问javascript -未检测到滑动手势

EN

document.body.addEventListener('touchstart', handleTouchStart(event), false);
document.body.addEventListener('touchmove', handleTouchMove(event), false);

var xDown = null;
var yDown = null;

function handleTouchStart(event) {
  if(screen && screen.width < 480 && !(document.getElementsByClassName('in')[0])) {
    xDown = event.touches[0].clientX;
    yDown = event.touches[0].clientY;
  }
};

function handleTouchMove(event) {
  if(screen && screen.width < 480 && !(document.getElementsByClassName('in')[0])) {
    if ( ! xDown || ! yDown ) {
        return;
    }
    var xUp = event.touches[0].clientX;
    var yUp = event.touches[0].clientY;

    var xDiff = xDown - xUp;
    var yDiff = yDown - yUp;

    if ( Math.abs( xDiff ) > Math.abs( yDiff ) ) {/*most significant*/
        if ( xDiff > 0 ) {
          red -= increment;
          if(red < 0) { red = 0; }
          document.getElementById("val-display").innerHTML = "Red: " + red + "<br>Green: " + green + "<br>Blue: " + blue + "<br>Hex: #" + fullColorHex(red, green, blue);
        } else {
          blue -= increment;
          if(blue < 0) { blue = 0; }
          document.getElementById("val-display").innerHTML = "Red: " + red + "<br>Green: " + green + "<br>Blue: " + blue + "<br>Hex: #" + fullColorHex(red, green, blue);
        }
    } else {
        if ( yDiff > 0 ) {
          green -= increment;
          if(green < 0) { green = 0; }
          document.getElementById("val-display").innerHTML = "Red: " + red + "<br>Green: " + green + "<br>Blue: " + blue + "<br>Hex: #" + fullColorHex(red, green, blue);
        }
    }
    setBackgroundColor();
    setTextColorFromBackground("title", false);
    setTextColorFromBackground("side_box", true);
      /* reset values */
    xDown = null;
    yDown = null;
  }
};