Question

Suppose some callback is registered for stdin.

fileevent stdin readable thatCallback

This means that during the execution of the update command it will evaluate thatCallback time after time while there is input available at stdin.

How can I make thatCallback to be evaluated only once during each call of update?

Answer 1

For each use of update, any particular event handler will be executed only once. That’s because what update does is to check for which event handlers are runnable (and which events are actually queued) and then executes all the ready ones, once. It doesn’t wait for further events to happen.

Now, the fileevent will still be registered so that the next time the event loop is entered it will possibly be runnable again. If you want a once-only fileevent, de-register the handler while it is running. For example:

fileevent stdin readable thatCallback
proc thatCallback {} {
    # First, de-register ourselves so we're a one-shot
    fileevent stdin readable {}
    # Now do the rest of it here...
}