Add scroll functionality to bevy_picking

[colepoirier]

Objective

bevy_picking currently does support scroll events.

Solution

This pr adds a new event type for scroll, and updates the default input system for mouse pointers to read and emit this event.

Testing

• Did you test these changes? If so, how?
• Are there any parts that need more testing?
• How can other people (reviewers) test your changes? Is there anything specific they need to know?
• If relevant, what platforms did you test these changes on, and are there any important ones you can't test?

I haven't tested these changes, if the reviewers can advise me how to do so I'd appreciate it!

[aevyrie]

this comment looks like a copy paste error

[colepoirier]

I removed that and added an appropriate comment mirroring the other events' implementations lower down in that code block.

[ickshonpe]

You need to add

 .add_event::<Pointer<Scroll>>()

to InteractionPlugin's Plugin impl, otherwise you get a panic because it can't find the scroll event resource.

[ickshonpe]

With the event registered, there don't seem to be any other problems.

To test the changes, I removed the update_scroll_position from the testbed_ui example (in examples/testbed/ui.rs) and added an observer to the scrollable node:

// Scrolling list
parent
    .spawn((
        Node {
            flex_direction: FlexDirection::Column,
            align_self: AlignSelf::Stretch,
            height: Val::Percent(50.),
            overflow: Overflow::scroll_y(),
            ..default()
        },
        BackgroundColor(Color::srgb(0.10, 0.10, 0.10)),
    ))
    .observe(|
        trigger: Trigger<Pointer<Scroll>>,
        mut scroll_position_query: Query<&mut ScrollPosition>,
    | {
        if let Ok(mut scroll_position) = scroll_position_query.get_mut(trigger.target) {
            let (dx, dy) = match trigger.unit {
                MouseScrollUnit::Line => (trigger.x * 20., trigger.y * 20.),
                MouseScrollUnit::Pixel => (trigger.x, trigger.y),
            };
    
            scroll_position.offset_x -= dx;
            scroll_position.offset_y -= dy;
        }
    })
    .with_children(|parent| {
        // List items
        for i in 0..25 {
            parent
                .spawn((
                    Text(format!("Item {i}")),
                    TextFont {
                        font: asset_server.load("fonts/FiraSans-Bold.ttf"),
                        ..default()
                    },
                    Label,
                    AccessibilityNode(Accessible::new(Role::ListItem)),
                ))
                .insert(Pickable {
                    should_block_lower: false,
                    ..default()
                });
        }
    });

scrolling working as before.

[ickshonpe]

Suggested change

	/// Fires while a pointer is scrolling over the `target` entity.
	/// Fires while a pointer is scrolling over the `target` entity.

The way this is written seems a bit odd, does a pointer scroll? Maybe I'm being fussy, I'm not sure.

[aevyrie]

This is just mirroring the language used in the other event docs. I think it's a valid description. The pointing device sends the scroll, but it is definitely tied to the pointer. That is, pointer scrolling events depend on where the pointer is located w.r.t. picking, it's not just an input.

[colepoirier]

Does that mean we should leave it as is?

[ickshonpe]

Yep it's fine, aervrie's explanation makes sense.

[colepoirier]

You need to add

 .add_event::<Pointer<Scroll>>()

to InteractionPlugin's Plugin impl, otherwise you get a panic because it can't find the scroll event resource.

Done!