Overview

Add scroll wheel support to SkiaSharp's Android MAUI views by handling ACTION_SCROLL events in SKTouchHandler and normalizing AXIS_VSCROLL values to the v120 standard (120 = one discrete mouse wheel notch).

Parent issue: #3533

Current State

The Android SKTouchHandler (source/SkiaSharp.Views.Maui/SkiaSharp.Views.Maui.Core/Platform/Android/SKTouchHandler.cs) currently:

• Handles: MotionEventActions.Down, Move, Up, PointerDown, PointerUp, Cancel
• Does NOT handle: ACTION_SCROLL
• All SKTouchEventArgs are constructed with wheelDelta = 0

Platform Details

Native API

MotionEvent with ACTION_SCROLL:

"The motion event contains relative vertical and/or horizontal scroll offsets."
— MotionEvent.ACTION_SCROLL

// Received via onGenericMotionEvent(), NOT onTouchEvent()
float vScroll = event.getAxisValue(MotionEvent.AXIS_VSCROLL);
float hScroll = event.getAxisValue(MotionEvent.AXIS_HSCROLL);

Note: ACTION_SCROLL events are delivered through onGenericMotionEvent(), not onTouchEvent(). This is a separate event path from touch/pointer events. Pointer movement uses ACTION_MOVE (not ACTION_POINTER_MOVE, which is not a standard public constant).

Raw Values Per Discrete Mouse Notch

⚠️ Note: The ±1.0 value per mouse notch is the empirically observed default across standard mice and Android devices. The Android documentation does not explicitly guarantee this value — it describes AXIS_VSCROLL as a "relative vertical scroll offset" without specifying units per notch. The ×120 normalization assumes this de-facto standard. If a device produces different values, the normalization will be proportionally off.

Sign Convention

"Positive values indicate scrolling forward (away from the user)"
— MotionEvent.AXIS_VSCROLL

• AXIS_VSCROLL > 0 = scroll up (away from user)
• AXIS_VSCROLL < 0 = scroll down (toward user)
• Already matches v120 sign convention — no negation needed

Official Documentation

• MotionEvent class reference — full class with all action types and axes
• MotionEvent.AXIS_VSCROLL — vertical scroll offset, "positive values indicate scrolling forward (away from the user)"
• MotionEvent.AXIS_HSCROLL — horizontal equivalent
• MotionEvent.ACTION_SCROLL — "the motion event contains relative vertical and/or horizontal scroll offsets"
• View.onGenericMotionEvent — the event handler that receives scroll events

Normalization Logic

// In SKTouchHandler.cs, handle ACTION_SCROLL:
float axisValue = motionEvent.GetAxisValue(Axis.Vscroll);
int wheelDelta = (int)Math.Round(axisValue * 120.0);
// Mouse notch (1.0) → 120
// Trackpad micro (0.1) → 12
// Trackpad half (0.5) → 60

Expected Results

Implementation Notes

• ACTION_SCROLL events come through OnGenericMotionEvent(), not OnTouchEvent(). You may need to register a generic motion listener or override the view's OnGenericMotionEvent.
• The MAUI Android handler is in source/SkiaSharp.Views.Maui/SkiaSharp.Views.Maui.Core/Platform/Android/SKTouchHandler.cs
• Fire SKTouchAction.WheelChanged with SKTouchDeviceType.Mouse
• Use motionEvent.GetX() / motionEvent.GetY() for pointer location
• Set inContact = false (wheel doesn't imply contact)
• Feed args.Handled back to return value of OnGenericMotionEvent (return true to consume)
• Reference: Windows handler in Platform/Windows/SKTouchHandler.cs lines 101-104, 122-124 for pattern