使用类别来为UIColor添加方法：
#import <UIKit/UIKit.h>
@interface UIColor(Random)
+(UIColor *)randomColor;
@end
实现文件：
@implementation UIColor(Random)
+(UIColor *)randomColor
{
static BOOL seeded = NO;
if (!seeded) {
seeded = YES;
srandom(time(NULL));
}
CGFloat red = (CGFloat)random()/(CGFloat)RAND_MAX;
CGFloat blue = (CGFloat)random()/(CGFloat)RAND_MAX;
CGFloat green = (CGFloat)random()/(CGFloat)RAND_MAX;
return [UIColor colorWithRed:red green:green blue:blue alpha:1.0f];
}
@end