Question

I’m trying to implement a simple custom scrolling method in my mobile web app. I’m having trouble with the vertical scrolling where I’d like to have a little momentum effect if the page is ‘flicked’.

The problem is detecting a “flick” gesture, (the velocity and perhaps length of that gesture) after a dragging gesture, changing direction and what not. Hope you understand what I mean, you can drag the page up or down and at the end of that drag, I’d like to detect if there is also a flick..

How do you separate the two? How does such a logic look?

Many thanks for any help.

The code: (sorry if this excerpt is a bit of a mess)

 var Device = function() {

    //define some private variablees
    var startY,
        startX,
        startTime,
        deltaY = 0,
        deltaX = 0,
        lastY,
        currentPage,
        nextPage,
        prevPage,
        directionY,
        lastTime,
        pages,
        pane,
        offsetX = 0,
        offsetY = 0,
        isPanning,
        isScrolling,
        isTouch = "ontouchstart" in window;


    return {



        init: function() {

        document.getElementById('frame').addEventListener(isTouch ? 'touchstart' : 'mousedown', Device.onTouchStart, false);


            //get all panes in an array
            panes = document.querySelectorAll('.pane');


        },     

onTouchStart: function (evt) {

            //get X and Y of the touch event
            var touch = isTouch ? event.touches[0] : event;
            startY = touch.clientY;

            //add listener for touch move and end
            document.addEventListener(isTouch ? 'touchmove' : 'mousemove', Device.onTouchMove, false);
            document.addEventListener(isTouch ? 'touchend' : 'mouseup', Device.onTouchEnd, false);

            startTime = new Date();
        },
        onTouchMove: function (evt) {

                //get X and Y of the touch event
                var touch = isTouch ? event.touches[0] : event;

                currentY = touch.clientY;

                //calc touch length
                deltaY = currentY - startY;             


                //Detect if scroll is bigger than threshold 5px
                 if (Math.abs(deltaY) > 5 && !isPanning) {



                        isScrolling = true;


                        //get the element
                           pane = panes[0];

                        //set new position
                        offsetY = pane.lastOffset + deltaY;

                                                    //call animation
                        Device.scrollTo(pane,0,offsetY);

                    }

            //detect last direction     
             directionY = (lastY >= currentY) ? 1 : 0;


                //roll over last variables  
                lastY = currentY;
                lastTime = new Date();
        },

        onTouchEnd: function () {

        //timing
                var endTime = new Date();
                var velocity = (endTime - lastTime).toFixed(0);


            console.log('velocity: ' + velocity);

//TEMPORARY
pane.lastOffset = offsetY;


                isScrolling = false;

                //housekeeping
                document.removeEventListener(isTouch ? 'touchmove' : 'mousemove', Device.onTouchMove, false);
                document.removeEventListener(isTouch ? 'touchend' : 'mouseup', Device.onTouchEnd, false);

           //call for momentum  
           Device.doMomentum(velocity);

        },
        scrollTo: function(el,x,y) {
        if (el) {

                el.style['-webkit-transition-timing-function'] = '';
                el.style['-webkit-transition-duration'] = '0ms';
                el.style[ 'WebkitTransform' ] = 'translate3d('+x+'px,'+y+'px, 0px)';
        }
        },
        animateTo: function(el,x,y) {
        if (el) {
                el.style['-webkit-transition-timing-function'] = 'cubic-bezier(0,0,0.25,1)';
                el.style['-webkit-transition-duration'] = '300ms';
                el.style[ 'WebkitTransform' ] = 'translate3d('+x+'px,'+y+'px, 0px)';
        }
        },
        doMomentum: function(velocity) {



        console.log((directionY == 1) ? 'up': 'down');
        console.log('pane.lastOffset: ' + pane.lastOffset);
        var endPosition;

            if (directionY == 1) {
         endPosition = pane.lastOffset - velocity;

        } else {

         endPosition = parseFloat(pane.lastOffset) + parseFloat(velocity);
        }

        console.log(endPosition);

        Device.animateTo(pane,0,endPosition);

        pane.lastOffset = endPosition;


        }

Answer 1

FYI, what I ended up doing was simply using the time elapsed between onTouchMove-event and the onTouchEnd-event.

Basically, I have a date variable that continuously overwrites itself in the onTouchMove callback:

onTouchMove: function (evt) {
   ...
 intermediateTime = new Date();
   ...
}

And then onTouchEnd I get an endTime value and evaluate the difference. If this time is very low, then I assume there was a “flick” at the end.

onTouchEnd: function (evt) {
   ...
 var endTime = new Date();

 var vel  = (endTime - intermediateTime);

 if (vel <= 100) {

    doMomentumOrSomething();

 }
   ...
}

This is very simple but actually works reasonably well.