[feature request]: Create a method on WidgetExt that allows the user to run some code when the widget is added.

[richard-uk1]

Follows on from #1478.

Sometimes it's necessary to do something when a widget is created. One example would be to set the cursor when the mouse is hovered over the widget. Currently this would require creating a custom controller. This issue proposes the following function:

pub trait WidgetExt<T: Data>: Widget<T> + Sized + 'static {
  // .... after on_click
  fn on_added(self, f: impl Fn(&mut LifeCycleCtx, &T, &Env) + 'static) -> ControllerHost<Self, Added<T>>;
  // ...
}

(I might have got the function signature wrong). The function would require a new type be created (Added<T>) and Controller implemented for it. The controller would react to the LifeCycle::WidgetAdded lifecycle event by running the closure.

EDIT I did get the function signature wrong. Corrected now.

[richard-uk1]

If anyone without much experience with druid wants to have a go, I'm happy to answer and questions you might have.

[arthmis]

I would like to do this.

[richard-uk1]

@arthmis go for it! :)

[arthmis]

I have run into an issue here.

The function parameter for on_added

f: impl Fn(&mut LifeCycleCtx, &mut T, &Env)

requires a &mut T for data, which is incompatible with Controller's lifecycle's function definition which is

fn lifecycle(
        &mut self,
        child: &mut W,
        ctx: &mut LifeCycleCtx,
        event: &crate::LifeCycle,
        data: &T,
        env: &Env,
    )

Does the closure have to receive a &mut T?

[cmyr]

No, on_added doesn't get mutable access to the data. We could also have a on_window_connected method that does get mutable data, if we wanted; the potential gotcha there is that not all widgets will see the WindowConnected event (it's only sent once, so widgets created after the window is created won't get it).

[richard-uk1]

@arthmis sorry my bad: on_added should get an immutable reference to T. I will update the issue accordingly.

The on_added method will be for setting things up inside the widget. It can use the data, but can't alter it.