Re: [gtk-list] (Tricky?) questions about GTK widget hierarchy and GNOME canvas

[Federico Mena Quintero <federico redhat com>]

>  It seems that GTK API allows that, but will the widget behave as
>  expected (e.g., can I have two "views" of that widget on the screen
>  simultaneously).

Nope.  A widget can only have one parent.  You have to use separate
widgets for each page.

>  In my library I have cells (what they call "cell masters" in CAD) and
>  they can be used in definitions of other cells (each such use is a
>  "cell instance"). I thought about reading each cell into its own
>  GnomeCanvasGroup (with natural instantiating of a sub-cell's Group
>  within a higher-level cell). For cell editing I want to flatten a
>  Group one level into a Canvas (with all sub-cells/Groups visible while
>  only sub-Group as a whole is manipulated).
>  
>  This possibility seems to be mentioned in GnomeCanvas.h:
>  
>   * Consider a circuit editor application that uses the canvas for its schematic
>   * display.  Hierarchically, there would be canvas groups that contain all the
>   * components needed for an "adder", for example -- this includes some logic
>   * gates as well as wires.  You can move stuff around in a convenient way by
>   * doing a gnome_canvas_item_move() of the hierarchical groups -- to move an
>   * adder, simply move the group that represents the adder.
>  
>  BUT: Can we have several identical adders on one canvas? 

Yes.  They are just different canvas items.  They are separate Gtk+
objects, too, just like widgets; a canvas item can only have one
parent group.

>       Can we have several identical adders on several canvases?

Yes, but again, they have to be different objects.

>       Can they be rotated/mirrowed/etc. independently?

Yes.  Each canvas item has its own independent affine transformation.

Caveat:  affines and the antialiased canvas are rather broken right
now.  I am fixing it in the CANVAS_CLEANUP branch of gnome-libs; if
you would like to keep track of that work, please get that branch from
cvs as well.

>      How do I create a CanvasGroup without a parent then attach it to
>  whatever parent(s) it is instantiated in? 

You can't.  Canvas items must always have a parent.  This means you
must create your item hierarchy in a top-down fashion.

>  And, finally the last easy one: How to I walk through all objects
>  attached to a Canvas?

Eeewww.  You could walk the group->item_list structures, but that is
ugly.

Normally you should not need this, though.  The canvas is a display
engine.  It is not designed to work as your primary data structure.
The idea is that you keep your own data structures, and they keep
pointers to the canvas items that form their visual representation.

  Federico