Cleaning up Messaging

Over time, libavg has accumulated support for a number of message callbacks. These are:

In addition, we’re currently adding some widget classes, and that adds more callbacks for button presses, list scrolling, etc.

While this allows you to get a lot of things done, it’s not consistent and hence not very easy to learn. The methods used to register for messages aren’t standardized. They have inconsistent names and varying parameters. Some allow you to register several callbacks for an event, some don’t. For an example, compare Node.connectEventHandler() to the gesture interface using constructor parameters. In addition, the implementation is just as problematic. We have multiple callback implementations in C++ and Python, which results in error-prone, high-maintanance code.


When work on the new widget classes promised to make things even more convulted, we decided to do something about the situation and implement a unified, consistent messaging system. The result is a publisher-subscriber system:

  • Publishers register MessageIDs.
  • Anyone can subscribe to these MessageIDs by registering callbacks. Several subscribers are possible in all cases.
  • When an event occurs, all registered callbacks are invoked.

We spent quite a bit of time to make a lot of things “just work”. The subscription interface is very simple. As an example, this is how you register for a mouse or touch down event:

node.subscribe(node.CURSOR_DOWN, self.onDown)

Any Python callable can be registered as a callback, including standalone functions, class methods, lambdas and even class constructors. In most cases, you don’t have to deregister messages to clean up either. Subscriptions are based on weak references wherever possible, so when the object they refer to disappears, the subscription will just disappear as well.

You can write your own publishers can be written in Python or C++ by simply deriving from the Publisher class. In Python, you need two lines of code to register a message:

class Button(avg.DivNode):
    CLICKED = avg.Publisher.genMessageID()
    def __init__(...):

and this line invokes all registered subscribers:

self.notifySubscribers(self.CLICKED, [])

The second parameter to notifySubscribers is a list of parameters to pass to the subscribers.


Transitioning old programs to the new interface is not very hard and involves replacing old calls to Node.connectEventHandler(), VideoNode.setEOFCallback(), Contact.connectListener() and so on with invocations of subscribe(). We’ll keep the old interfaces around for a while, but they’ll probably be removed when we release ver 2.0.

The End of Touch Jitter

On lots of multitouch devices, input suffers from jitter: The actual touch location is reported imprecisely and changes from frame to frame. This has obvious negative effects, since it’s much harder to hit a target this way. For years, people have been telling me that a lowpass filter would help. In its simplest form, a lowpass filter averages together the location values from the last few frames. This removes most of the jitter – because the jitter is random, there’s a good chance that the errors in successive frames cancel each other out. On the other hand, it adds latency because the software is not using the latest data. This tradeoff didn’t seem like a good one to me, so I didn’t add a jitter filter to libavg.

However, at this year’s CHI conference, Géry Casiez and coauthors published a paper on a 1€ Filter. This filter is based on an extremely simple observation: Precise positions are only important when the user is moving his finger slowly, while latency is important at fast speeds. So, the solution to the dilemma I described in the first paragraph is to build a filter that adjusts its latency depending on speed. Their filter is extremly simple to implement, and the results are really nice.

libavg can now process the touch input positions using this filter. The filter parameters are configurable in avgrc, and there’s a configuration utility ( that helps in finding correct filter values. The complete implementation is in the libavg_uilib branch – I’ll merge it to trunk in the next few weeks.