On 26 Feb 2003 17:29:37 +0100
Sven Neumann <[EMAIL PROTECTED]> wrote:

> Hi,
> Ernst Lippe <[EMAIL PROTECTED]> writes:
> > The point is the following. In the current implementation the preview
> > widget consists of the following components from top to bottom: the image, 
> > the progress bar and the zoom controls. When scrollbars should be added
> > the horizontal scrollbar should be located between the progress bar and
> > the image. So it is not possible to add scrollbars by simply wrapping
> > the entire current preview but the preview itself must be modified.
> > Adding scrollbars makes the layout algorithm more complex.
> IMO the preview widget should only be the preview, nothing else. If
> possible it should expose two adjustments so that you can easily add
> scrollbars. Progress-bar, zoom-control and scrollbars don't belong to
> the preview widget itself. They can be added by a composite widget.

That is already in my current design. There is a bare preview widget
that can do scaling and scrolling but has no GUI for these operations.
There is another composite widget that can include a progress bar and
zoom buttons. Having a standard composite widget makes life easier for
developers and gives more uniformity among the plug-ins.

> > > Actually if you go for two adjustments and
> > > expose them in your API you don't need to deal with signals at all
> > > since it should be sufficient to connect to the "value_changed" and
> > > "changed" signals of the two adjustments.
> >
> > The reason for a separate signal is that this makes it possible to
> > distinguish between between modifications that are initiated by the
> > user via the preview GUI and modifications that were initiated
> > programmatically via the API.  When this distinction is never
> > important the requirement could be dropped.
> Why should a widget behave differently if changed by the user or
> programmatically via the API? That sounds like a broken concept.

Why? This is simply a method to get a hook for trapping operations that
the user has performed on the GUI of the widget. 

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.

> > > > Requirement 12. The preview must support an API to scroll the preview
> > > > and change the magnification.
> > > > 
> > > > This functionality is needed to synchronize multiple previews.
> > > 
> > > and again you get this all for free if you go for two adjustments.
> > > Synchronizing two previews would boil down to synchronizing the
> > > preview's adjustements.
> > 
> > When you have an API call to change both coordinates at the same time,
> > this makes it easier to avoid unnecessary refreshes, otherwise the widget
> > might try to refresh itself between modification of the first and second
> > coordinate. 
> you will have to delegate the actual refresh to idle functions anyway
> so that shouldn't be a problem.
I've worked quite a bit with Delphi that extensively uses its own variant
 of signals and I found there that is was very usefull to wait with sending
signals when you were updating multiple variables. Anyhow, this is more
a design than a requirements discussion.

BTW, I will leave the requirement as it stands, exposing adjustments to
for scrolling and zooming is an API.



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