Custom functions workaround for Google Workspace add-ons - google-apps-script

Question

I'm looking for a workaround to the limitation that custom functions are not available in Google Workspace Add-ons. As proven by #Rubén in this SO answer.
I am building a Google Workspace Add-on. I want to get data from a specified range in the Spreadsheet, then run a function in the code, then output the data back to the user in the Spreadsheet.
I'm guessing I might use SpreadsheetApp.getActiveRange() to facilitate the spreadsheet data interactions. Something similar to the following pseudocode.
Pseudocode
const sourceRange = SpreadsheetApp.getActiveRange();
// do stuff
const destinationRange = foo;
destinationRange.setValues( bar, );
Is this a viable path forward?

Answer 1

Yes, you might use code like the shown in the question in Workspace add-ons for Google Sheets to make that your add-on UI interacts with the active spreadsheet.
The below code snippet is a complete helper function used in sample Workspace Add-on provided by Google Developers. It was taken from Translate text from Google Docs, Sheets, and Slides.
Breadcrumb: Samples by project type > Workspace Add-ons > Translate text
Specific URL: https://developers.google.com/apps-script/add-ons/translate-addon-sample#code.gs.
Please note that it uses
var ranges = SpreadsheetApp.getActive().getSelection().getActiveRangeList().getRanges();
/**
* Helper function to get the text of the selected cells.
* #return {CardService.Card} The selected text.
*/
function getSheetsSelection(e) {
var text = '';
var ranges = SpreadsheetApp.getActive().getSelection().getActiveRangeList().getRanges();
for (var i = 0; i < ranges.length; i++) {
const range = ranges[i];
const numRows = range.getNumRows();
const numCols = range.getNumColumns();
for (let i = 1; i <= numCols; i++) {
for (let j = 1; j <= numRows; j++) {
const cell = range.getCell(j, i);
if (cell.getValue()) {
text += cell.getValue() + '\n';
}
}
}
}
if (text !== '') {
var originLanguage = e.formInput.origin;
var destinationLanguage = e.formInput.destination;
var translation = LanguageApp.translate(text, e.formInput.origin, e.formInput.destination);
return createSelectionCard(e, originLanguage, destinationLanguage, text, translation);
}
}