Sample code for events management

Question

I'd like to write a simple loop that catches and prints the incoming event while the script is running. I've tried this:

       (let ((key 0))
         (while (= key 0)
           (setq event (read-char nil nil 0))
           (if event (print event))))

The script runs but no event data is printed. After C-g all characters pressed were printed in the buffer. What's wrong?

Answer 1

Passing a value of 0 for the SECONDS arg of read-char means it will immediately return nil, always.

(And your code never changes the value of key, so it remains 0. So the iteration goes on forever, till you hit C-g. Maybe that's what you wanted, though.)

(defun foo ()
  (interactive)
  (let ((key 0))
    (while (= key 0)
      (setq event (read-char "Char: " nil 5.0))
      (if event (print event)))))

Look in buffer *Messages* to see the chars read and printed.

C-h f read-char says, about SECONDS:

If the optional argument SECONDS is non-nil, it should be a number specifying the maximum number of seconds to wait for input. If no input arrives in that time, return nil. SECONDS may be a floating-point value.