Dynamically registering reactive handlers at runtime

[Stuart Bishop]

> The possible values for package-names are not known during build time. We do
> know that _all_ queued packages have to be installed. Would it be possible
> to have an `apt.installed.all` state?

@when_not('apt.queued_installs') tells you this.

@when_not('packages_queued')
def queue_package_installations():
    apt.queue_install(config()['packages'].split())
    reactive.set_state('packages_queued')

@when('packages_queued')
@when_not('apt.queued_installs')
def do_setup():
    # packages installed. Lets do this
    [...]


If you can calculate the packages at import time, you could also do this:

package_states = ['apt.installed.{}'.format(p) for p in
config()['packages'].split()]

@when(*package_states)
def do_setup():
    # Packages installed. Lets do this
    [...]


And for more potential solutions, the documentation for the apt layer
has been updated which states you can call charms.apt.install_queued()
yourself to install queued packages right now. It does nothing if
nothing is queued, so you could just call it at the start of your
handler instead of waiting on an apt.installed.* state at all.

-- 
Stuart Bishop <stuart.bishop at canonical.com>