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直接上代码，注释很详细

#import "CaptchaView.h"#define kRandomColor  [UIColor colorWithRed:arc4random() % 256 / 256.0 green:arc4random() % 256 / 256.0 blue:arc4random() % 256 / 256.0 alpha:1.0];//#define kRandomColor  [UIColor grayColor];#define kLineCount 6#define kLineWidth 1.0#define kCharCount 4#define kFontSize [UIFont systemFontOfSize:arc4random() % 5 + 15]@implementation CaptchaView@synthesize changeString,changeArray;- (instancetype)initWithFrame:(CGRect)frame{    if (self = [super initWithFrame:frame]) {        self.layer.cornerRadius = 5.0; //设置layer圆角半径        self.layer.masksToBounds = YES; //隐藏边界        self.backgroundColor = kRandomColor;        //        [UIColor grayColor]        //显示一个随机验证码        [self changeCaptcha];    }    return self;}#pragma mark 更换验证码,得到更换的验证码的字符串-(void)changeCaptcha{    //
   <一>
    从字符数组中随机抽取相应数量的字符，组成验证码字符串    //数组中存放的是全部可选的字符，可以是字母，也可以是中文    self.changeArray = [[NSArray alloc] initWithObjects:@"0",@"1",@"2",@"3",@"4",@"5",@"6",@"7",@"8",@"9",@"A",@"B",@"C",@"D",@"E",@"F",@"G",@"H",@"I",@"J",@"K",@"L",@"M",@"N",@"O",@"P",@"Q",@"R",@"S",@"T",@"U",@"V",@"W",@"X",@"Y",@"Z",@"a",@"b",@"c",@"d",@"e",@"f",@"g",@"h",@"i",@"j",@"k",@"l",@"m",@"n",@"o",@"p",@"q",@"r",@"s",@"t",@"u",@"v",@"w",@"x",@"y",@"z",nil];    //如果能确定最大需要的容量，使用initWithCapacity:来设置，好处是当元素个数不超过容量时，添加元素不需要重新分配内存    NSMutableString *getStr = [[NSMutableString alloc] initWithCapacity:kCharCount];    self.changeString = [[NSMutableString alloc] initWithCapacity:kCharCount];    //随机从数组中选取需要个数的字符，然后拼接为一个字符串    for(int i = 0; i < kCharCount; i++)    {        NSInteger index = arc4random() % ([self.changeArray count] - 1);        getStr = [self.changeArray objectAtIndex:index];        self.changeString = (NSMutableString *)[self.changeString stringByAppendingString:getStr];    }}#pragma mark 点击view时调用，因为当前类自身就是UIView，点击更换验证码可以直接写到这个方法中，不用再额外添加手势-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{    //点击界面，切换验证码    [self changeCaptcha];    //setNeedsDisplay调用drawRect方法来实现view的绘制    [self setNeedsDisplay];}#pragma mark 绘制界面（1.UIView初始化后自动调用； 2.调用setNeedsDisplay方法时会自动调用）- (void)drawRect:(CGRect)rect {    // 重写父类方法，首先要调用父类的方法    [super drawRect:rect];    //设置随机背景颜色    self.backgroundColor = kRandomColor;    //获得要显示验证码字符串，根据长度，计算每个字符显示的大概位置    NSString *text = [NSString stringWithFormat:@"%@",self.changeString];    CGSize cSize = [@"S" sizeWithAttributes:@{NSFontAttributeName:[UIFont systemFontOfSize:20.0]}];    int width = rect.size.width / text.length - cSize.width;    int height = rect.size.height - cSize.height;    CGPoint point;    //依次绘制每一个字符,可以设置显示的每个字符的字体大小、颜色、样式等    float pX, pY;    for (int i = 0; i < text.length; i++)    {        pX = arc4random() % width + rect.size.width / text.length * i;        pY = arc4random() % height;        point = CGPointMake(pX, pY);        unichar c = [text characterAtIndex:i];        NSString *textC = [NSString stringWithFormat:@"%C", c];        [textC drawAtPoint:point withAttributes:@{NSFontAttributeName:kFontSize}];    }        //调用drawRect：之前，系统会向栈中压入一个CGContextRef，调用UIGraphicsGetCurrentContext()会取栈顶的CGContextRef        CGContextRef context = UIGraphicsGetCurrentContext();        //设置画线宽度        CGContextSetLineWidth(context, kLineWidth);        //绘制干扰的彩色直线        for(int i = 0; i < kLineCount; i++)        {            //设置线的随机颜色            UIColor *color = kRandomColor;            CGContextSetStrokeColorWithColor(context, [color CGColor]);            //设置线的起点            pX = arc4random() % (int)rect.size.width;            pY = arc4random() % (int)rect.size.height;            CGContextMoveToPoint(context, pX, pY);            //设置线终点            pX = arc4random() % (int)rect.size.width;            pY = arc4random() % (int)rect.size.height;            CGContextAddLineToPoint(context, pX, pY);            //画线            CGContextStrokePath(context);        }}@end
   

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