移动端js判断手指滑动方向

var startx, starty;
//获得角度
function getAngle(angx, angy) {
return Math.atan2(angy, angx) * 180 / Math.PI;
};

//根据起点终点返回方向 1向上 2向下 3向左 4向右 0未滑动
function getDirection(startx, starty, endx, endy) {
var angx = endx - startx;
var angy = endy - starty;
var result = 0;

//如果滑动距离太短
if (Math.abs(angx) < 2 && Math.abs(angy) < 2) {
return result;
}

var angle = getAngle(angx, angy);
if (angle >= -135 && angle <= -45) {
result = 1;
} else if (angle > 45 && angle < 135) {
result = 2;
} else if ((angle >= 135 && angle <= 180) || (angle >= -180 && angle < -135)) {
result = 3;
} else if (angle >= -45 && angle <= 45) {
result = 4;
}

return result;
}
//手指接触屏幕
document.addEventListener("touchstart", function(e) {
startx = e.touches[0].pageX;
starty = e.touches[0].pageY;
}, false);
//手指离开屏幕
document.addEventListener("touchend", function(e) {
var endx, endy;
endx = e.changedTouches[0].pageX;
endy = e.changedTouches[0].pageY;
var direction = getDirection(startx, starty, endx, endy);
switch (direction) {
case 0:
alert("未滑动！");
break;
case 1:
alert("向上！")
break;
case 2:
alert("向下！")
break;
case 3:
alert("向左！")
break;
case 4:
alert("向右！")
break;
default:
}
}, false);