[Sugar-devel] Communication between two Activities

[James Cameron]

On Wed, May 20, 2015 at 10:43:23AM -0300, Gustavo Duarte wrote:
> Notificator: There is a Sugar Activity who shows Notification messages
> for the kid.
> 
> Daemon: There is an script running on cron, who get new notification
> from a Server.
> 
> I need communicate the Daemon with the Notificator, so when Daemon
> receive a new notification from Server, it'll send a signal to
> Notificator.
> 
> Notificator when receive the signal, should refresh a widget.
> 
> I know a way to do that, using system signals, but i wonder if Sugar
> has another way to achieve that.

What you describe is interprocess communication.

Sugar uses several methods for communication with system daemons.

The most frequently coded is D-Bus.
http://en.wikipedia.org/wiki/D-Bus

Other methods in Python are
https://docs.python.org/2/library/ipc.html

You can use files, as Gonzalo says, but try to place them in a path
that does not cause writes to disk; e.g. /tmp ... otherwise sometimes
the kernel will reschedule the process that is writing, and this can
slow performance.

My preference for your scenario is signals:

1.  Notificator calls os.getpid and writes to /tmp/notificator.pid

2.  Notificator calls signal.signal to request a callback on SIGUSR1,

3.  Daemon reads /tmp/notificator.pid and calls os.kill(pid, SIGUSR1),
kill(1), or kill(3) to send signal.

An example here:
http://dev.laptop.org/git/projects/runin/tree/runin-gtk

Signals become more complex if the communication is complex, and there
are a limited number of different signals.  That's when I would
suggest D-Bus.

-- 
James Cameron
http://quozl.linux.org.au/