Hi, Ernst Lippe <ernstl@xxxxxxxxx> writes: > A set of previews that you want to synchronize is an example of a > constraint based system where you want to solve a set of constraints > among multiple objects. The naive implementation of such a system is > to let each object synchronize with all others when its value is changed. > In general this is not a very good architecture: > * It is expensive, you need at least n * (n - 1) synchronizations. > * It frequently leads to oscillatory behaviour. > > As an example where you could get funny behavior, take two previews that > show the area around a certain point at different magnifications. > Assume that the user scrolls in preview A. Now A will update the > position of B. Because B is updated it will attempt to update > A's position. In all implementations that I can think of there > are choices for the scale factor such that the new position for > A is different from the position that was set by the user. > So A's position changes again and A will try to update B a > second time. Eventually, this will probably stabilize, but > when there are 3 previews with different magnifications there > are probably cases where the oscillations never stabilize. > > The standard solution for these problems is to have some > central arbitrator that makes global decisions for all objects. > > When you have a seperate signal for user operations this is > a nice hook for such an arbitrator. > It is of course possible to implement an arbitrator without these > signals but its implementation seems a lot messier. Probably > you would need some global arbitration flag and change the way > that "value-changed" signals are handled based on the value of > this flag. You would also have to be careful about subsequent > operations by the user before the arbitration computations > are finished. we usually solve this problem blocking the signal handlers when doing the update: http://developer.gnome.org/doc/API/2.0/gobject/gobject-Signals.html#g-signal-handlers-block-by-func This is IMO cleaner and simpler than adding an extra signal. Salut, Sven
