WheelPicker stops between two items when fling is interrupted by touch

[daihieptn97]

Fix: WheelPicker stops between two items when fling is interrupted by touch

Problem

When the user swipes the wheel strongly (triggering a fling), then immediately taps the wheel to stop it, the picker halts at an intermediate position between two items. As a result:

• No item gets selected (OnItemSelectedListener is not triggered)
• The wheel visually sits between two rows with no clear selection
• The user must scroll again to land on a valid item

This happens because ACTION_DOWN calls scroller.abortAnimation(), which stops the scroll immediately at whatever scrollOffsetY value the fling was at — without snapping to the nearest item boundary. When the finger lifts (ACTION_UP), the event is treated as a click (isClick = true), so the snap logic inside the ACTION_UP block is never executed.

Root Cause

In onTouchEvent, the ACTION_DOWN branch:

MotionEvent.ACTION_DOWN -> {
    if (!scroller.isFinished) {
        scroller.abortAnimation()  // stops at arbitrary scrollOffsetY
        isForceFinishScroll = true
        // ← no snap correction here
    }
    ...
}

After abortAnimation(), scrollOffsetY can be a value like 347 when itemHeight is 100, leaving the wheel stuck 47px between two items. Since the subsequent ACTION_UP detects a tap (not a scroll), it skips the snap logic entirely.

Fix

After calling scroller.abortAnimation() in ACTION_DOWN, immediately correct scrollOffsetY to the nearest item boundary using the existing computeDistanceToEndPoint() helper:

MotionEvent.ACTION_DOWN -> {
    parent?.requestDisallowInterceptTouchEvent(true)
    if (!scroller.isFinished) {
        scroller.abortAnimation()
        isForceFinishScroll = true

        // Snap scrollOffsetY to the nearest item boundary
        // so the wheel does not freeze between two items
        val remainder = scrollOffsetY.rem(itemHeight)
        val snapOffset = computeDistanceToEndPoint(remainder)
        if (snapOffset != 0) {
            scrollOffsetY += snapOffset
            if (!isCyclic) {
                scrollOffsetY = scrollOffsetY.coerceIn(minFlingY, maxFlingY)
            }
            invalidate()
        }
    }
    isScrolling = false
    lastPointY = event.y.toInt()
    downPointY = lastPointY
}

Why this approach

• Reuses the existing computeDistanceToEndPoint() method — no new logic introduced
• Applies the clamp guard (coerceIn) already used in ACTION_UP for non-cyclic mode
• Calls invalidate() so the wheel visually snaps before the user lifts their finger
• Does not affect normal scroll or fling behaviour — only runs when a fling is actively interrupted

Steps to Reproduce

1. Populate the WheelPicker with any dataset (5+ items)
2. Swipe the wheel quickly downward to trigger a fling
3. While the wheel is still spinning, tap and hold briefly, then release
4. Expected: Wheel snaps to the nearest item and fires OnItemSelectedListener
5. Actual: Wheel stops between two items; no selection is reported

Testing

Tested on:

• Android API 26 (Oreo)
• Android API 10 (Quince Tart)
• Both cyclic and non-cyclic modes
• Physical device and emulator

No regressions observed in normal tap, slow scroll, or fast fling scenarios.