Proposal: Add OnThumbDragStarted and OnThumbDragCompleted Events to WinUI3 Slider Control

[HHaoWang]

Proposal: Add OnThumbDragStarted and OnThumbDragCompleted Events to WinUI3 Slider Control

Summary

Propose adding OnThumbDragStarted and OnThumbDragCompleted events to the WinUI3 Slider control to enable developers to track when a user begins and ends thumb dragging. This aligns with UWP/WPF's Thumb.DragStarted, Thumb.DragCompleted/OnThumbDragStarted , OnThumbDragStarted events.

Rationale

• Migrating UWP/WPF Developers: Developers porting apps from UWP/WPF to WinUI3 require feature parity. The absence of drag-tracking events forces functions loss, workarounds, complicating code and reducing maintainability.
• User Experience Enhancements: Enables scenarios like deferred UI updates until dragging completes (e.g., pausing real-time data rendering during drags to optimize performance).
• Consistency with XAML Ecosystem: Matches standard Slider behavior in other Microsoft XAML frameworks, reducing learning curve friction.

Scope

Capability	Priority
Expose OnThumbDragStarted event when thumb dragging begins	Must
Expose OnThumbDragCompleted event when thumb dragging ends	Must
Provide event arguments containing drag offset data	Should
Include a OnThumbDragDelta event for intermediate drag updates	Could

Important Notes

API Proposal:

public class Slider : RangeBase  
{  
    public event DragStartedEventHandler OnThumbDragStarted;  
    public event DragCompletedEventHandler OnThumbDragCompleted;  
}  

// Event arguments (reuse existing WinUI types):  
public class DragStartedEventArgs : RoutedEventArgs { /* Horizontal/VerticalOffset */ }  
public class DragCompletedEventArgs : RoutedEventArgs { /* Canceled, HorizontalChange, VerticalChange*/ }  

Usage Example:

mySlider.OnThumbDragStarted+= (s, e) => {  };  
mySlider.OnThumbDragCompleted+= (s, e) => {  };  

Design Alignment:

• Mirrors UWP’s Slider event model.
• Avoids breaking changes by reusing Microsoft.UI.Xaml.Controls.Primitives event handlers.

Open Questions

1. Should OnThumbDragDelta (continuous drag updates) be included in this proposal?
2. Is there a need for cancelation support (e.g., if a drag is interrupted by system events)?
3. Should these events be added to other RangeBase controls (e.g., ScrollBar, ProgressBar)?

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This proposal directly addresses the gap in WinUI3’s Slider control while prioritizing migration compatibility and user experience.

[HHaoWang]

A common scenario is, for example, in a music player where a scheduled task periodically updates the playback progress bar. Users can drag the progress bar to adjust the music playback position, requiring monitoring of the dragging behavior to prevent the scheduled task from updating it.

[castorix]

WinUI 3 slider is like UWP slider where you can handle PointerPressed, PointerMoved, ... events (with AddHandler)
(MSDN : https://learn.microsoft.com/en-us/windows/uwp/debug-test-perf/optimize-animations-and-media#improve-media-scrubbing)

[HHaoWang]

WinUI 3 slider is like UWP slider where you can handle PointerPressed, PointerMoved, ... events (with AddHandler) (MSDN : https://learn.microsoft.com/en-us/windows/uwp/debug-test-perf/optimize-animations-and-media#improve-media-scrubbing)

Even so, adding the aforementioned events to the Slider is beneficial for enhancing the developer experience. It's not reasonable to expect all features to be implemented through workarounds, as such code tends to be too messy and hard to maintain.