Development Team/Almanac

The tasks below are random useful techniques that have come up as I write code and documentation for this reference. They have yet to be categorized, but will be as a sufficient set of related entries are written.

How do I know when my activity is "active" or not?

You can set an event using the VISIBILITY_NOTIFY_MASK constant in order to know when your activity changes visibility. Then in the callback for this event, you simply compare the event's state to gtk-defined variables for activity visibility. See the GDK Visibility State Constants section of gtk.gdk.Constants for more information.

        # Notify when the visibility state changes by calling self.__visibility_notify_cb
        # (PUT THIS IN YOUR ACTIVITY CODE - EG. THE __init__() METHOD)
        self.add_events(gtk.gdk.VISIBILITY_NOTIFY_MASK)
        self.connect("visibility-notify-event", self.__visibility_notify_cb)
    ...
    # Callback method for when the activity's visibility changes
    def __visibility_notify_cb(self, window, event):
        if event.state == gtk.gdk.VISIBILITY_FULLY_OBSCURED:
            print "I am not visible"
        elif event.state in [gtk.gdk.VISIBILITY_UNOBSCURED, gtk.gdk.VISIBILITY_PARTIAL]:
            print "I am visible"

How do I get the amount of free space available on disk under the /home directory tree?

The following function uses the statvfs module. The following code demonstrates how to get the total amount of free space under /home.

    #### Method: getFreespaceKb, returns the available freespace in kilobytes. 
    def getFreespaceKb(self):
        stat = os.statvfs("/home")
        freebytes  = stat[statvfs.F_BSIZE] * stat[statvfs.F_BAVAIL]
        freekb = freebytes / 1024
        return freekb

Note, however, that assuming anything about "/home" is a bad idea, better use os.environ['HOME'] instead. Rainbow will put your actual files elsewhere, some on ramdisks, some on flash. Be clear about which filesystem's free space you actually care about.

How do I know whether my activity is running on a physical XO?

Sugar runs on ordinary computers as well as on XO's. While your activity is typically going to be run on a real XO, some people will indeed run it elsewhere. Normally you shouldn't write your activity to care whether it's on an XO or not. If for some odd reason, you need to care, the easiest way to tell if you are on a physical XO is to check whether /sys/power/olpc-pm, an essential power management file for the XO, exists. ^[1] ^[2]

import os
...
      #Print out a boolean value that tells us whether we are on an XO or not. 
      print os.path.exists('/sys/power/olpc-pm')

How do I know the current language setting on my XO?

The system variable 'LANG' tells you which language is currently active on the XO. The following code shows how to look at the value of this variable.

import os
...
       _logger.debug(os.environ['LANG'])

How do I repeatedly call a specific method after N number of seconds?

The gobject.timeout_add() function allows you to invoke a callback method after a certain amount of time. If you want to repeatedly call a method, simply keep invoking the gobject.timeout_add function in your callback itself. The code below is a simple example, where the callback function is named repeatedly_call. Note that the timing of the callbacks are approximate. To get the process going, you should make an initial call to repeatedly_call() somewhere in your code.

You can see a more substantive example of this pattern in use when we regularly update the time displayed on a pango layout object.


	#This method calls itself ROUGHLY every 1 second 
	def repeatedly_call(self):
		now = datetime.datetime.now()
		gobject.timeout_add(self.repeat_period_msec, self.repeatedly_update_time)

How do I update the current build version of code that is running on my XO?

There are several pages that give you instructions on how to install/update your current build.

If you already have a working build installed and an internet connection, first try olpc-update.
If that doesn't work, you can look at instructions for an Activated upgrade that can be done via USB] boot.

As the instructions on the pages linked above note, make sure to install your activities separately after you have upgraded to a specific base build.

I am developing on an XO laptop, but my keyboard and language settings are not ideal. How can I change them?

Internationalized laptops will often have settings that might slow you down while developing. To change around the language settings so you can better understand environment messages, use the Sugar Control Panel

Keyboard settings on internationalized laptops^[3] can also be suboptimal, especially as characters like "-" and "/" are in unfamiliar positions. You can use the setxkbmap command in the Terminal Activity to reset the type of keyboard input used and then attach a standard U.S. keyboard that will allow you to type normally. The command below sets the keyboard to the US mapping (it will reset to the default internationalized mapping upon restart).

setxkbmap us

My Python activity wants to use threads; how do I do that?

Early versions of Sugar (pre-"Update.1" release) would automatically configure Python, gobject, and glib for threaded operation. Due to the overhead imposed on every activity by those initializations, Sugar no longer does that. If you are writing an activity for a later release, and you want threads to work, start off your activity (just after the copyright comments) with:

import os
import logging

# Sugar does this but only in pre-update.1.
import gobject
gobject.threads_init()
#import dbus.mainloop.glib
#dbus.mainloop.glib.threads_init()

Comment out, or uncomment, the glib threads initialization, depending on whether you need it. (I don't know how to tell if you need it, except by trying it both ways.)

Then follow that with the rest of your imports (such as "import gtk"). In my activity (SimCity), the pygame sound player would not produce sound reliably unless I did this.

In SimCity, I have tested this to make sure it works in the *old* XO software releases as well as the new ones. The documentation for these threads_init functions is really bad, and doesn't say whether you can call them twice, nor does it say how you can tell if they've already been called so you could avoid calling them twice. Apparently it's pretty harmless in release 650.

Notes

[1] reliably detecting if running on an XO

[2] OLPC Power Management Interface

[3] Keyboard layouts#OLPC keyboard layouts

[1]

[2]

[3]