The tree_source example is problematic

[rmartin16]

Describe the bug

The tree_source example creates a lazy-loading tree view of the current directory.

1. When used with Briefcase (dev or run), the cwd is $HOME.....the app loaded 10GB in to memory before i killed it
2. It doesn't seem to actually be lazy.....it tried to read my entire home directory before showing the window
3. It completely bombs out on a node if anything causes an OSError...for instance, a broken symlink in my home dir

Steps to reproduce

Run briefcase dev for the tree_source example

Expected behavior

The directory used for the tree is enforced to be the directory of the app and it is actually lazy.

Screenshots

No response

Environment

• Operating System: pop os 22.04
• Python version: 3.10.12
• Software versions:
  • Briefcase: 0.3.15.dev361+g02f412be.d20230619
  • Toga: 0.3.2.dev383+g7d68e74d.d20230619

Logs

No response

Additional context

No response

[rmartin16]

Additionally, the tree doesn't seem to load anything on Windows and there isn't any visible error. I can't identify the breakdown though.

[freakboy3742]

The breakdown is that tree isn't implemented on Windows :-)

Or, rather, the implementation is known to be broken, because Winforms doesn't support multiple columns of content in a Tree. Some of the recent discussions about a "Simple Tree" widget extend from this limitation.

[freakboy3742]

Ok - so... the tree_source demo does do lazy loading (at least, as of #2017)... but that's only as good as the backend implementation.

Cocoa's implementation uses a view on the data source - it only renders what is visible, and asks for data on a cell-by-cell basis. Cells aren't expanded by default, and expandability is determined by the can_have_children attribute, which is determined based on whether the node is a file or a directory. The children aren't evaluated until you try to access the list of children, and this doesn't happen until a child is actually rendered.

GTK builds a full tree data model when it is loaded - which means creating all the children. #1978 has flagged that GTK has deprecated Gtk.TreeStore in favour of Gtk.ListModel, and from a casual inspection, it looks like that is more amenable to an "interface" based implementation; this might provide a way to fix the problem by making the GTK ListModel feeding the tree an API wrapper around the Toga TreeSource.