RxJS: distinguish single click from drag

All we need is an easy explanation of the problem, so here it is.

I have an AngularJS component that should react to either a single click or a drag (resizing an area).

I started to use RxJS (ReactiveX) in my application, so I try to find a solution using it. The Angular side of the request is minor…

To simplify the problem (and to train myself), I made a slider directive, based on the rx.angular.js drag’n’drop example: http://plnkr.co/edit/UqdyB2
See the Slide.js file (the other code is for other experiments). The code of this logic is:

    function(scope, element, attributes)
    {
      var thumb = element.children(0);
      var sliderPosition = element[0].getBoundingClientRect().left;
      var sliderWidth = element[0].getBoundingClientRect().width;
      var thumbPosition = thumb[0].getBoundingClientRect().left;
      var thumbWidth = thumb[0].getBoundingClientRect().width;

      // Based on drag'n'drop example of rx-angular.js
      // Get the three major events
      var mousedown = rx.Observable.fromEvent(thumb,     'mousedown');
      var mousemove = rx.Observable.fromEvent(element,   'mousemove');
      var mouseup   = rx.Observable.fromEvent($document, 'mouseup');

      // I would like to be able to detect a single click vs. click and drag.
      // I would say if we get mouseup shortly after mousedown, it is a single click;
      // mousedown.delay(200).takeUntil(mouseup)
      //   .subscribe(function() { console.log('Simple click'); }, undefined, function() { console.log('Simple click completed'); });

      var locatedMouseDown = mousedown.map(function (event) 
      {
        event.preventDefault();
        // console.log('Click', event.clientX - sliderPosition);
        // calculate offsets when mouse down
        var initialThumbPosition = thumb[0].getBoundingClientRect().left - sliderPosition;
        return { from: initialThumbPosition, offset: event.clientX - sliderPosition };
      });

      // Combine mouse down with mouse move until mouse up
      var mousedrag = locatedMouseDown.flatMap(function (clickInfo) 
      {
        return mousemove.map(function (event)
        {
          var move = event.clientX - sliderPosition - clickInfo.offset;
          // console.log('Move', clickInfo);
          // calculate offsets from mouse down to mouse moves
          return clickInfo.from + move;
        }).takeUntil(mouseup);
      });

      mousedrag 
        .map(function (position)
        {
          if (position < 0)
            return 0;
          if (position > sliderWidth - thumbWidth)
            return sliderWidth - thumbWidth;
          return position;
        })
        .subscribe(function (position) 
        {
          // console.log('Drag', position);
          // Update position
          thumb.css({ left: position + 'px' });
        });
    }

That’s mostly D’n’D constrained horizontally and to a given range.

Now, I would like to listen to mousedown, and if mouse up happens within a short while (say 200 ms, to adjust), I see it as a click and I do a specific treatment (eg. resetting the position to zero).

I tried with delay().takeUntil(mouseup), as seen in another SO answer, without success. Perhaps a switch() might be needed, too (to avoid going the drag route).

Any idea? Thanks in advance.

How to solve :

I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.

Method 1

You can use timeout (timeoutWith if you are using ReactiveX/RxJS)

var click$ = mousedown.flatMap(function (md) {
  return mouseup.timeoutWith(200, Observable.empty());
});

If the mouseup doesn’t occur before the timeout it will just propagate an empty Observable instead. If it does then the downstream observer will receive an event.

Method 2

Isn’t the trick with delay(Xms).takeUntil(mouseup) doing the opposite of what you want? I mean, you want to detect when the mouseup event happens before the countdown, while the aformentioned trick detect when the mouseup event happens after.

I would try something around those lines (untested for now, but hopefully it will orient you in some positive direction):

var click$ = mousedown.flatMap(function ( mouseDownEv ) {
  return merge(
      Rx.just(mouseDownEv).delay(Xms).map(function ( x ) {return {event : 'noclick'};}),
      mouseup.map(function ( mouseUpEv ) {return {event : mouseUpEv};})
      ).first();
});

The idea is to race the mouseup event against a dummy emission happening after your delay, and see who wins. So if click$ emits ‘noclick’ then you can consider that no click happened.

Hopefully that works, i will test soon but if you do before me, let me know.

Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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