Standardized 'watch'-like mechanism rather than separate wait_ functions

[dimo414]

Original report by Michael Diamond (Bitbucket: dimo414).

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watch isn't reasonable to use with Bash functions since it invokes the given command as a subprocess. It would be good to emulate a subset of its features in Bash, however, so functions like wait_ext and wait_port could be more standard (and other gems could add other similar utils).

[dimo414]

Proposal:

Users can define functions in the polling:: namespace, like polling::port_listening, and functions in the notify:: namespace like notify::desktop or notify::email. These can then be composed arbitrarily, and helper wrappers like wait_for or email_once can simplify common use-cases. Bonus if the callback functions can be tab-completable.

Examples:

# Invoke notify::echo once polling::port_listening succeeds, indicating port 80 is up
$ util::watch echo port_listening 80

# Helper for blocking in the current shell (basically just wrapping `util::watch echo ...`)
$ wait_for url http://google.com

# Send an email once task is complete; doesn't block
# wraps util::watch notify::email within a disown ... & subshell
$ email_once process_complete 1234