Detect Beginning and Ending state of Scroll Wheel event in Cocoa

Question

Cocoa provides many useful methods to detect the beginning and ending states of gestures and mouse clicks. For mouse clicks, we can easily override mouseDown: and mouseUp: methods. However, we don't seem to have a way to do the same for mouse scrolling event -- we are only provided with scrollWheel: method which is activated when scrolling happens.

I have tried beginGestureWithEvent: but it only responds to touch events.

Is there a way to detect the beginning and the ending states of mouse scrolling in Cocoa?

This is for generic NSView. I plan to use scrollView event for something non-related to manipulating the view, like changing certain member variables. — HuaTham, Apr 03 '15 at 23:31

Daniyar · Answer 1 · 2015-04-08T05:58:04.760

As I see there's no such a simple solution. But you still can do this.

The most evident is using timer to check when the wheel did scroll last time. I prefer using lightweight GCD-timers for this purposes (they also ARC-capable objects instead of NSTimer's which are storing strong target-references).

- (void) scrollWheel:(NSEvent *)event
{
  _lastStamp = mach_absolute_time(); // get current timestamp
  if (!_running)
  {
    if (!_timer)
      [self createTimerSource];
    if (!_running)
      dispatch_resume(_timer); 
    _running = YES;
  }
}

- (void)createTimerSource
{
  NSTimeInterval timeout = ...;
  _timer=dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, _timerQueue);
  dispatch_source_set_timer(_timer, DISPATCH_TIME_NOW, timeout * NSEC_PER_SEC, 0);
  // set the event handler
  dispatch_source_set_event_handler(_timer, ^{
     uint64_t stamp = mach_absolute_time(); // current stamp
     if (stamp - _lastStamp > some_tolerance_value)
     {
       // wheel did end scroll
       if (_running)
         dispatch_source_cancel(_timer);
       _running = NO;
     }
  });
}

Check the article out to know more about Dispatch Sources.

Thank you for the approach. I can see now that it's possible to do but not sure if this is the best answer. I upvote your answer for appreciation. — HuaTham, Apr 07 '15 at 09:08

Sentry.co · Answer 2 · 2016-02-28T12:49:35.970

The best way I've found to detect onScrollWheelDown and onScrollWheelUp is to use event.phase == MayBegin for onDown event.phase == Cancelled and event.phase == Ended for onUp. This even works when your gesture goes off the trackpad and should work for MagicMouse as well. I Will test the MagicMouse later.

You can read a more in-depth analysis of how I came to this conclusion here: http://stylekit.org/blog/2016/02/28/Scroll-wheel/

score 0 · Answer 3 · answered Oct 04 '22 at 14:21

0

I solved the problem with these 2 lines of code

- (void)scrollWheel:(NSEvent*)theEvent
{
    BOOL    begin = theEvent.phase & NSEventPhaseBegan;
    BOOL    end   = theEvent.phase & NSEventPhaseEnded || theEvent.momentumPhase & NSEventPhaseEnded;

    if(begin){
       // do something when the scroll begins
       // as store some initial value
    }
    else if(end)
    {
        //... do something when the scroll ends
    }
    else{
       //... do something when scrolling
    }

}

answered Oct 04 '22 at 14:21

Leonardo

751
8
15

For me theEvent.phase and theEvent.momentumPhase are always always 0 . Any idea why ? can this happen because i'm not using Apple mouse ? – user1105951 Oct 16 '22 at 11:37
May be. It could depend on the macOS version too. I can tell you that it works with the trackpad on my MacBook Pro with macOS 12.6.1. – Leonardo Nov 03 '22 at 20:34

Detect Beginning and Ending state of Scroll Wheel event in Cocoa

3 Answers3