用Javascript检测iOS Safari中的抖动？

这说明了这个例子：

if (typeof window.DeviceMotionEvent != 'undefined') {
    // Shake sensitivity (a lower number is more)
    var sensitivity = 20;

    // Position variables
    var x1 = 0,y1 = 0,z1 = 0,x2 = 0,y2 = 0,z2 = 0;

    // Listen to motion events and update the position
    window.addEventListener('devicemotion',function (e) {
        x1 = e.accelerationIncludingGravity.x;
        y1 = e.accelerationIncludingGravity.y;
        z1 = e.accelerationIncludingGravity.z;
    },false);

    // Periodically check the position and fire
    // if the change is greater than the sensitivity
    setInterval(function () {
        var change = Math.abs(x1-x2+y1-y2+z1-z2);

        if (change > sensitivity) {
            alert('Shake!');
        }

        // Update new position
        x2 = x1;
        y2 = y1;
        z2 = z1;
    },150);
}