Add delta to CursorMoved event

[Sebbl0508]

What problem does this solve or what need does it fill?

Since MouseMotion (afaik) intentionally doesn't take mouse acceleration or (maybe also?) scaling into account, one would have to use CursorMoved to stay "up-to-date" to the actual cursor position(s).
Unfortunately the CursorMoved event doesn't contain deltas. That means if one needs that, they would need to implement that themselves in their code.

What solution would you like?

If possible and/or wanted delta to be added to the CursorMoved event.

What alternative(s) have you considered?

Implementing the delta calculation in my own game code or trying to solve my problem without using deltas.

Additional context

If this is nothing against this addition, i'd like to try implementing this myself. I haven't gone into the bevy internals so idk how possible this is, but i'd like to try :)

For additionaly context why i want this change: Added is a video of my WIP app, where i'm trying to implement a mouse-dragging camera controller. At the moment i'm using the deltas from MouseMotion, which causes issues since apparently on my laptop there's some kind of mouse acceleration or something.

2024-02-04.14-08-08.mp4

[Sebbl0508]

For the moment, this is the approach i took in my own game code:

Code

use bevy::prelude::*;

pub struct CursorMovedDeltaPlugin;

impl Plugin for CursorMovedDeltaPlugin {
    fn build(&self, app: &mut App) {
        app.add_event::<CursorMovedWithDelta>()
            .init_resource::<LastCursorPosition>()
            .add_systems(PreUpdate, translate_cursor_moved_events);
    }
}

#[derive(Event, Debug)]
pub struct CursorMovedWithDelta {
    pub window: Entity,
    pub position: Vec2,
    pub delta: Vec2,
}

/// `None` if not set yet
#[derive(Resource, Debug, Default)]
pub struct LastCursorPosition(Option<Vec2>);

fn translate_cursor_moved_events(
    mut last_cursor_position: ResMut<LastCursorPosition>,
    mut cursor_motion: EventReader<CursorMoved>,
    mut cursor_motion_with_delta: EventWriter<CursorMovedWithDelta>,
) {
    for evt in cursor_motion.read() {
        if let Some(last_cursor_position) = last_cursor_position.0 {
            let delta = evt.position - last_cursor_position;
            cursor_motion_with_delta.send(CursorMovedWithDelta {
                window: evt.window,
                position: evt.position,
                delta,
            });
        }

        last_cursor_position.0 = Some(evt.position);
    }
}

[lynn-lumen]

I agree that this might be useful, but since CursorMoved is a translated version of the winit::CursorMoved struct which does not include delta, I don't know if this is the place to add it.

[Sebbl0508]

I agree that this might be useful, but since CursorMoved is a translated version of the winit::CursorMoved struct which does not include delta, I don't know if this is the place to add it.

That's what i thought too.
Nice PR btw. Thanks a lot. Hope it gets merged!