How to listen to cell execution?

I would like to perform some action in the UI of the extension when the user executes a cell with a particular piece of code. How do I listen to such events and take action accordingly in my extension code?

Pre or post? Might not be the cleanest code (and I parsed the args two different ways apparently)

Scheduled -

NotebookActions.executionScheduled.connect((_, args) => {
  let cell: Cell;
  let notebook: Notebook;
  notebook = args["notebook"];
  cell = args ["cell"];

Post Execution

NotebookActions.executed.connect((_, args) => {
  const { cell, notebook, success } = args;