This command is used to bring the application
up to date
by entering the event loop repeatedly until all pending events
(including idle callbacks) have been processed.
If the idletasks keyword is specified as an argument to the
command, then no new events or errors are processed; only idle
callbacks are invoked.
This causes operations that are normally deferred, such as display
updates and window layout calculations, to be performed immediately.
The update idletasks command is useful in scripts where
changes have been made to the applications state and you want those
changes to appear on the display immediately, rather than waiting
for the script to complete. Most display updates are performed as
idle callbacks, so update idletasks will cause them to run.
However, there are some kinds of updates that only happen in
response to events, such as those triggered by window size changes;
these updates will not occur in update idletasks.
The update command with no options is useful in scripts where
you are performing a long-running computation but you still want
the application to respond to events such as user interactions; if
you occasionally call update then user input will be processed
during the next call to update.