

The attached ZKUDFFinder class allow registration of a function from java code.
It can be used from a composer to add a new function to the function finder, which have access to the evalution context.

@Wire
	private Spreadsheet kk;
	
	@Override
	public void doAfterCompose(Component comp) throws Exception {
		super.doAfterCompose(comp);
		if(ZKUDFFinder.instance.findFunction("TESTFN") == null){
			FreeRefFunction fn = new FreeRefFunction() {
				@Override
				public ValueEval evaluate(ValueEval[] args, OperationEvaluationContext ec) {
					  ...
				}
			};
			((ZKUDFFinder) ZKUDFFinder.instance).putFunction("TESTFN", fn);
		}