1 window.onload = function(){
 2     //canvas initialization
 3     var canvas = document.getElementById("canvas");
 4     var ctx = canvas.getContext("2d");
 5     //dimensions
 6     var W = canvas.width;
 7     var H = canvas.height;
 8     //Variables
 9     var degrees = 0;
10     var new_degrees = 0;
11     var difference = 0;
12     var color = "green"; //green looks better to me
13     var bgcolor = "#E8E8E8";
14     var text;
15     var animation_loop, redraw_loop;
16     
17     function init()
18     {
19         //Clear the canvas everytime a chart is drawn
20         ctx.clearRect(0, 0, W, H);
21         
22         //Background 360 degree arc
23         ctx.beginPath();
24         ctx.strokeStyle = bgcolor;
25         ctx.lineWidth = 10;    //预填充环的宽度
26         ctx.arc(W/2, H/2, 100, 0, Math.PI*2, false); //you can see the arc now
27         ctx.stroke();
28         
29         //gauge will be a simple arc
30         //Angle in radians = angle in degrees * PI / 180
31         var radians = degrees * Math.PI / 180;
32         ctx.beginPath();
33         ctx.strokeStyle = color;
34         ctx.lineWidth = 10;    //填充环的宽度
35         //The arc starts from the rightmost end. If we deduct 90 degrees from the angles
36         //the arc will start from the topmost end
37         ctx.arc(W/2, H/2, 100, 0 - 90*Math.PI/180, radians - 90*Math.PI/180, false); 
38         //you can see the arc now
39         ctx.stroke();
40         
41         //Lets add the text
42         ctx.fillStyle = color;
43         ctx.font = "50px bebas";
44         text = Math.floor(degrees/360*100) + "%";
45         //Lets center the text
46         //deducting half of text width from position x
47         text_width = ctx.measureText(text).width;
48         //adding manual value to position y since the height of the text cannot
49         //be measured easily. There are hacks but we will keep it manual for now.
50         ctx.fillText(text, W/2 - text_width/2, H/2 + 15);
51     }
52     
53     function draw()
54     {
55         //Cancel any movement animation if a new chart is requested
56         if(typeof animation_loop != undefined) clearInterval(animation_loop);
57         new_degrees=new_degrees+1;
58         difference = new_degrees - degrees;
59         animate_to();
60     }
61     
62     //function to make the chart move to new degrees
63     function animate_to()
64     {
65         //clear animation loop if degrees reaches to new_degrees
66         if(degrees == new_degrees) 
67             clearInterval(animation_loop);
68         
69         if(degrees==360)//如果加载到了360度，就停止
70             return;
71         
72         if(degrees < new_degrees)
73             degrees++;
74         else
75             degrees--;
76             
77         init();
78     }
79     
80     //调用各个函数，实现动态效果
81     draw();
82     redraw_loop = setInterval(draw, 10); //Draw a new chart every 2 seconds
83     
84 }