docs: function onTouchStart(event) {
        var touches = event.touches && event.touches.length ? event.touches : [event];
        var x = touches[0].clientX;
        var y = touches[0].clientY;
        touchCoordinates.push(x, y);
  
        $timeout(function() {
          // Remove the allowable region.
          for (var i = 0; i < touchCoordinates.length; i += 2) {
            if (touchCoordinates[i] === x && touchCoordinates[i+1] === y) {
              touchCoordinates.splice(i, i + 2);
              return;
            }
          }
        }, PREVENT_DURATION, false);
      }