Reference-counted AreaHandlers for interactive canvas scenarios

[shankuniyogi]

I'm using this library to implement widgets for a STEM toolkit I'm building. It's perfect, super simple, and has just what I need. Thank you!

Some of my components require both custom painting and interaction, but when I started to use Area and AreaHandler, I ran into some limitations. The AreaHandler object passed into Area::new is boxed, and there's no way to access that object from a reference to an Area, so its state can't be changed after the area is created.

let meter = Box::new(Meter { value: 100.0 });
let area = Area::new(&ui, meter);
// Can't change it anymore!
meter = 50.0;

In my fork, I've modified area.rs to also support reference-counted handlers (Rc<RefCell<dyn AreaHandler>>).

let meter = Rc::new(RefCell:new(Meter { value: 100.0 }));
let area = Area::new(&ui, meter.clone());
// Can change this any time.
meter.borrow_mut().value = 50.0;
area.queue_redraw_all(&ui);

The change still supports boxed handlers, and requires no changes to existing code - the existing canvas example runs perfectly - but I've also created a new example that shows how to build an interactive canvas with a ref-counted handler. The handler also implements a changed event that can be hooked by the application. In effect, someone could use this technique to build simple controls on top of an Area.

Would you be interested in a PR to add this change? I'd be happy to do the work to get it PR-ready.

Thanks again for this project!

[duffrecords]

Here's an idea for anyone in need of an immediate workaround. What I do is create a Tokio watch channel and pass the receiver half of the channel as a parameter in the new Area. Then I can update it through the sender half at any time. I spawn any tasks that need to run independently of the UI and then run a main loop that repeatedly evaluates a Tokio select macro. It has a branch for receiving state from each spawned task and a default branch that sleeps asynchronously for a few milliseconds and sends these updates to the Area before calling queue_redraw_all().