A helper mod for the Rust Driuid GUI library, to bypass the AppDelgate in order to mutate data on a data change event

on_change_controller.rs

use druid::{widget::Controller, Data, Env, Event, EventCtx, Selector, UpdateCtx, Widget};

/// Bypass the `AppDelegate` entirely by appending the return from this function as a
/// `Controller` wrapping whatever you wish to perform a desired action when there is a change in the data
///
/// # Example
///
/// ```
///TextBox::new()
///    .lens(AppData::foo)
///    .controller(on_change_controller(|x| {
///        dbg!(x);
///    }))
///```
pub fn on_change_controller<T: Data + PartialEq, F: FnMut(&mut T) + 'static>(
    f: F,
) -> OnChangeController<F> {
    OnChangeController(f)
}

const FIELD_CHANGED: Selector = Selector::new(concat!(
    file!(),
    "-",
    line!(),
    "-",
    column!(),
    "-",
    "field_changed"
));

pub struct OnChangeController<F>(F);
impl<T: Data + PartialEq, W: Widget<T>, F: FnMut(&mut T)> Controller<T, W>
    for OnChangeController<F>
{
    fn event(&mut self, child: &mut W, ctx: &mut EventCtx, event: &Event, data: &mut T, env: &Env) {
        if let Event::Command(command) = event {
            if command.is(FIELD_CHANGED) {
                (self.0)(data);
                return;
            }
        }
        child.event(ctx, event, data, env);
    }

    fn update(
        &mut self,
        child: &mut W,
        ctx: &mut UpdateCtx<'_, '_>,
        old_data: &T,
        data: &T,
        env: &Env,
    ) {
        if old_data != data {
            ctx.submit_command(FIELD_CHANGED.to(ctx.widget_id()));
        }
        child.update(ctx, old_data, data, env);
    }
}