Example Rust widget library design

widgets.rs

use std::cell::RefCell;
use std::rc::Rc;

trait Widget {
	fn add(&self, w: Rc<Widget>) -> Rc<Widget> { w }
	fn children(&self) -> Vec<Rc<Widget>> { vec!() }
	fn type_name(&self) -> &'static str;

	fn print_tree(&self, depth: i32) {
		for _ in 0..depth { print!("  "); }
		println!("{}", self.type_name());
		for c in self.children() { c.print_tree(depth + 1); }
	}
}

// Button

struct Button;

impl Button {
	fn new() -> Rc<Button> { Rc::new(Button) }
}

impl Widget for Button {
	fn type_name(&self) -> &'static str { "Button" }
}

// HBox

struct HBox {
	children: RefCell<Vec<Rc<Widget>>>,
}

impl HBox {
	fn new() -> Rc<HBox> { Rc::new(HBox { children: RefCell::new(vec!()) }) }
}

impl Widget for HBox {
	fn add(&self, w: Rc<Widget>) -> Rc<Widget> { self.children.borrow_mut().push(w.clone()); w }
	fn children(&self) -> Vec<Rc<Widget>> { self.children.borrow_mut().clone() }
	fn type_name(&self) -> &'static str { "HBox" }
}

// An example widget hierarchy

fn main() {
	let mut hbox = HBox::new();
	hbox.add(Button::new());

	let mut hbox2 = hbox.add(HBox::new());	
	hbox2.add(Button::new());

	hbox.print_tree(0);
	
	// Prints:
	//
	// HBox
	//   Button
	//   HBox
	//     Button
}