Google app script monitor spreadsheet selected ranges

Question

I want to write a app script that can get the selected cells

and show it on the html input text.

example:

when I selected A1 cell, then the input text wi

Answer 1

New event added in April 2020:

Ref: https://developers.google.com/apps-script/guides/triggers#onselectionchangee

/**
 * The event handler triggered when the selection changes in the spreadsheet.
 * @param {Event} e The onSelectionChange event.
 */
function onSelectionChange(e) {
  // Set background to red if a single empty cell is selected.
  var range = e.range;
  if(range.getNumRows() === 1 
      && range.getNumColumns() === 1 
      && range.getCell(1, 1).getValue() === "") {
    range.setBackground("red");
  }
}