微信小程序实现图形验证码

<canvas canvas-id="canvas" bindtap='change' style="width:90px;height: 30px;"></canvas>

var app = getApp();
var baseUrl = getApp().baseUrl;
var ctx;
data:{
    text:""
},
onLoad: function(options) {
    var that = this;
    this.drawPic(that);
},
change: function() {
    var that = this;
    this.drawPic(that);
},
mobile(e) {
    this.setData({
        mobile: e.detail.value
    })
},
randomNum(min, max) {
return Math.floor(Math.random() * (max - min) + min);
},
/**生成一个随机色**/
randomColor(min, max) {
let r = this.randomNum(min, max);
let g = this.randomNum(min, max);
let b = this.randomNum(min, max);
return "rgb(" + r + "," + g + "," + b + ")";
},
 
/**绘制验证码图片**/
drawPic(that) {
ctx = wx.createCanvasContext('canvas');
/**绘制背景色**/
ctx.fillStyle = this.randomColor(180, 240); //颜色若太深可能导致看不清
ctx.fillRect(0, 0, 90, 28)
/**绘制文字**/
var arr;
var text = '';
var str = 'ABCEFGHJKLMNPQRSTWXY123456789';
for (var i = 0; i < 4; i++) {
   var txt = str[this.randomNum(0, str.length)];
   ctx.fillStyle = this.randomColor(50, 160); //随机生成字体颜色
   ctx.font = this.randomNum(20, 26) + 'px SimHei'; //随机生成字体大小
   var x = 5 + i * 20;
   var y = this.randomNum(20, 25);
   var deg = this.randomNum(-20, 20);
   //修改坐标原点和旋转角度
   ctx.translate(x, y);
   ctx.rotate(deg * Math.PI / 180);
   ctx.fillText(txt, 5, 0);
   text = text + txt;
   //恢复坐标原点和旋转角度
   ctx.rotate(-deg * Math.PI / 180);
   ctx.translate(-x, -y);
}
/**绘制干扰线**/
for (var i = 0; i < 4; i++) {
   ctx.strokeStyle = this.randomColor(40, 180);
   ctx.beginPath();
   ctx.moveTo(this.randomNum(0, 90), this.randomNum(0, 28));
   ctx.lineTo(this.randomNum(0, 90), this.randomNum(0, 28));
   ctx.stroke();
}
/**绘制干扰点**/
for (var i = 0; i < 20; i++) {
   ctx.fillStyle = this.randomColor(0, 255);
   ctx.beginPath();
   ctx.arc(this.randomNum(0, 90), this.randomNum(0, 28), 1, 0, 2 * Math.PI);
   ctx.fill();
}
ctx.draw(false, function() {
   that.setData({
       text: text
   })
});