On 01 Mar 2003 17:39:56 +0100
Sven Neumann <[EMAIL PROTECTED]> wrote:

> Hi,
> Ernst Lippe <[EMAIL PROTECTED]> 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.

But blocking the signals on a GtkAdjustment only prevents the propagation of
signals, it does not prevent an update of the underlying value. When
the signal handlers are not sufficiently fast it is possible that the
user has scrolled or zoomed while the signal handlers were blocked.
In that case the underlying adjustment of the preview in which the user
performed the operation will be updated but other components will not
be notified. With scrolling this might be acceptable but with zooming
the differences in magnification between multiple, supposedly synchronized,
previews are very obvious. The only fundamental solution would be to block all
updates by the user to each preview adjustment. I don't think that that
this can be done on the adjustements themselves, the only solution I can
see at the moment is to "freeze" all GUI components that could modify
these adjustments. 


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