[Gtk-sharp-list] NodeStore and lazy loading

[Ben Maurer]

> I don't believe this is wrong: from an MVC point of view, the model
> shouldn't know/care about which branches are visible or not. Ideally
> you'd just add an event handler for the expand/collapse events and
> *tell* the model to load the specific branch for real.
>
> This is also how the Swing tree widget works btw (for reference, see:
> http://java.sun.com/docs/books/tutorial/uiswing/events/treeexpansionlistener.html)

That's one way of doing it, and it is doable with the swing api. But
consider this from the java documentation:

http://java.sun.com/products/jfc/tsc/articles/jtree/src/FileSystemModel.txt

The lazy load structure is implemented without any need to look at the
view. IMHO, this method is easier to reuse (the code for using the model
is alot easier). Also, we could do some fancy things this way. Imagine we
have a property in TreeNode

protected virtual bool ShouldLoadAsync { get; }

A class could override this to say "hey, I will be slow to load". The
person coding the tree view could set a property (or maybe we just do it
by default) that would cause the hourglass cursor and a new tree node that
says "loading...' to be created when such nodes are created.

The "wait for a listener" model is also messy because non-leaf nodes need
a dummy child. As a consequence of this, people have two options: they can
either load all the children of visible nodes. This can hurt performance,
but is often simpler to implement. For example, monodoc does this. The
other option is to create a dummy child node. This is messy code wise, but
helps out performance alot. With the swing model in the above url, one
does not even need to think about this.

-- Ben