利用Quartz2D实现：
绘图步骤: 1.获取上下文 2.描述路径 3.把路径添加到上下文 4.渲染上下文
drawRect：画线必须要在drawRect方法实现，因为只有在drawRect方法中才能获取到根view相关联上下文；当前控件即将显示的时候调用，只调用一次；
注意： drawRect不能手动调用；drawRect只能系统调用,每次系统调用drawRect方法之前,都会给drawRect方法传递一个跟当前view相关联上下文；
执行[self.view setNeedsDisplay];会调用drawRect。

效果图

#define kRandomColor [UIColor colorWithRed:arc4random()%255/255.0 green:arc4random()%255/255.0 blue:arc4random()%255/255.0 alpha:1.0] //随机颜色
#define kLineCount 6        //干扰线的个数
#define kLineWidth 1.0      //干扰线的宽度
#define kCharCount 5        //需要验证码的个数
#define kFontSize [UIFont systemFontOfSize:arc4random() % 5 + 15]   //字体大小

获取随机验证码

#pragma mark - 获取随机验证码
- (void)getAuthcode {

    _authCodeStr = [[NSMutableString alloc] initWithCapacity:kCharCount];
    _dataArray = @[@1,@2,@4,@5,@7,@8,@0,@9,@"Z",@"W",@"E",@"R",@"L",@"w",@"g",@"j",@"t",@"v"];

    //随机从素材数组中取出需要个数的字符串，拼接为验证码字符串存入验证码数组中
    for (int i = 0; i < kCharCount; i ++){
        NSInteger index = arc4random() % self.dataArray.count;
        [self.authCodeStr insertString:[NSString stringWithFormat:@"%@",self.dataArray[index]] atIndex:i];
    }
    
}

点击view刷新验证码

//点击界面切换验证码
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{

    [self getAuthcode];
    [self setNeedsDisplay];
}

绘制验证码和线条

- (void)drawRect:(CGRect)rect {
    [super drawRect:rect];
    self.backgroundColor = [UIColor whiteColor];
    
    /*计算每个字符串显示的位置*/
    NSString *text = [NSString stringWithFormat:@"%@",_authCodeStr];
    
    CGSize cSize = [@"A" sizeWithAttributes:@{NSFontAttributeName:[UIFont systemFontOfSize:20]}];
    
    int width = rect.size.width / text.length - cSize.width;
    int height = rect.size.height - cSize.height;
    CGPoint point;
    
    /*绘制字符*/
    float tempX,tempY;
    for (int i = 0 ; i < text.length; i ++) {
        tempX = arc4random() % width + rect.size.width / text.length * i;
        tempY = arc4random() % height;
        point = CGPointMake(tempX, tempY);
        
        unichar c = [text characterAtIndex:i];
        NSString *textC = [NSString stringWithFormat:@"%C", c];
        
        [textC drawAtPoint:point withAttributes:@{NSFontAttributeName:kFontSize}];
    }
   
    //绘制kLineCount条随机色干扰线
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetLineWidth(context,kLineWidth);
    for(int cout = 0; cout < kLineCount; cout++)
    {
        
        CGContextSetStrokeColorWithColor(context, kRandomColor.CGColor);
        tempX = arc4random() % (int)rect.size.width;
        tempY = arc4random() % (int)rect.size.height;
        CGContextMoveToPoint(context, tempX, tempY);
        tempX = arc4random() % (int)rect.size.width;
        tempY = arc4random() % (int)rect.size.height;
        CGContextAddLineToPoint(context, tempX, tempY);
        CGContextStrokePath(context);
    }
   
    
}