example.rs

use druid::{AppLauncher, PlatformError, Widget, WidgetExt, WindowDesc};

use druid::{
    widget::{Flex, Label, Painter, TextBox},
    Color, Data, Lens, RenderContext,
};

/// state data.
/// `string` is updated via the `TextBox`
#[derive(Clone, Debug, Data, Lens)]
pub struct State {
    string: String,
}

impl State {
    fn good(&self) -> bool {
	is_even(self.string.len())
    }
}

/// builds a widget to show path information.
pub fn path_widget() -> impl Widget<State> {
    let painter = Painter::new(|ctx, state: &State, _env| {
        let bounds = ctx.size().to_rect();
	let color = if state.good() {
            Color::rgb(0, 1, 0)
        } else {
            Color::rgb(1, 0, 1)
        };
        // draw the border
        ctx.stroke(bounds.inset(-0.5), &color, 2.0);
    });

    let textbox = TextBox::new()
        .with_placeholder("./directory")
        .expand_width()
        .lens(State::string)
        .padding(4.0)
        .background(painter);

    // a dummy label that triggers the expected draw
    let state_label = Label::dynamic(|state: &State, _| {
        if state.good() {
            String::from("good state!")
        } else {
            String::from("not good :(")
        }
    });

    Flex::column().with_flex_child(textbox, 1.0)
    // uncomment this line to fix the drawing!
    //.with_flex_child(state_label, 1.0)
}

fn main() -> Result<(), PlatformError> {
    simple_logger::init().expect("unable to initialize logger");

    let ui_builder = path_widget;

    // new state at root
    let state = State::from("/");

    AppLauncher::with_window(WindowDesc::new(ui_builder)).launch(state)?;
    Ok(())
}

// details
// -------------------------------------------------
fn is_even(n: usize) -> bool {
    n & 1 == 0
}

impl<S: Into<String>> From<S> for State {
    fn from(s: S) -> State {
        let string = s.into();
        State { string }
    }
}

impl Default for State {
    fn default() -> State {
        State::from("starting string")
    }
}