[Mac Catalyst] Mouse wheel / trackpad scroll events not delivered by SKCanvasView

[mattleibow]

Description

On Mac Catalyst, SKCanvasView with EnableTouchEvents = true does not deliver mouse wheel or trackpad scroll events. The Touch event never fires with ActionType = SKTouchAction.WheelChanged, even though the SKTouchAction enum includes WheelChanged and SKTouchEventArgs has a WheelDelta property.

Expected Behavior

When the user scrolls with the mouse wheel or two-finger trackpad gesture over the SKCanvasView, the Touch event should fire with:

• ActionType = SKTouchAction.WheelChanged
• WheelDelta set to the scroll amount
• Location set to the cursor position

Actual Behavior

No WheelChanged events are delivered. Scrolling with mouse wheel or trackpad two-finger scroll over the canvas produces no touch events at all.

Root Cause Analysis

On Mac Catalyst (UIKit), scroll wheel events are delivered via UIScrollView integration or by overriding scrollWheel(with:) on the UIView. The standard touch handling (touchesBegan/touchesMoved/touchesEnded) does not capture scroll wheel input.

Suggested Fix

In the Mac Catalyst platform handler for SKCanvasView, override ScrollWheel:

// In the native Mac Catalyst view for SKCanvasView
#if MACCATALYST
public override void ScrollWheel(UIEvent uiEvent)
{
    base.ScrollWheel(uiEvent);
    
    // UIEvent on Mac Catalyst provides scroll delta through the underlying CGEvent
    // For Mac Catalyst 13.4+, we can use UIScrollEvent (subclass of UIEvent)
    if (uiEvent is not null)
    {
        var allTouches = uiEvent.AllTouches;
        // Alternative: use the scrolling delta from the event
        // On Mac Catalyst, you can access scroll data via the native event
        
        var location = /* get cursor location */;
        var wheelDelta = /* extract from event */;
        
        var args = new SKTouchEventArgs(
            id: -1,
            type: SKTouchAction.WheelChanged,
            mouse: SKMouseButton.Unknown,
            device: SKTouchDeviceType.Mouse,
            position: new SKPoint((float)location.X, (float)location.Y),
            inContact: false,
            wheelDelta: wheelDelta,
            pressure: 0);
        OnTouch(args);
    }
}
#endif

Alternatively, for Mac Catalyst 15.0+, a UIPanGestureRecognizer with allowedScrollTypesMask set to .all can capture trackpad scroll gestures and convert them to wheel events.

Reproduction Steps

1. Create a MAUI app targeting net9.0-maccatalyst
2. Add an SKCanvasView with EnableTouchEvents="True"
3. Subscribe to the Touch event and log all events (including ActionType and WheelDelta)
4. Run on macOS
5. Scroll with mouse wheel or two-finger trackpad gesture over the canvas
6. Observe: no WheelChanged events are logged, WheelDelta is always 0

Environment

• SkiaSharp: 3.119.1
• SkiaSharp.Views.Maui.Controls: 3.119.1
• .NET: 9.0
• macOS (Mac Catalyst)

Related

• [Mac Catalyst] Mouse hover (non-contact move) events not delivered by SKCanvasView #3523 (Mouse hover events also not delivered on Mac Catalyst)