1 var pointA = {
 2     X: 500,
 3     Y: 300
 4 };
 5 var pointB = {};
 6 var pointC = {};               // A,B,C分别代表中心点，起始点，结束点坐标
 7                                        // 这里通过鼠标的移动获取起始点和结束点
 8 var typeMouse = false;
 9 var allA = 0;                     // 存放鼠标旋转总共的度数
10 $(document).on('mousedown', function(e) {
11     pointB.X = e.pageX;
12     pointB.Y = e.pageY;
13     typeMouse = true;          //获取起始点坐标
14 });
15 $(document).on('mousemove', function(e) {
16     if(typeMouse) {
17     pointC.X = e.pageX;
18     pointC.Y = e.pageY;         // 获取结束点坐标
19     var AB = {};
20     var AC= {};
21     AB.X = (pointB.X - pointA.X);
22     AB.Y = (pointB.Y - pointA.Y);
23     AC.X = (pointC.X - pointA.X);
24     AC.Y = (pointC.Y - pointA.Y);                   // 分别求出AB,AC的向量坐标表示
25     var direct = (AB.X * AC.Y) - (AB.Y * AC.X);           // AB与AC叉乘求出逆时针还是顺时针旋转
26     var lengthAB = Math.sqrt( Math.pow(pointA.X - pointB.X, 2) +
27     Math.pow(pointA.Y - pointB.Y, 2)),
28     lengthAC = Math.sqrt( Math.pow(pointA.X - pointC.X, 2) +
29     Math.pow(pointA.Y - pointC.Y, 2)),
30     lengthBC = Math.sqrt( Math.pow(pointB.X - pointC.X, 2) +
31     Math.pow(pointB.Y - pointC.Y, 2));
32     var cosA = (Math.pow(lengthAB, 2) + Math.pow(lengthAC, 2) - Math.pow(lengthBC, 2)) /
33     (2 * lengthAB * lengthAC);                 //   余弦定理求出旋转角
34     var angleA = Math.round( Math.acos(cosA) * 180 / Math.PI );
35     if (direct < 0){
36         allA -= angleA;                       //叉乘结果为负表示逆时针旋转， 逆时针旋转减度数
37     } else {
38         allA += angleA;                       //叉乘结果为正表示顺时针旋转，顺时针旋转加度数
39     }
40     pointB.X = pointC.X;
41     pointB.Y = pointC.Y;                   //运算结束后将起始点重新赋值为结束点，作为下一次的起始点
42     }
43 });
44 $(document).on('mouseup', function(e) {
45     typeMouse = false;
46 });