group layers design

[Bart Coppens]

On Saturday 24 December 2005 12:00, Gábor Lehel wrote:
> - KisLayer has methods prevSibling() and nextSibling() -- I think it'd
> be excessive and inefficient (and require housekeeping to keep it all
> consistent) to have m_prevSibling and m_nexSibling to store these,
> when you could just find out via the parent(); but this needs parent()
> to be of a type that's guarenteed to have the info (either m_layers
> accessible, or KisLayerSP itemAtIndex(int i) / int
> indexOfItem(KisLayerSP), or something of the sort).
Well whereas I don't really oppose a doubly linked list internally, I can see 
the point that it's indeed some housekeeping. But the point was made over irc 
that just having a list of layers in the parent would make this quite 
inefficient. That's why maybe we should add some kind of methods index() + 
setIndex(int) to KisLayer, that return and set the position of the layer in 
its parent layer. This way, KisGroupLayer can, in constant time if it uses a 
regular vector, return next and previous siblings of the layer. This also 
makes sense from a design point of view, since _all_ layers are the child of 
some other layer and thus have an index in it (except, of course, the root 
layer, but that's a special case anyway).

> - Only KisGroupLayers can have child layers; this means no
> add/removeLayer() methods in KisLayer. If you have a KisLayer and want
> to add sublayers, you have to dynamic_cast. parent()s of layers are
> forced/guarenteed to be KisGroupLayers. [...]
This seems like the most clean solution at the moment without further 
complicating matters. If need be we can later on always add some kind of 
interface class 'KisChildHavingLayer' or so, having the needed methods, which 
KisGroupLayer and KisVWLT could both derive from. Then we just say that every 
layer is the child of such a class, and be done with it.

> - As above, except also have virtual bool supportsChildLayers(), which
> you can check before adding child layers.
I think this is a good idea in any case, since you could use this anyway when 
you need to move layers around in the layerbox and would like to find out if 
you can move a layer to be the child of another. Constantly dynamic_casting 
wouldn't be needed then with this function, so that it would look a bit 
cleaner.

Bart Coppens