1 // pages/bind/bind.js
 2 var app = getApp();
 3 var baseUrl = getApp().baseUrl;
 4 var ctx;
 5 Page({
 6  
 7     /**
 8      * 页面的初始数据
 9      */
10     data: {
11         text: ‘‘,
12     },
13  
14     /**
15      * 生命周期函数--监听页面加载
16      */
17     onLoad: function(options) {
18         var that = this;
19         drawPic(that);
20     },
21     change: function() {
22         var that = this;
23         drawPic(that);
24     },
25     mobile(e) {
26         this.setData({
27             mobile: e.detail.value
28         })
29     },
30 })

 1 function randomNum(min, max) {
 2     return Math.floor(Math.random() * (max - min) + min);
 3 }
 4 /**生成一个随机色**/
 5 function randomColor(min, max) {
 6     var r = randomNum(min, max);
 7     var g = randomNum(min, max);
 8     var b = randomNum(min, max);
 9     return "rgb(" + r + "," + g + "," + b + ")";
10 }
11  
12 /**绘制验证码图片**/
13 function drawPic(that) {
14     ctx = wx.createCanvasContext(‘canvas‘);
15     /**绘制背景色**/
16     ctx.fillStyle = randomColor(180, 240); //颜色若太深可能导致看不清
17     ctx.fillRect(0, 0, 90, 28)
18     /**绘制文字**/
19     var arr;
20     var text = ‘‘;
21     var str = ‘ABCEFGHJKLMNPQRSTWXY123456789‘;
22     for (var i = 0; i < 4; i++) {
23         var txt = str[randomNum(0, str.length)];
24         ctx.fillStyle = randomColor(50, 160); //随机生成字体颜色
25         ctx.font = randomNum(20, 26) + ‘px SimHei‘; //随机生成字体大小
26         var x = 5 + i * 20;
27         var y = randomNum(20, 25);
28         var deg = randomNum(-20, 20);
29         //修改坐标原点和旋转角度
30         ctx.translate(x, y);
31         ctx.rotate(deg * Math.PI / 180);
32         ctx.fillText(txt, 5, 0);
33         text = text + txt;
34         //恢复坐标原点和旋转角度
35         ctx.rotate(-deg * Math.PI / 180);
36         ctx.translate(-x, -y);
37     }
38     /**绘制干扰线**/
39     for (var i = 0; i < 4; i++) {
40         ctx.strokeStyle = randomColor(40, 180);
41         ctx.beginPath();
42         ctx.moveTo(randomNum(0, 90), randomNum(0, 28));
43         ctx.lineTo(randomNum(0, 90), randomNum(0, 28));
44         ctx.stroke();
45     }
46     /**绘制干扰点**/
47     for (var i = 0; i < 20; i++) {
48         ctx.fillStyle = randomColor(0, 255);
49         ctx.beginPath();
50         ctx.arc(randomNum(0, 90), randomNum(0, 28), 1, 0, 2 * Math.PI);
51         ctx.fill();
52     }
53     ctx.draw(false, function() {
54         that.setData({
55             text: text
56         })
57     });
58 }

<canvas canvas-id="canvas" bindtap=‘change‘></canvas>

canvas {
    width: 90px;
    height: 28px;
    border-radius: 14px;
}